BRAIN

Humans and other vertebrates possess a central nervous system (CNS)—the brain and spinal cord—containing specialized cells called neurons. The nervous system is essential for virtually every aspect of life and, along with the body's other systems (muscular-skeletal, endocrine, etc.), performs the following seven basic, interrelated tasks:

1. Maintenance of vital functions, including control of the cardiovascular system and homeostasis (regulation of temperature, weight, internal milieu in general).
2. Obtaining information via the sensory systems (auditory, visual, somatosensory, olfactory, etc.) and processing that information. Information about ourselves and the world is provided by the sensory portions of the nervous system.
3. Storage and retrieval of information by the processes of learning and memory. Changes occur in the brain every time something new is learned, and the changes often last in the form of memories. Moreover, the brain must be adept at retrieving that information from storage when needed.
4. Production of behavior, including movement, locomotion, autonomic responses, and communicative behavior such as language. The brain's motor systems operate skeletal muscles that move the limbs, facial muscles, mouth, vocal cords, and so on.
5. Integration of information and output: tying things together to make "decisions" ranging from simple reflexes to complex social and cognitive processes (intelligence, language, spatial orientation, etc.).
6. Modulation of the overall activity levels of the brain and body associated with emotion, arousal, and sleep.
7. Carrying out the genetic mandate to pass on one's genes to the next generation, especially with respect to sex, reproductive behavior, parenting, and aggression.

It is evident from our everyday observations that any or all of the seven functions may operate at sub-par levels as people get older. For example, maintaining body temperature may become more difficult under extreme conditions; the eyes, ears, and other senses may not pick up as much as they used to; it can become harder to remember names; athletic skills decline; "intelligence" for new technological concepts seems poorer compared to that of young people; a good night's sleep is often harder to get; and the frequency of sexual activity may change. Because all seven functions are beholden to the nervous system, it follows that age-related changes in the system's components are part and parcel of these problems.

Understanding how the nervous system and its components fare as individuals age, how agerelated neural changes are manifested behaviorally, and how this knowledge may be used to improve the quality of life is an immensely daunting task because, irrespective of aging, the nervous system is bafflingly complex.

The nervous system and its complexity

The number of neurons in the human brain is vast—many billions (although glial cells, which provide various support functions, are even more numerous). A thread-like axon (nerve fiber) extends from the neuron, often branching repeatedly, to provide functional connections to other neurons located at its endings (terminals), sometimes at remote locations within the nervous system. Electrical impulses (action potentials) are generated in or near a neuron's cell body and travel outward along the axons, much as telegraph impulses are sent from an operator, traversing wires to receiving destinations (other neurons). A unique feature of neurons that greatly increases the number of axon terminals that can contact them are dendrites—elaborate tree-like arrays emanating from the cell body. The evolution of extensive dendritic trees, coupled with branching axons, has led to the development of neural "wiring diagrams" of enormous complexity. The situation is further complicated by the properties of synapses, the sites where dendrites and axon terminals "communicate." For the most part, the communication between neurons uses chemical neurotransmitters that are typically stored in the axon terminals. Packets of neurotransmitter molecules are released by mechanisms associated with the arrival of nerve impulses generated by the axons' parent neuron. The neurotransmitter quickly diffuses across the narrow synaptic cleft to reach a dendrite or cell body of the target neuron. The neurotransmitter molecules find their way to synaptic receptors, specialized sites in or on the receiving neuron that react with the neurotransmitter. The reaction between neurotransmitter and synaptic receptors alters some physiological properties of the receiving neuron, changing its activity and output. This neuron is, of course, connected to other neurons via the synapses made by its own axon terminals, which are in turn affected. And so on.

A number of different types of neurotransmitters are used by the nervous system, such as acetylcholine, dopamine, serotonin, and many others. Moreover, for a particular neurotransmitter there can be a variety of receptor types on one receiving neuron or another, so that the same neurotransmitter can produce different effects. The large number of possible permutations of neurotransmitter and receptors confer yet another layer of complexity upon the brain.

All of these highly varied, interacting aspects of neural circuits somehow work together in a manner that miraculously allows the nervous system to accomplish its basic functions. By the same token, they provide many "targets" for deleterious age-related changes. Reviewing some basics of the nervous system structure and function and their neurogerontological implications will help impart a sense of the mischief that aging can visit upon the nervous system. Then, after addressing a few of the basic functions, some possible ways by which age-related changes in the nervous system might be modified for the better will be outlined.

Organization of neurons into a nervous system and basic neuroanatomy

For meaningful behavior to occur, information from the body and environment must get into the brain, and instructions from the brain must be delivered to the muscles and glands so that the body can perform. The portion of the nervous system that provides this interface is called the peripheral nervous system. It has two basic subsystems: the somatic system and the autonomic system. The somatic system brings information into the brain via axons originating in the sensory organs and sends messages outward through axons to skeletal muscles causing them to contract. The autonomic system is linked to our emotions and arousal states, and activates glands (e.g., sweat glands), smooth muscles (e.g., controlling the pupils of the eye, blood vessels, etc.), and heart. The autonomic system contains two subsystems: the sympathetic system readies us for action (increases heart rate, dilates pupils), whereas the parasympathetic system has the opposite effect, restoring us toward a resting level when warranted. Some age-related changes often occur in the peripheral systems, such as loss or thinning of nerve fibers or changes in the "target organs" (muscles and glands), and these may result in poorer performance.

In the central nervous system (CNS), the spinal cord extends below the brain, encased by the vertebral bones of the neck and back. The spinal cord relays incoming sensory messages from the periphery to the brain and outgoing instructions from the brain to spinal cord neurons whose axons leave the CNS to activate muscles. However, in addition to being a relay to and from the brain, the spinal cord itself contains impressive assemblies of neuronal circuitry that perform a number of sensory-motor behaviors, such as withdrawal from a painful stimulus or rhythmic movements associated with locomotion. Changes in the spinal cord can occur in older people that make it less efficient at relaying the information up and down.

Merging with the spinal cord is the brainstem, the lowest region of the brain. Its basic subdivisions (moving upwards) are the medulla, pons, and midbrain. At the top of the brainstem is the thalamus. The neural traffic between the brain and spinal cord travels along axons that traverse the brainstem. In addition, sensory information (coded in trains of action potentials) enters the brainstem from the head and the special sense organs (ears, eyes, etc.), while other messages leave the brainstem to control the face, mouth, eyes, and so on. The brainstem controls a number of activities without a necessary contribution from higher levels of the brain. For example, comparison of acoustic input from the two ears to compute the location of sounds in space is a function of brainstem circuits. Some regions of the brainstem are especially vulnerable to agerelated changes that can affect a variety of behaviors.

Behind the brainstem, near the base of the skull, is the cerebellum. It plays key roles in coordinating movements, balance, and even some types of learning. The cerebellum has numerous axons communicating with the brainstem that, in turn, communicate with the higher regions of the brain and the spinal cord below. The cerebellum is required for balance, posture, gait, and the adjustment and coordination of movements. Age-related changes observed in neurons of the cerebellum include loss of dendrites and spines and changes in neurotransmitter systems, and these are likely to affect movements.

The bulk of the brain lies above and around the thalamus. The basal ganglia are prominent parts of the interior brain adjacent to the thalamus, and deal with movement (they play important roles in cognition, as well). To the side, but folded over so as to sit deep in the middle of the brain, is the hippocampus, a structure that is essential for certain types of learning and memory as well as spatial behaviors. The amygdala, along with the hypothalamus and other parts of the limbic system, comprise the "emotional brain." At the base of the brain is the hypothalamus, a collection of small subdivisions that regulate a number of essential functions such as eating and drinking, body temperature, biological rhythms, and reproductive behavior. The hypothalamus produces hormones that influence the release of other hormones by the neighboring pituitary gland. At least some of these hormone systems become less responsive with age because of reduced production by hypothalamic neurons, changes in receptor sensitivity for the hormones, and/or changes in the endocrine glands. The fact that relatively small hypothalamic subdivisions control important biological functions has suggested that subtle changes might contribute to aging in a fundamental way.

If one looks at a human brain, it is dominated by two large, folded cerebral hemispheres. On the surface of the hemispheres (and folded into the creases or sulci) are several layers of neurons that comprise the cerebral cortex. The cerebral cortex, working in concert with the rest of the brain, is capable of incredible feats, most notably in humans, in which its size and complexity far exceeds that of other species. Language, complex thoughts, logic, and many other "higher" functions are beholden to a highly advanced cerebral cortex. The left and right hemispheres communicate with one another with millions of axons, most of which are contained in a huge band of axons called the corpus callosum. Studies have found evidence that the transfer of information between cerebral hemispheres across the corpus callosum can be slowed or diminished with age.

Neurobiology and aging

In order to understand how the "slings and arrows" of aging can affect the brain, some basics of neurobiology must be appreciated.

Neurons lack the capacity to regenerate. With a few exceptions, new neurons are not produced once the maximum number is established early in life, and the ability of CNS neurons to be repaired when damaged is quite limited. We can lose neurons as we age, but we cannot grow new ones. The exact number of neurons lost by the human brain during aging has been elusive, plagued by methodological issues that include technical difficulty in counting neurons, post-mortem changes that can occur in human brains from autopsy, differences in the pre-mortem condition of young and old people who are autopsied (for example, older people are more likely to have died from chronic illnesses that could have resulted in brain pathology), and the unwitting inclusion of patients with undetected dementia. Even when nonhuman animals are studied, inconsistencies arise, stemming from species differences, variability among genetic strains within the same species, and the fact that different parts of the brain often show different age changes. All of this suggests that no general pattern of neuron loss occurs in aging nervous systems. However, there is a growing consensus that some older studies probably overestimated the degree to which neurons die as people age. The current view is more optimistic: at least in the neocortex, many (perhaps most) healthy older people exhibit a minimal loss of neurons, although other brain regions may be more vulnerable.

Neurons require a disproportionate share of the blood supply. Neurons have a ravenous appetite for the blood's precious cargo of glucose and oxygen, and the percentage of the body's blood and oxygen consumption in the brain at any time is far out of proportion with the rest of the body. It has to be this way because reducing the supply of blood/oxygen to neurons results in impairment, damage, or destruction depending on the severity and duration. Thus, conditions that reduce the brain's blood supply, such as atherosclerosis, diabetes, and, of course, stroke are cause for concern. Each of these conditions becomes more prevalent with age, as do other changes in the vascular system serving the brain, even in the absence of diseases.

Neurons are at risk from various toxins. Over a lifetime, neurons, like other cells, are exposed to toxins. These can be environmental or endogenous—produced by the brain itself. For example, glutamate is the major neurotransmitter used by neurons to synaptically activate (excite) other neurons. Under certain conditions, such as hypoxia or tissue damage, the effects of glutamate can become exaggerated, resulting in excessive entry of calcium into the neurons, and such excitotoxic events prove to be damaging to neurons. If aging were associated with weakening of the defenses against excitotoxicity, negative age effects could accrue. Indeed, this process appears to be involved in certain types of dementia and neurodegenerative conditions that can accompany aging.

The neuron's nucleus regulates many functions. The synthesis of proteins is coded by DNA, the genetic material found in the nucleus of neurons and other cells. Many varieties of protein are produced for use as structural components of neurons (e.g., the microtubules and microfilaments in axons that transport molecules used for neurotransmitters and provide structural support), enzymes that control the numerous biochemical reactions necessary for cellular activities, synaptic receptors, and many other uses. Damage to DNA that can accrue in cells with age has the potential to alter many facets of neuronal physiology.

Dendritic branches and spines are at risk with aging. The size, shape, orientation, and complexity of the neuron's dendritic tree have a great deal to do with the number of functioning contacts that can be made with other neurons. Dendritic spines are small extensions that provide many additional sites for synapses. One of the best documented age-related changes in neurons is a reduction in the number of dendritic branches and spines. Even if neurons do not die off, a loss of synaptic contacts is likely to reduce the information-processing capacity of neural circuits, negatively affecting brain function.

Parts of the brain are differentially vulnerable to aging. Age effects vary greatly among different components of the nervous system. Various behavioral and cognitive functions are affected to different extents, depending on how each brain region fares. For example, the hippocampus is very important for storing memories. Research has shown that portions of the hippocampus are often damaged during aging, and this may be responsible for learning and memory deficits.

The speed of information processing slows with age. Behavior and cognition tend to become slower with age. Indeed, behavioral/cognitive slowing has been proposed as a marker of aging (i.e., a measure that can differentiate chronological age from functional age). There is a good deal of research indicating a general slowing of brain processes, with cognitive slowing likely to reflect the sluggishness of smaller components (sensory, motor, and interconnected central circuits). Possible causes of slowing might include slower conduction of action potentials because of changes in the axons; slower synaptic transmission because of structural and/or chemical changes; diminished intracellular metabolism (e.g., associated with damage to energy-producing mitochondria); reduced production of neurotransmitters or other critical products; impaired gene expression (e.g., associated with DNA damage); and many other potential changes that would interfere with optimal neural performance. Changes in the peripheral sensory and motor systems (e.g., loss or thinning of axons) probably make only small contributions to slowing. More salient are the central neural circuits that intervene between stimuli and responses.

Aging and the basic functions of the nervous system

Only a few examples of how aging affects the seven functions of the nervous system are presented, but they show the types of age effects that have been observed.

Obtaining information with the sensory systems. The neural means by which sensory stimuli are experienced involve multistage processes requiring high-quality representation of stimuli by the peripheral sensory apparatus, undistorted neural messages carried by action potentials into the brain, and accurate processing of the information by the central sensory systems. Disruption of any of these processes with age would have the potential to cause problems in the sensory domain. The sad truth is that our sensory abilities almost inevitably decline with age. The rate and severity of the decline may vary considerably among individuals and across sensory modalities within individuals, but few, if any, octogenarians possess the same sensory capacities they started out with. All the sensory modalities suffer with age, including hearing and the auditory system.

The term "presbycusis" or "presbyacusis" is typically used to describe the changes in hearing associated with aging. Whereas the most commonly mentioned manifestation of presbycusis is a loss of sensitivity for high frequency sounds, the types of hearing problems confronting older listeners extend to speech perception, hearing in noisy backgrounds, distorted loudness of sounds, and tinnitus ("ringing in the ears"). Presbycusis typically involves progressive damage to the inner ear: the cochlea (where acoustic events are ultimately translated to neural events) and the cochlear neurons (where sounds are coded as trains of action potentials and sent, via the auditory nerve, to the brain for processing). Damage to any part of the cochlea diminishes the amount and quality of auditory input to the brain, with deleterious effects on hearing.

It is in the CNS where the action potential–coded sensory information originating in the inner ear is somehow transformed into auditory perception and experience. The central components of the auditory system are threatened by two adverse correlates of aging. First, changes in the structure or function of the brain's neurons occur in the context of biological aging discussed above. Second, an otherwise "healthy" central auditory system may be secondarily affected by damage to the cochlea. It has been shown that, when certain central neurons are deprived of their normal synaptic input, physiological and anatomical changes are induced. The effects can produce additional hearing deficits. Because the altered neurons provide input to other neurons, the effects could spread. Because the central sensory systems of older individuals might be affected in two rather different ways, it is useful to differentiate two types of age-related central changes. The term central effects of biological aging (CEBA) refers to sensory changes stemming from age-related changes in neurons, metabolism, support systems, and so on. The term central effects of peripheral pathology (CEPP) also refers to sensory changes associated with modifications of neurons and neural circuits in the brain. However, these are secondary to the removal or alteration of peripheral sensory input. It would be expected that CEBA and CEPP often occur in combination, since many older people have some loss of receptor function as well as various CNS deficits.

Whether CEBA, CEPP, or both are at work, the changes that occur in the auditory CNS are multifaceted. Some neurons die off or come to perform less efficiently, becoming "sluggish" in their responses to sound. By contrast, other neurons come to respond more vigorously, probably because aging is accompanied by deficits in inhibitory neurotransmitters, which normally dampen the responses of neurons and prevent hyperactivity. A combination of these and other types of central changes are likely to contribute to difficulties that many older people have in understanding speech, even when it is loud enough for them to hear.

The storage of information (learning and memory). Research indicates that, to varying extents (according to individual differences, genotype, species, etc.), circuits and neurotransmitter systems relevant for learning and memory often exhibit deleterious changes with age; deficiencies in any of these can cause some sort of learning/memory deficit. Learning and memory involve modifications (plasticity) of synapses in neural circuits. For example, one type of synaptic change associated with learning is long-term potentiation (LTP): lasting changes in neural responses induced by situations similar to those involved with learning. Experiments have shown changes in LTP in hippocampus neurons of old rats that have learning deficits. Thus, in addition to the general types of changes that occur in aging nervous systems, processes specific to learning may be affected as well.

Production of behavior (movement, etc.). Whereas some of the age-related declines in motor skill are associated with a decrease in muscle mass and a loss of strength, the most important and interesting stories are found in the workings of the nervous system. A large portion of the nervous system is devoted to movement—deciding what to do, planning how to do it, and carrying it out. Each has been demonstrated to exhibit some degree of age-related change that might result in less effective movement. For example, the primary motor cortex is the major source of descending axons to the motor neurons of the spinal cord that control the muscles. Several studies have described abnormalities in the large neurons of the primary motor cortex of older brains, including a loss of dendrites and dendritic spines. In addition to the motor cortex, the basal ganglia (the next lower set of structures controlling movement) are involved in self-initiated, complex movements, the control of postural adjustments, and other aspects of motor behavior. The effects of damage to the basal ganglia are evident in the motor symptoms of Parkinson's and Huntington's diseases, disorders affecting these structures. Some of the mild motor disturbances that occur in healthy older people could be a consequence of less severe basal ganglia damage that has been observed during normal aging.

Modulation of behavior (emotion, arousal, stress). Our behavior varies constantly—up and down, this way and that way—in accordance with emotions, arousal, and biological clocks. One powerful modulator of behavior is stress: Various stimuli, events, or situations that are actually or potentially threatening (stressors) elicit activation of the sympathetic nervous system and a sequence of hormonal reactions, including the release of glucocorticoid hormones from the adrenal gland. Whereas the stress response is adaptive (e.g., it increases the probability of surviving dangerous situations), too much stress is generally considered to be a bad thing. Indeed, high blood pressure, suppression of the immune system, and exacerbation of diseases are known concomitants of stress. Thus, the relationship between stress and aging is potentially important. Moreover, there is evidence that, over time, glucocorticoids can actually damage the hippocampus, contributing to negative changes in the aging brain.

Modifying changes in the aging nervous system

Neural plasticity is a term that describes the ability of synapses, dendrites, axons, and other aspects of neurons to change—usually in an adaptive fashion. Plasticity is very potent in developing organisms, and it is now established that older brains retain much capacity for change as well. Research has shown that new synapses can form in older brains in response to injury or environmental manipulations, and that dendrites continue to be modifiable. However, the process generally takes longer and may not reach the magnitude typical of younger brains. The ability of the adult nervous system to engage mechanisms of synaptic plasticity has at least two important implications. First, degenerative tendencies may be counteracted by replacement of damaged synapses and repair of neural circuits. Second, the nervous system can continue to manifest the normal, adaptive types of synaptic plasticity exhibited by young individuals.

The dynamic properties of the older nervous system provide potential opportunities for the development of strategies aimed at modulating the direction or severity of negative age-related changes. A number of approaches are being investigated by researchers. In one way or another, most approaches attempt to enhance neural functioning by promoting the activity of various neurotransmitters or other physiologically important substances that protect neurons from age-related damage or improve neural functioning per se. For example, diets that promote the general health of cardiovascular and other systems are also good for the nervous system.

Neurotrophic factors such as nerve growth factor (NGF) are essential for the maintenance, growth, and survival of neurons both during development and in adults. Administration of neurotrophic factors has been shown to retard or prevent neural degeneration in experimental animals, and infusion of NGF may be able to prevent shrinkage of neurons typically observed with age. It appears that neurotrophic factors may have a variety of potentially beneficial effects on the aging nervous system. Some of these may be harnessed for clinical use.

Calorically restricted diets can extend longevity of rodents, slow certain age-related physiological declines, and decrease tumors and diseases. Although much of this research has focused on non-neural systems, there is ample evidence that dietary restriction modulates aging of the brain. Effects of dietary restriction on some of the general concomitants of neural aging, such as accumulation of lipofuscin ("age pigment") in neurons, the efficacy of glial cells, and loss of dendritic spines, have been reported.

Relatively simple environmental manipulations can have beneficial effects on the brain. Young and old rats living in an "enriched" environment (e.g., ten rats per cage, large space, toys) may exhibit a thicker cerebral cortex, compared to like-aged unenriched rats. Enhancement of dendritic growth and complexity have also been demonstrated in studies of environmental enrichment. Some evidence has linked neurotrophic factors to environmental enrichment and improved cognitive performance. The expression of NGF has been found to increase under these conditions. It could be that enriched environments or behaviors are associated with increased neural activity, which results in an upregulation of nerve growth factors, which in turn leads to enhanced neuronal survival, growth, and plasticity.

Unfortunately, age-related damage to neurons can be too severe to be managed by neurotrophins or environmental manipulations. This is especially true of neurodegenerative diseases. In such cases, transplantation or grafting of new neurons into the damaged site might prove to be feasible approach. The main problems are survival of the graft and, more importantly, appropriate rewiring of circuitry with the host brain. There is a tendency of the grafted tissue to make contacts appropriate for their neurotransmitters and circuits, although this depends on brain region and other variables. The possibility of replacing brain tissue lost to aging—thereby restoring function—is intriguing. Although controversial and inconsistent, improvements have been obtained by grafting tissue from the adrenal gland or fetal substantia nigra into Parkinson's patients. Encouraging results have been obtained from animal research in other brain regions as well, and several studies have shown that fetal brain tissue can be successfully transplanted into the brains of aged rodents. A big issue is whether complex behaviors and cognitive processes of humans might ever benefit from neural grafting. It is one thing to enhance dopamine activity in Parkinson's patients and another to replace intricate neural circuitry underlying cognitive processes. The latter may never be attainable. For now, the utility of neural grafts is likely to be found in their capacity to generate growth factors and other beneficial substances, or boost the activity of certain circuits by replenishing neurotransmitters.

James Willott

See also Alzheimer's Disease; Balance and Mobility; Cellular Aging; Dementia; Emotion; Hearing; Intelligence; Language Disorders; Memory; Neurobiology; Neurotransmitters; Parkinsonism; Stress and Coping.

BIBLIOGRAPHY

Albert, M. L., and Knoefel, J. E., eds. Clinical Neurology of Aging, 2d ed. New York: Oxford University Press, 1994.

Amenta, F. Aging of the Autonomic Nervous System. Boca Raton, Fla.: CRC Press, 1993.

Craik, F. I. M., and Salthouse. T. A. The Handbook of Aging and Cognition. Hillsdale, N.J.: Erlbaum, 1992.

Mann, D. M. A. Sense and Senility: The Neuropathology of the Aged Human Brain. New York: Chapman and Hall, 1997.

Morley, J. E., and Korenman, Stanleyg. Endocrinology and Metabolism in the Elderly. Boston: Blackwell Scientific Publishers, 1992.

Nussbaum, P. D. Handbook of Neuropsychology and Aging. New York: Plenum, 1997.

Sapolsky, R. M. Stress, the Aging Brain, and the Mechanisms of Neuron Death. Cambridge: MIT Press, 1992.

Schneider, E. L., and Rowe, J. W. Handbook of the Biology of Aging, 3d ed. San Diego: Academic Press, 1990.

Schneider, E. L., and Rowe, J. W. Handbook of the Biology of Aging, 4th ed. San Diego: Academic Press, 1996.

Willott, J. F. Aging and the Auditory System: Anatomy, Physiology, and Psychophysics. San Diego: Singular Press, 1991.

Willott, J. F. Neurogerontology: Aging and the Nervous System. New York: Springer Publishing Co., 1999.

Brain

Invertebrate brain

Vertebrate brain

Human brain

The brain stem

The diencephalon

The cerebrum

The cerebellum

Studying the brain

Resources

The brain is a mass of nerve tissue located in an animal’s head that controls the body’s functions. In simple animals, the brain functions like a switchboard picking up signals from sense organs and passing information to muscles. The brain is also responsible for a variety of involuntary behavior, including keeping the heart beating, maintaining blood pressure, and temperature. In more advanced forms, particularly vertebrates, a more analytical brain coordinates complex behaviors. In higher vetebrates, the brain coordinates thinking, memory, learning, and emotions. The brain is part of an animal’s central nervous system, which receives and transmits impulses. It works with the peripheral nervous system, which carries impulses to and from the brain and spinal cord via nerves running throughout the body.

Invertebrate brain

Nematodes (roundworms) have a simple brain and nervous system consisting of approximately 300 nerve cells, or neurons. Sensory neurons located in the head end of the animal detect stimuli from the environment and pass messages to the brain. The brain then sends out impulses through a ventral nerve cord to muscles, which respond to the stimulus. The way that the interneurons of the brain process the data determines the response.

The earthworm and other annelids, as well as insects and other arthropods, have more complex nervous systems. In these animals, there are paired ventral nerve cords that run from head to tail on the animal’s underside. Cell bodies of neurons in the cords form pairs of ganglia in each body segment. Four of the most anterior ganglia fuse in the head to form a brain. As a result, the brain ganglia are larger than the segmental ganglia, and also contain a larger proportion of sensory motor neurons. The brain ganglia have some dominance over the segment ganglia. The ventral nerve cords, brain, and segmental ganglia comprise the central nervous system. Neuronal fibers in the cords, bundled into nerves that carry communications between ganglia, make up the peripheral nervous system.

The earthworm’s brain consists of paired ganglia in the head end. An impulse, such as touch, light, or moisture, is detected by receptor cells in the skin. A pair of nerves in each of the earthworm’s segments carries the signal to the brain and smaller ganglia in each segment, where the signals are analyzed. The central nervous system then transmits impulses on nerves that coordinate muscle action, causing the earthworm to move.

In insects, specialized sense organs detect information from the environment and transmit it to the central nervous system. Such sense organs include simple and compound eyes, sound receptors on the thorax or in the legs, and taste receptors. The brain of an insect consists of a ganglion in the head. Some of the segmental ganglia are fused, allowing better communication between the segments. The information

that insects use for behaviors such as walking, flying, mating, and stinging is stored in the segmental ganglia. In experiments in which heads are cut off of cockroaches and flies, these insects continue to learn.

Vertebrate brain

The central nervous system of vertebrates consists of a single spinal cord, which runs in a dorsal position along the back, and a highly developed brain. The brain is the dominant structure of the nervous system. It is the master controller of all body functions, and the analyzer and interpreter of complex information and behavior patterns. We can think of the brain as a powerful neural computer. The peripheral nervous system, composed of nerves that run to all parts of the body, transmits information to and from the central nervous system.

The vertebrate brain is divided into three main divisions: the forebrain, the midbrain, and the hind-brain. The hindbrain connects the brain to the spinal cord, and a portion of it, called the medulla oblongata, controls important body functions such as breathing rate and the heart rate. The cerebellum, also in the hindbrain, controls balance. The forebrain consists of the cerebrum, thalamus, and hypothalamus. The fore-brain controls, among other things, the sense of smell in vertebrates.

During the first few weeks of development, the brain of a vertebrate looks like a series of bulges in the neural tube. It is hard to see a difference when we examine the early embryonic brains of fish, amphibians, reptiles, birds, and mammals. As the brain develops, the bulges enlarge, and each type of vertebrate acquires its own specific adult brain that helps it survive in its environment. In the forebrain of fish, the olfactory (smell) sense is well developed, whereas the cerebrum serves merely as a relay station for impulses. In mammals, on the other hand, the olfactory division is included in the limbic system, which also controls emotions, and the cerebrum is highly developed, operating as a complex processing center for information. Optic lobes are well developed in the midbrain of non-mammalian vertebrates, whereas in mammals the vision centers are mainly in the forebrain. In addition, a bird’s cerebellum is large compared to the rest of its brain, since it controls coordination and balance in flying.

Human brain

The living human brain is a soft, shiny, grayish white, mushroom-shaped structure. Encased within the skull, it is a 3 lb (1.4 kg) mass of nerve tissue that keeps us alive and functioning. On average, the brain weighs 13.7 ounces (390 g) at birth, and by age 15 grows to approximately 46 ounces (1,315 g). The human brain is composed of up to one trillion nerve cells—100 billion of them are neurons, and the remainder are supporting (glial) cells. Neurons receive, process, and transmit impulses, while glial cells (neuroglia) protect, support, and assist neurons. The brain is protected by the skull and by three membranes called the meninges—the outermost the dura mater, the middle the arachnoid, and the innermost the pia mater. Also protecting the brain is cerebrospinal fluid, a liquid that circulates between the arachnoid and pia mater in the subarachnoid space. Many bright red arteries and bluish veins on the surface of the brain penetrate inward. Glucose, oxygen, and certain ions pass easily from the blood into the brain, whereas other substances, such as antibiotics, do not. The capillary walls are believed to create a blood-brain barrier that protects the brain from a number of biochemicals circulating in the blood.

The parts of the brain can be studied in terms of structure and function. Four principal sections of the human brain are the brain stem (the hindbrain and midbrain), the diencephalon, the cerebrum, and the cerebellum.

The brain stem

The brain stem is the stalk of the brain, and is continuous with the spinal cord. It consists of the medulla oblongata, pons, and midbrain. A part of the brain stem, the medulla oblongata is a continuation of the spinal cord. All the messages that are transmitted between the brain and spinal cord pass through the medulla via fibers in the white matter. The fibers on the right side of the medulla cross to the left and those on the left cross to the right. The result is that each side of the brain controls the opposite side of the body. There are three vital centers in the medulla, which control the heartbeat, the rate of breathing, and the diameter of the blood vessels. Centers that help coordinate swallowing, vomiting, hiccoughing, coughing, and sneezing are also located in the medulla. The reticular formation occurs partially in the medulla and in other parts of the central nervous system. The reticular formation operates in maintaining our conscious state. The pons (meaning “bridge”) conducts messages between the spinal cord and the rest of the brain, and between the different parts of the brain. The midbrain conveys impulses from the cerebral cortex to the pons and spinal cord. It also contains visual and audio reflex centers involving the movement of eyeballs and head.

Twelve pairs of cranial nerves originate in the underside of the brain, mostly from the brain stem. They leave the skull through openings and extend as peripheral nerves to their destinations. Cranial nerves include the olfactory nerve that brings messages about smell from the nose and the optic nerve that conducts visual information from the eyes.

The diencephalon

The diencephalon lies above the brain stem, and embodies the thalamus and hypothalamus. In the diencephalon, the thalamus is an important relay station for sensory information for the cerebral cortex from other parts of the brain. The thalamus also interprets sensations of pain, pressure, temperature, and touch, and is concerned with some of our emotions and memory. It receives information from the environment in the form of sound, smell, and taste. The hypothalamus performs numerous important functions. These include the control of the autonomic nervous system (a branch of the nervous system involved with control of a number of body functions, such as heartbeat rate and digestion). The hypothalamus helps regulate the endocrine system and controls normal body temperature. It tells us when we are hungry, full, and thirsty. It helps regulate sleep and wakefulness, and is involved when we feel angry and aggressive.

The cerebrum

The cerebrum, constituting about 87.5% of the brain weight, spreads over the diencephalon. The cerebral cortex is the outer layer of the brain and is composed of gray matter made up of nerve cell bodies. It is about 0.08 inch (2 mm) thick and its surface area is about 5 square feet (1.5 sq m)—around half the size of an office desk. White matter, composed of nerve fibers covered with myelin sheaths, lies beneath the gray matter. With the rapid growth of the brain during embryonic development, the gray matter grows faster than the white matter and folds on itself. The folds are called convolutions or gyri, and the grooves between them are known as sulci. A deep longitudinal fissure separates the cerebrum into a left and right hemisphere. Each cerebral hemisphere is divided into frontal, temporal, parietal, and occipital lobes. The corpus callosum, a large bundle of fibers, connects the two cerebral hemispheres. The thalamus and subcortical nuclei, or basal ganglia, are areas of gray matter that exist below the white matter.

Sensory areas of the cerebrum interpret sensory impulses. Spoken and written language are transmitted to a part of the cerebrum called Wernicke’s area where meaning is extracted, and sent to Broca’s area, one of the motor areas of the cerebrum. Motor areas of the cerebrum control muscle movements. Within Broca’s area, thoughts are translated into speech, and muscles are coordinated for speaking. Impulses from other motor areas direct our hand muscles when we write, and our eye muscles when we scan the page for information.

Association areas of the cerebrum are concerned with emotions and intellectual processes, by connecting sensory and motor functions. In our association areas, innumerable impulses are processed that result in memory, emotions, judgment, personality, and intelligence.

Certain structures in the cerebrum and diencephalon make up the limbic system. These regions function in memory and emotions, and are associated with pain and pleasure.

By studying patients whose corpus callosa were destroyed, scientists realized that differences existed between the left and right sides of the cerebral cortex. The left side of the brain functions mainly in speech, logic, writing, and arithmetic. The right side of the brain, on the other hand, is more concerned with imagination, art, symbols, and spatial relations.

The cerebellum

The cerebellum is located below the cerebrum and behind the brain stem, and is shaped like a butterfly. The “wings” are the cerebellar hemispheres, and each consists of lobes that have distinct grooves or fissures. The cerebellum controls the movements of our muscular system needed for balance, posture, and maintaining posture.

Studying the brain

At the end of the nineteenth century, Santiago Ramón y Cajal (1852–1934), a Spanish scientist, studied neurons using stain developed by Camillo Golgi. Cajal realized that the brain was made up of individual units and not a continuous net as was believed at the time. His studies uncovered a large variety of neurons that differed in size and shape. He explained that neurons received signals on dendrites and transmitted impulses on axons. Since his work, researchers have learned that neurons carry information in the form of brief electrical impulses called action potentials that result when positively charged sodium ions travel across the axon membrane from the fluid outside to the cytoplasm inside. When a nerve impulse reaches the end of an axon, neurotransmitters are released at junctions called synapses. The neurotransmitters are chemicals that bind to receptors on the receiving neurons, triggering the continuation of the impulse. Fifty different neurotransmitters have been discovered since the first one was identified in 1920. By studying the chemical effects of neurotransmitters in the brain, scientists have made advances in finding medicines for the treatment of mental disorders, and determining the actions of drugs on the brain.

Researchers today are able to trace various molecules that are transported along axons during action potentials. Microelectrodes are used to detect the currents that cross synapses. Using this information, wiring diagrams are created that model the patterns of information flow within the brain.

Considerable knowledge about the human brain has been obtained during brain surgery by stimulating specific areas with a mild electric current, and from the observation of patients with brain damage. In the 1920s, a Canadian neurosurgeon named Wilder Penfield electrically stimulated different parts of the brains of some of his patients. He found this caused them to remember specific events from the past. For example, one patient heard someone from the past singing a particular song. From this and other studies, scientists realized that specific functions are localized in specific parts of the brain. Recently, scientists observed the behavior of a woman whose amygdala (an almond-shaped group of cells in the cerebrum) was destroyed. The amygdala plays a role in emotions and social relationships. The researcher realized that without an amygdala, the patient could not read facial expressions. As a result, she couldn’t judge the intentions of others, and often made poor social decisions.

Until recently, scientists believed that brain cells do not regenerate, thereby making brain injuries and brain diseases untreatable. Researchers are now trying to help such patients with neuron transplants, introducing nerve tissue into the brain. They are also studying substances, such as nerve growth factor (NGF), that someday may be used to help regrow nerve tissue.

Since the 1950s, scientists have begun to understand the process of sleep. They find that sleep occurs in different stages. One stage is called rapid eye movement (REM) sleep. We dream during REM sleep, a period when there is a lot of brain activity and eye movements and the body is inactive. The pons, an area of the brainstem, sets off REM sleep and dreaming. During REM sleep, the brain emits characteristic brain waves. Non-REM sleep usually comes first, takes up about 75% of our sleep, and is much quieter. Its stages get deeper and deeper. Non-REM sleep also has its own particular brain waves. The two types of sleep alternate during the night. Scientists are beginning to understand the factors that control sleep and wakefulness. These include a biological clock, a group of about 10,000 neurons in the hypothalamus that trigger off waking up; homeostasis, the body’s tendency to maintain equilibrium in physiological systems; and changes in the level of norepinephrine and serotonin, neurotransmitters in the brain.

Where and how does memory occur? This is another question that has puzzled scientists for decades. Recent information suggests that memory is not stored in a single brain center, but instead is part of numerous processing systems in the cerebral cortex. Scientists believe that memory involves chemical and structural changes in neurons, as well as changes in the strength of synapses.

Technology provides useful tools for researching the brain and helping patients with brain disorders. An electroencephalogram (EEG) is a record of brain waves, electrical activity generated in the brain. An EEG is obtained by positioning electrodes on the head and amplifying the waves with an electroencephalograph, and is valuable in diagnosing brain diseases such as epilepsy and tumors.

Scientists use three different techniques that involve scans to study and understand the brain and diagnose disorders:

(1) Magnetic resonance imaging (MRI) uses a magnetic field to display the living brain at various depths as if in slices. Not blocked by bone, MRI allows the viewer to zoom in on any region and obtain reliable pictures of brain tissue.

(2) Positron emission tomography (PET) produces color images of the brain on the screen of a monitor. During this test, a technician injects a small amount of a substance, such as glucose, that is marked with a radioactive tag. The marked substance shows where glucose is consumed in the brain. PET is used to study the chemistry and activity of the normal brain and to diagnose abnormalities such as tumors.

(3) Magnetoencephalography (MEG) measures the electromagnetic fields created between neurons as electrochemical information is passed along. When under the machine, if the subject is told, “wiggle your toes,” the readout is an instant picture of the brain at work. Concentric colored rings appear on the computer screen that pinpoint the brain signals even before the toes are actually wiggled.

Using an MRI along with MEG, physicians and scientists can look into the brain without using surgery. They hope to use these techniques for the early diagnosis of disorders such as Alzheimer and Parkinson diseases. They hope that these techniques could help paralysis victims move by supplying information on how to stimulate their muscles, or indicating the signals needed to control an artificial limb. Furthermore, by understanding what areas of the brain are active while performing particular tasks, and by mapping information regarding increased or decreased metabolism in particular regions of the brain, a variety of questions regarding various disease states, as well as a variety of questions regarding the phenomenonal potential of the human brain, may be answered.

Researchers are studying any number of issues regarding brain functioning. Fascinating research revealing the presence of tiny magnetic bits within the brain has suggested that humans (like some insects and birds) have the potential to navigate via interactions with Earth’s magnetic field. Technological research into computers and artificial intelligence are furthered by a good understanding of the amazing,

KEY TERMS

Broca’s area— Area in the cerebrum that organizes thought and coordinates muscles for speech.

Ganglion— A cluster of nerve cell bodies, usually found outside of the central nervous system. Glial cells—Nerve cells (other than neurons) located in the brain that protect, support, and assist neurons.

REM sleep— Rapid eye movement sleep that is characterized by dreaming, active brain activity, and numerous eye movements.

Wernicke’s area— The portion of the left side of the brain that stores and retrieves words and word patterns.

computerlike intelligence of the human brain; conversely, studies of the human brain have been furthered by efforts to create artificial intelligence. Research into the human brain is actually thought to be in its infancy; myriad topics of investigation must be explored to grasp the complexities and intricacies of the human brain.

See also Electrocardiogram (ECG); Neurotransmitter; Nuclear medicine.

Resources

BOOKS

Jackson, Carolyn, ed. How Things Work: The Brain. Alexandria, VA: Time-Life Books, 1990.

Rumbaugh, Duane M., and D.A. Shaw. Intelligence of Apes and Other Rational Beings (Current Perspectives in Psychology). New Haven, CT: Yale University Press, 2003.

PERIODICALS

Carey, Joseph, ed. Brain Facts Washington, DC: Society for Neuroscience, 1993.

Friedrich, M.J. “A Bit of Culture for Children: Probiotics May Improve Health and Fight Disease.” Journal of the American Medical Association no. 284 (September 2000): 1365-1366.

Golden, Frederic. “Mental Illness: Probing the Chemistry of the Brain.” Time 157 (January 2001). “Mind and Brain.” Scientific American (September, 1992).

OTHER

Harvard Medicine. “The Whole Brain Atlas” <http://www.med.harvard.edu/AANLIB/home.html> (accessed November 6, 2006).

The Nature of the Nerve Impulse. Films for the Humanities and Sciences, 1994-95. Videocassette.

Public Broadcasting Service (PBS). “The Secret Life of the Brain” <http://www.pbs.org/wnet/brain> (accessed November 6, 2006).

Bernice Essenfeld

Brain

The brain is a mass of nerve tissue located in an animal's head that controls the body's functions. In simple animals, the brain functions like a switchboard, picking up signals from sense organs and passing the information to muscles. In more advanced animals, particularly vertebrates, the brain is much more complex and is capable of far more advanced behaviors.

The brain is part of an animal's central nervous system, which receives and transmits impulses. It works with the peripheral nervous system, which carries impulses to and from the brain and spinal cord via nerves that run throughout the body.

Words to Know

Blood-brain barrier: A modification of capillaries in the brain stem that prevent certain chemicals from entering the brain through the bloodstream.

Broca's area: An area in the cerebrum that organizes thought and coordinates muscles for speech.

Ganglion: A structure comprised of nerve cell bodies, usually located outside the central nervous system.

Glial cells: Nerve cells (other than neurons) located in the brain that protect, support, and assist neurons.

Neuron: A type of nerve cell.

Stimulus: Anything that incites an organism to action, such as light, sound, or moisture.

Wernicke's area: An area in the cerebrum that processes information from written and spoken language.

Invertebrate brain

The simplest brains are those found in invertebrates, animals that lack a backbone. For example, roundworms have a simple brain and nervous system consisting of approximately 300 nerve cells, or neurons. Sensory neurons located in the head end of the animal detect stimuli from the environment and pass messages to the brain. The brain then sends out impulses through a nerve cord to muscles, which respond to the stimulus. The way that neurons in the brain process the data received determines the response made by the animal.

Somewhat more advanced organisms have more complex nervous systems. A key component of such systems are ganglia, masses of neurons that can take in and process information. The brain of an earthworm, for example, consists of a pair of ganglia at the animal's head end.

Suppose that an earthworm encounters some external stimulus, such as touch, light, or moisture. That stimulus is detected by receptor cells in the skin, which send out a message along a pair of nerves in each of the earthworm's segments. These nerves carry the message to the brain and also to smaller ganglia in each of the worm's segments, where the signals

are analyzed. The central nervous system then transmits impulses along nerves that coordinate muscle action, causing the earthworm to move toward or away from the stimulus.

Insect brain. In insects, specialized sense organs detect information from the environment and transmit it to the central nervous system. Such sense organs include simple and compound eyes, sound receptors on the thorax (the main body) or in the legs, and taste receptors. The brain of an insect consists of a ganglion in the head. Ganglia are also found in some segments of the insect's body. The information that insects use for behaviors such as walking, flying, mating, and stinging is stored in these segmental ganglia. In experiments in which heads are cut off of cockroaches and flies, these insects continue to learn.

Vertebrate brain

The central nervous system of vertebrates (animals with backbones) consists of a single spinal cord, which runs along the animal's back, and a highly developed brain. The brain is the dominant structure of the nervous system. It is the master controller of all body functions, and the analyzer and interpreter of complex information and behavior patterns. One can think of the brain as a powerful computer that uses nerve cells rather than silicon chips. The peripheral nervous system, composed of nerves which run to all parts of the body, transmits information to and from the central nervous system.

The vertebrate brain is divided into three main divisions: the fore-brain, the midbrain, and the hindbrain. The hindbrain connects the brain to the spinal cord, and a portion of it, called the medulla oblongata, controls important body functions such as the breathing rate and the heart rate. Also located in the hindbrain is the cerebellum, which controls balance.

The forebrain consists of the cerebrum, thalamus, and hypothalamus. Among its other functions, the forebrain controls the sense of smell in vertebrates. The midbrain is the location of the optic lobes, responsible for receiving and interpreting visual signals. The midbrain is also the source of an organism's motor responses.

During the first few weeks of development, the brain of a vertebrate looks like a series of bulges in the tube of nerve cells. There is very little difference among early brains of fish, amphibians, reptiles, birds, and mammals. As the brain develops, however, the bulges enlarge. Each type of vertebrate acquires its own specific adult brain that helps it survive in its environment. In the forebrain of fish, for example, the olfactory (smell) sense is well developed, whereas the cerebrum serves merely as a relay station for impulses. In mammals, on the other hand, the olfactory division is included in the system that also controls emotions, and the cerebrum is highly developed, operating as a complex processing center for information. Optic lobes are well developed in the midbrain of nonmammalian vertebrates, whereas in mammals the vision centers are mainly in the forebrain. In addition, a bird's cerebellum is large compared to the rest of its brain, since it controls coordination and balance in flying.

Human brain. The living human brain is a soft, shiny, grayish white, mushroom-shaped structure. Encased within the skull, it is a 3-pound (1.4-kilogram) mass of nerve tissue that keeps us alive and functioning. On average, the brain weighs 13.7 ounces (390 grams) at birth, and by age 15 grows to approximately 46 ounces (1,300 grams). The human brain is composed of up to one trillion nerve cells. One hundred billion of these are neurons, and the remainder are supporting (glial) cells. Neurons receive,

process, and transmit impulses, while glial cells (neuroglia) protect, support, and assist neurons.

The brain is protected by the skull and by three membranes called the meninges. The outermost membrane is known as the dura mater; the middle as the arachnoid; and the innermost as the pia mater. Also protecting the brain is cerebrospinal fluid, a liquid that circulates between the arachnoid and pia mater. Many bright red arteries and bluish veins on the surface of the brain penetrate inward. Glucose, oxygen, and certain ions pass easily from the blood into the brain, whereas other substances, such as antibiotics, do not. Capillary walls are believed to create a blood-brain barrier that protects the brain from a number of biochemicals circulating in the blood.

The parts of the brain can be studied in terms of structure and function. Four principal sections of the human brain are the brain stem (the hindbrain and midbrain), the diencephalon, the cerebrum, and the cerebellum.

The brain stem. The brain stem is the stalk of the brain, and is a continuation of the spinal cord. It consists of the medulla oblongata, pons, and midbrain. The medulla oblongata is actually a portion of the spinal cord that extends into the brain. All messages that are transmitted between the brain and spinal cord pass through the medulla. Nerves on the right side of the medulla cross to the left side of the brain, and those on the left cross to the right. The result of this arrangement is that each side of the brain controls the opposite side of the body.

Three vital centers in the medulla control heartbeat, rate of breathing, and diameter of the blood vessels. Centers that help coordinate swallowing, vomiting, hiccuping, coughing, and sneezing are also located in the medulla. A region within the medulla helps to maintain the conscious state. The pons (meaning "bridge") conducts messages between the spinal cord and the rest of the brain, and between the different parts of the brain. The midbrain conveys impulses from the cerebral cortex to the pons and spinal cord. It also contains visual and audio reflex centers involving the movement of eyeballs and head.

Twelve pairs of cranial nerves originate in the underside of the brain, mostly from the brain stem. They leave the skull through openings and extend as peripheral nerves to their destinations. Cranial nerves include the olfactory nerve that brings messages about smell from the nose and the optic nerve that conducts visual information from the eyes.

The diencephalon. The diencephalon lies above the brain stem, and includes the thalamus and hypothalamus. The thalamus is an important relay station for sensory information coming to the cerebral cortex from other parts of the brain. The thalamus also interprets sensations of pain, pressure, temperature, and touch, and is concerned with some of our emotions and memory. It receives information from the environment in the form of sound, smell, and taste.

The hypothalamus performs numerous important functions. These include the control of the autonomic nervous system. The autonomic nervous system is a branch of the nervous system involved with control of a number of body functions, such as heart rate and digestion. The hypothalamus helps regulate the endocrine system (which produces hormones, chemical messengers that regulate body functions) and controls normal body temperature. It tells us when we are hungry, full, and thirsty. It helps regulate sleep and wakefulness, and is involved when we feel angry and aggressive.

The cerebrum. The cerebrum makes up about 80 percent of the brain's weight. It lies above the diencephalon. The cerebral cortex is the outer layer of the brain and is made up of a material known as gray matter, consisting of many nerve cell bodies. The tissue of the cerebral cortex is about 0.08 to 0.16 inches (2 to 4 millimeters) thick, and if spread out would have a surface area of about 5 square feet (1.5 square meters), about one-half the size of an office desk. White matter, composed of nerve fibers covered with a fatty-like coating known as myelin sheaths, lies beneath the gray matter.

A deep fissure separates the cerebrum into a left and right hemisphere (halves). Each cerebral hemisphere is divided into regions known as frontal, temporal, parietal, and occipital lobes. The corpus callosum, a large bundle of fibers, connects the two cerebral hemispheres.

The cerebral cortex is the portion of the brain that provides the most important distinctions between humans and other animals. It is responsible for the vast majority of functions that define what we mean by "being human." It enables us not only to receive and interpret all kinds of sensory information, such as color, odor, taste, and sound, but also to remember, analyze, interpret, make decisions, and perform a host of other "higher" brain functions.

By studying animals and humans who have suffered damage to the cerebral cortex, scientists have found that various parts of this region have specific functions. For example, spoken and written language are transmitted to a part of the cerebrum called Wernicke's area, where meaning is extracted. Instructions are then sent to Broca's area, which controls the movement of muscles throughout the body. Within Broca's area, thoughts are translated into speech and muscles are coordinated for speaking. Impulses from other motor areas direct our hand muscles when we write and our eye muscles when we scan the page for information.

Association areas of the cerebrum are concerned with emotions and intellectual processes, by connecting sensory and motor functions. In our association areas, innumerable impulses are processed that result in memory, emotions, judgment, personality, and intelligence.

Memory

One of the most fascinating of all brain functions is memory. Memory refers to the brain's ability to recall events that have taken place at some time in the past. Scientists have learned that two kinds of memory exist, short-term and long-term memory. They believe that the way in which these two kinds of memory function are somewhat different from each other. People who have a condition known as retrograde amnesia, for example, lose the ability to remember events that occurred immediately before some kind of shock, such as a blow to the head. Yet, they can easily remember events that occurred days, weeks, months or years before that shock.

Scientists are still uncertain as to how the brain remembers things. They use the term memory trace to describe changes in the brain that correspond to the creation of memory. But no one really knows exactly what a memory trace corresponds to in terms of brain structure, chemistry, or function.

According to the most popular current theory of memory, exposure to stimuli can cause changes in the connections that neurons make with each other. These changes may be the "memory traces" that scientists talk about. Those neural connections appear to be able to survive for very long periods of time and can be recalled when a person decides to recall them or when some stimulus causes them to reappear.

Some exciting research on memory has suggested that nerve cells may actually grow and change as they are exposed to light, sounds, chemicals, and other stimuli. The new patterns they form may in some way be connected to the development of a memory trace in the brain.

Two surprisingly small areas at the front of the cerebrum, located on each hemisphere roughly above the outer edge of the eyebrow, are the brain's centers for high-level thinking. In 2000, scientists announced that this paired region, called the lateral prefrontal cortex, was activated in people who were given tests involving verbal and spatial problems. This is the same region of the brain that previous research studies had shown to be important for solving novel tasks, keeping many things in mind at once, and screening out irrelevant or unimportant information. Scientists also believe the lateral prefrontal cortex acts as a global workspace for organizing and coordinating information and carrying it back to other parts of the brain as needed.

Certain structures in the cerebrum and diencephalon make up the limbic system. These regions are responsible for memory and emotions, and are associated with pain and pleasure.

By studying patients whose corpus callosum had been destroyed, scientists have learned that differences exist between the left and right sides of the cerebral cortex. The left side of the brain functions mainly in speech, logic, writing, and arithmetic. The right side of the brain, on the other hand, is more concerned with imagination, art, symbols, and spatial relations.

The cerebellum. The cerebellum is located below the cerebrum and behind the brain stem, and is shaped like a butterfly. The "wings" are the cerebellar hemispheres, and each consists of lobes that have distinct grooves or fissures. The cerebellum controls the movements of our muscular system needed for balance, posture, and maintaining posture.

Brain disorders

As with any other part of the body, the brain is subject to a variety of disfunctions and disorders. Four of the most common of these are coma, epilepsy, migraine, and stroke.

Coma. The term coma comes from the Greek word koma, meaning "deep sleep." Medically, coma is a state of extreme unresponsiveness in which an individual exhibits no voluntary movement or behavior. In a deep coma, stimuli, even painful stimuli, are unable to effect any response. Normal reflexes may be lost.

The term coma is used for a side variety of conditions ranging from drowsiness or numbness at the least extreme to brain death at the worst extreme.

Coma is the result of something that interferes with the functioning of the cerebral cortex and/or the functioning of the structures that make up the reticular activating system (RAS). The RAS is a network of structures (including the brain stem, the medulla, and the thalamus) that work together to control a person's tendency to remain awake and alert.

A large number of conditions can result in coma, including damage to the brain itself, such as brain tumors, infections, and head injuries. They may also involve changes in the way the brain functions, such as a decrease in the availability of substances necessary for appropriate brain functioning, such as oxygen, glucose, and sodium; the presence of certain substances disrupting the functioning of neurons, such as drugs or alcohol in toxic (poisonous) quantities; or changes in the levels of certain essential brain chemicals due to seizures.

Split-brain Research

The two hemispheres of the human brain are not created equal. Scientists have suspected for centuries that each hemisphere of the brain has specialized functions. As early as the 1860s, French physician Paul Broca (1824–1880) showed that patients with speech problems had damage to the left side of their brains.

Additional research on split-brain functions later came from many other sources. For example, some patients with severe epilepsy have had the corpus callosum in their brain severed to relieve their discomfort. This surgery has, as a by-product, produced information on the way the two hemispheres function. A simpler way to study the two hemispheres is simply to protect one hemisphere from receiving information, such as placing a card over one eye, covering one ear, or providing sensory inputs to only one hand, arm, or foot.

As a result of studies of this kind, scientists have been able to assign certain types of function to one hemisphere of the brain or the other. Perhaps the most obvious of these functions is handedness. In general, about 90 percent of all humans are right-handed, a characteristic that can now be traced to the control of that function in the left hemisphere of most human brains.

Scientists now believe that language functions, such as the ability to speak, read, name objects, and understand spoken language are a function of the left hemisphere. The left hemisphere is also thought to be responsible for numerical and analytical skills. In contrast, the right hemisphere in most humans is thought to control non-verbal activities, such as the ability to draw and copy geometric figures, various musical abilities, visual-spatial reasoning and memory, and the recognition of form using vision and touch.

There is also evidence that the two hemispheres of the brain process information differently. It seems that the right hemisphere tends to process information in a more simultaneous manner, processing and bringing diverse pieces of information together. The left hemisphere seems to process information in a logical and sequential manner, proceeding in a more step-by-step manner than the right hemisphere.

In terms of emotions, there are some very intriguing findings. Some of these findings come from studies of individuals who showed uncontrollable laughter or crying. This evidence suggests that the left hemisphere is highly involved in the expression of positive emotions, while the right hemisphere is highly involved in the expression of negative emotions. Some researchers believe that the two hemispheres of the brain usually inhibit each other so that there is a balance, making uncontrollable emotional outbursts rare.

The ultimate results of coma depend on a number of factors. In general, it is extremely important for a physician to determine quickly the cause of a coma, so that potentially reversible conditions are treated immediately. For example, an infection may be treated with antibiotics, a brain tumor may be removed, brain swelling from an injury can be reduced with certain medications.

Outcome from a coma depends on its cause and duration. In drug poisonings, for example, extremely high rates of recovery can be expected, following prompt medical attention. Patients who have suffered head injuries tend to do better than patients whose coma was caused by other types of medical illnesses. Excluding drug poisoning-induced comas, only about 15 percent of patients who remain in a coma for more than a few hours make a good recovery. Adult patients who remain in a coma for more than four weeks have almost no chance of regaining their previous level of functioning. However, children and young adults have regained functioning after even two months in a coma.

Epilepsy. The term epilepsy is derived from the Greek word for seizure. It describes a condition marked by irregularities in the body's electrical rhythms and is characterized by convulsive attacks (violent involuntary muscle contractions) during which a person may lose consciousness. The outward signs of epilepsy may range from only a slight smacking of the lips or staring into space to a generalized convulsion. It is a condition that can affect anyone, from the very young to adults, of both sexes and any race. Epilepsy was first described by the Greek physician Hippocrates, known as the father of medicine, who lived in the late fifth century b.c.

The number of people who have epilepsy is not known. Some authorities say that up to one-half of 1 percent of the population are epileptic. But other experts believe this estimate to be too low. Many cases of epilepsy, those with very subtle symptoms, are not reported.

The cause of epilepsy remains unknown. However, scientists are often able to determine the area of the brain that is affected by the manner in which the condition is demonstrated. For example, Jacksonian seizures, which are localized twitching of muscles, originate in the frontal lobe of the brain in the motor cortex. A localized numbness or tingling indicates an origin in the parietal lobe on the side of the brain in the sensory cortex.

The recurrent (repeated) symptoms associated with epilepsy, then, are the result of unusually large electrical discharges from neurons in a particular region of the brain. These discharges can be seen on the standard brain test called the electroencephalogram (EEG). For this test, electrodes (devices that conduct electrical current) are applied to specific areas of the head to pick up the electrical waves generated by the brain. If the patient experiences an epileptic episode while he or she is wired to the EEG, the abnormal brain waves can easily be seen and the determination made as to their origin in the brain. If the patient is not experiencing a seizure, however, abnormalities will usually not be found in the EEG.

Perhaps the best known examples of epilepsy known to the general public are grand mal and petit mal. The term mal comes from the French word of "illness," while grand and petit refer respectively to "large" and "small" episodes of the illness. In the case of grand mal, an epileptic is likely to have some indication that a seizure is imminent immediately prior to the seizure. This feeling is called an aura. Very soon after feeling the aura, the person will lapse into unconsciousness and experience generalized muscle contractions that may distort the body position. The thrashing movements of the limbs that follow in a short time are caused by opposing sets of muscles alternating in contractions. The person may also lose control of the bladder and/or bowels. When the seizures cease, usually after three to five minutes, the person may remain unconscious for up to half an hour. Upon awakening, the person may not remember having had a seizure and may be confused for a time.

In contrast to the drama of the grand mal seizure, the petit mal may seem insignificant. The person interrupts whatever he or she is doing and for up to about 30 seconds may show subtle outward signs, such as blinking of the eyes, staring into space, or pausing in conversation. After the seizure has ended, the person resumes his or her previous activity, usually totally unaware of the interruption that took place. Petit mal seizures are associated with heredity, and they never occur in people over the age of 20 years. Oddly, though the seizures may occur several times a day, they do so in most cases when the person is quiet and not during periods of activity. After puberty these seizures may disappear or they may be replaced by the grand mal type of seizure.

Treatment. A number of drugs are available for the treatment of epilepsy. The oldest is phenobarbital, which has the unfortunate side effect of being addictive. Other drugs currently on the market are less addictive, but all have the possibility of causing unpleasant side effects such as drowsiness or nausea or dizziness.

The epileptic person needs to be protected from injuring himself or herself during an attack. For the person having a petit mal seizure, little usually needs to be done. Occasionally these individuals may lose their balance and need to be helped to the ground to avoid hitting their head. Otherwise, they need little attention.

The individual in a grand mal seizure should not be restrained, but may need to have some help to avoid striking his or her limbs or head on the floor or any nearby objects. If possible, the person should be rolled to one side. This action will maintain an open airway for the person to breathe by allowing the tongue to fall to one side.

Epilepsy is a recurrent, lifelong condition that must be managed on a long-term basis. Medication can control seizures in a substantial percentage of people, perhaps up to 85 percent of those with grand mal manifestations. Some people will experience seizures even with maximum dosages of medication. These individuals need to wear an identification bracelet to let others know of their condition. Epilepsy is not a reflection of insanity or mental retardation in any way. In fact, many who experience petit mal seizures are of above-average intelligence.

Migraine. Migraine is a particularly severe form of headache. It was first described during the Mesopotamian era, about 3000 b.c. Migraine is a complex condition that is still poorly understood. The term does not apply to a single medical condition, but is applied to a variety of symptoms that are often numerous and changeable. Migraine sufferers find that their headaches are provoked by a particular stimulus, such as stress, loud noises, missed meals, or eating particular foods such as chocolate or red wine.

A migraine condition can generally be divided into four distinct phases. The first phase is known as the prodrome. Symptoms develop slowly over a 24-hour period preceding the onset of the headache, and often include feelings of heightened or dulled perception, irritability or withdrawal, cravings for certain foods, and other features.

The second phase, known as the aura, features visual disturbances that may be described as flashing lights, shimmering zig-zag lines, spotty vision, and other disturbances in one or both eyes. Other sensory symptoms may occur as well, such as pins and needles or numbness in the hands. All of these symptoms can be acutely distressing to the patient. This phase usually precedes the onset of headache by one hour or less.

Phase three consists of the headache itself, usually described as severe, often with a throbbing or pulsating quality. The pain may occur on one or both sides of the head, and may be accompanied by nausea and vomiting and intolerance of light (photophobia), noise (phonophobia), or movement. This phase may last from 4 to 72 hours. During the final phase, called the postdrome, the person often feels drained and washed-out. This feeling generally subsides within 24 hours.

Migraines appear to involve changes in the patterns of blood circulation and of nerve transmissions in the brain. Scientists currently believe that migraines develop in three phases. The first step takes place in the midbrain. For reasons not fully understood, cells that are otherwise functioning normally in this region begin sending abnormal electrical signals along their projections to other brain centers, including the visual cortex. The second step is activation of the blood vessels in the brain, wherein arteries may contract or dilate (expand). The third step is activation of nerve cells that control the sensation of pain in the head and face. Some patients may experience only one of these three stages. This fact could explain individuals who experience only the aura, without the pain phase, for example.

Some recent research suggests a connection between migraine and levels of serotonin, a neurotransmitter found in the brain and numerous other cells and tissues. Migraine attacks have been correlated with falling levels of serotonin in the body. The connection has been strengthened by the observation that the drug sumatriptan, which closely resembles serotonin chemically, is highly effective in treating migraine.

Stroke. Stroke is a medical condition characterized by the sudden loss of consciousness, sensation, and voluntary movement caused by the loss of blood flow to the brain. Stroke is also called a cerebral vascular accident or CVA. It is caused by one of two events, a ruptured artery or an artery that has become closed off because a blood clot has lodged in it. Stroke resulting from a burst blood vessel is called a hemorrhagic stroke, while one caused by a clot is called a thrombotic stroke. Blood circulation to the area of the brain served by that artery stops at the point of disturbance, and brain tissue beyond that point is damaged by the lack of oxygen and begins to die.

Stroke is the third leading cause of death in the United States after heart attack and all forms of cancer. Approximately 500,000 strokes, new and recurrent, are reported each year. Of these, about 150,000 will be fatal. Today approximately 3,000,000 Americans who have had a stroke are alive.

How a stroke occurs. The brain requires a constant and steady flow of blood in order to carry out its functions. Blood delivers the oxygen and nutrients needed by the brain cells. If this blood flow is interrupted for any period of time and for any reason, brain cells begin to die quickly.

A burst blood vessel may occur in a weak area in the artery, or a blood vessel that becomes plugged by a floating blood clot. In either case, blood is no longer supplied to brain tissue beyond the point of the occurrence. The effect of the interruption in circulation to the brain depends upon the area of the brain that is involved. Interruption of a small blood vessel may result in a speech impediment or difficulty in hearing or an unsteady gait. If a larger blood vessel is involved, the result may be total paralysis of one side of the body. Damage to the right hemisphere of the brain results in disruption of function on the left side of the body, and vice versa. The onset of the stroke may be so sudden and severe that the patient is literally struck down in his tracks. Some patients have early warnings that a stroke may be developing, however.

People who are known to form blood clots in their circulatory system can be given medications to prevent it. Also, current therapy includes medications that can be given to dissolve clots, thereby removing the barrier to blood flow. If blood flow can be reinstated quickly enough, brain tissue may suffer less damage—and less function may be lost.

Strokes can be prevented by effective treatment of high blood pressure and by taking an aspirin tablet every day, for those who can tolerate such medication. The aspirin helps to prevent clot formation, and a number of clinical trials have shown it to be effective in stroke reduction.

Recovery from a stroke varies from one person to the next. Swift treatment followed by effective physical therapy may restore nearly full function of an affected area of the body. Some individuals have experienced severe enough damage that their recovery is minimal and they may be confined to a wheelchair or bed for the remainder of their lives.

[See also Circulatory system; Cognition; Nervous system ]

Brain

The vertebrate brain is the large anterior portion of the central nervous system . The "cranial vault" of the skull encases the brain in most vertebrates. In invertebrates, the enlarged and specialized anterior ganglion of the central nervous system is often referred to as a brain, although not all scientists regard it as a true brain.

The brain receives and processes sensory information, initiates and controls movement, and executes cognitive (thought) processes. The human brain has an extraordinary capacity, correlated with the great enlargement of the cerebrum, for information storage and retrieval, thought, emotions, and initiation of behavior.

Gross Anatomy

The mammalian brain has three primary subdivisions: the cerebrum (including the outer, wrinkled cortex), cerebellum, and brainstem. The brain-stem is further divided into the diencephalon, midbrain, pons, and medulla. The human brain is about 85 percent cerebrum, 11 percent cerebellum, and 4 percent brainstem.

The human brain has more than 100 billion neurons , with 14 to 16 billion in the cerebral cortex and nearly 100 billion in the cerebellum alone. In addition, there are perhaps nine times as many glial cells, whose exact roles are unclear, but which help to support and maintain neurons. Most neurons are present shortly after birth, and as the brain continues to grow, the number and complexity of neuronal connections increases. These neurons are arranged into gray matter and white matter. Gray matter composes areas rich in neurons, their dendrites, and synapses. White matter is tissue rich in axons (nerve fibers), but with a few cell bodies or dendrites. It gets its color from an insulating wrap called myelin around the nerve fibers. The high lipid content of white matter makes it light and easily distinguished from gray matter in fresh, unstained tissue.

Multiple sclerosis is an autoimmune disease in which myelin degenerates.

The cerebrum and cerebellum each have a multilayered sheet of cells on the surface called the cortex, composed of gray matter. The white matter lies deep to this and consists of axons that send information to and from the cortex or connect different regions of the cortex to each other. Deeper masses of gray matter are also found embedded in the white matter.

The central nervous system (brain and spinal cord) develops as a hollow tube whose internal space eventually forms a system of fluid-filled cavities called ventricles . The first two ventricles are a pair of C-shaped lateral ventricles, one in each cerebral hemisphere. Each of these communicates through a small pore with a slitlike third ventricle between the two hemispheres, surrounded by the diencephalon. From here, a slender canal, the cerebral aqueduct, passes down the middle of the midbrain and leads to a triangular fourth ventricle, between the cerebellum and the brainstem. Pores from the fourth ventricle open into a subarachnoid space that surrounds the brain. These ventricles are filled with a liquid, the cerebrospinal fluid (CSF), which also bathes the outside of the brain and cushions the organ in the cranial cavity. The CSF is secreted in part by a complex of blood vessels, the choroid plexus, in each ventricle.

Around the brain and spinal cord, between the nervous tissue and bone, are found three membranes called meninges: the dura mater just under the bone; a middle arachnoid; and a delicate pia mater on the surface of the tissue.

Inflammation of the meninges occurs in meningitis, which may be caused by a viral or bacterial infection.

The brain receives most of its input from, and sends most of it output to, the spinal cord, which merges with the brainstem at the base of the brain. The twelve cranial nerves provide input and output pathways to and from the structures in the head.

The Cerebrum

The cerebrum, the largest subdivision of the human brain, consists of a pair of cerebral hemispheres. Each hemisphere consists of an outer mantle of gray matter (the cerebral cortex), an extensive underlying of white matter, and deep aggregations of gray matter, the basal nuclei, or ganglia . Each hemisphere develops from a lateral outgrowth of the embryonic forebrain. Near its attachment to the forebrain, immature neurons aggregate to form the basal nuclei. As the basal nuclei grow, the remainder of the hemisphere continues to balloon outward and posteriorly, forming the cerebral cortex. This outgrowth is hollow, and its cavity becomes the lateral ventricle.

In adults, the right and left hemispheres are separated from each other by a deep midline cleft, the longitudinal fissure, and are separated from the cerebellum by a deep horizontal groove, the transverse fissure. The hemispheres are connected to each other by a massive bundle of nerve fibers, the corpus callosum, on the floor of the longitudinal fissure. Many of these fibers connect regions of one hemisphere to corresponding points in the opposite hemisphere.

As the cortex continues to grow, it is thrown into folds called gyri (singular, gyrus), separated by shallow grooves called sulci (singular, sulcus). A few especially prominent sulci appear early in development and are consistent from brain to brain. They serve as landmarks to divide the cortex into areas called lobes. (Gyri are not as numerous or pronounced in most other mammals.)

Imbalance between production and drainage of cerebrospinal fluid can lead to hydrocephalus, a potentially fatal disorder.

The frontal, parietal, temporal, and occipital lobes are visible on the surface of the brain. The frontal lobe extends from the region of the forehead to a groove called the central sulcus at the top of the head. The parietal lobe begins there and progresses posteriorly as far as the parieto-occipital sulcus, which is visible only on the medial surface of the brain. The occipital lobe extends from there to the rear of the head. A conspicuous lateral fissure separates the temporal lobe, in the region of the ear, from the frontal and parietal lobes above it. The insula is a fifth lobe of the cerebrum not visible from the surface. It lies deep to the lateral fissure between portions of the frontal, parietal, and temporal lobes.

The limbic system is a ring of tissue on the medial surface of each hemisphere, surrounding the corpus callosum and diencephalon and incorporating parts of the frontal, parietal, and temporal lobes. A major component of this system is the hippocampal formation, deep in the temporal lobe.

Functional Areas of the Cerebral Cortex

Considerable knowledge of cortical function has come from patients with damage to specific cortical areas, and from electrical stimulation and recording from the cortex, often as a necessary prelude to neurosurgery. Imaging procedures developed in the 1980s and 1990s, such as positron emission tomography (PET), enable neuroscientists to follow changes in cortical activity over time. PET scans can show sequential changes in brain activity during such tasks as planning and executing movement and learning and storing information.

Motor Areas. Four motor areas collectively occupy almost half of the frontal lobe. One of these, the primary motor cortex, is the precentral gyrus just anterior to the central sulcus. The motor areas are extensively connected to the basal ganglia and cerebellum. Working together in complex feedback loops, these areas are essential for motor coordination, postural stability and balance, learned movements, and the planning and execution of voluntary movement.

Sensory Areas. Primary sensory areas receive incoming sensory information. One of these, the primary somatosensory cortex, receives input for pain, temperature, touch, and pressure. It is located in the postcentral gyrus, the first gyrus of the parietal lobe posterior to the central sulcus. The primary auditory cortex, for hearing, is on the super (upper) margin of the temporal lobe, deep in the lateral fissure. The primary visual cortex, for sight, is in the occipital lobe, especially the medial surface.

Primary sensory areas are organized into precise sensory maps of the body. The primary somatosensory cortex, for example, has a point-for-point correspondence with the opposite (contralateral) side of the body, so that, for instance, the first and second fingers of the left hand send sensory information to adjacent areas of the right primary somatosensory cortex. Similarly, the primary visual cortex has a point-for-point map of the contralateral visual field. The primary auditory cortex has a tonotopic map of the cochlea of the inner ear, with different points in the cortex representing different sound frequencies.

Association Areas. Once received by a primary sensory area, information is sorted and relayed to adjacent sensory association areas for processing. Association areas identify specific qualities of a stimulus and integrate stimulus information with memory and other input. To hear a piece of music, for example, involves the primary auditory cortex, but to recognize that music as Mozart or Elvis Presley involves the auditory association area just below the primary auditory cortex.

The human brain differs from that of other primates in its large amount of association cortex. Association areas not only integrate immediate sensory data with other information, but are also responsible for human ingenuity, personality, judgment, and decision making.

Cortical Lesions

The posterior region of the parietal lobe integrates motor and sensory information. Damage to this region often results in neglect or unawareness of the contralateral side of the body and the space around that side of the body. This can be reflected in such oversights as forgetting to shave one side of the face or dress one side of the body. The degree of behavioral dysfunction depends on the specific areas of the brain that are damaged and the extent of the damage. Temporal lobe lesions often cause difficulty performing tasks that require keen visual discrimination. Damage of the inferior (lower) area of the temporal lobe may produce short-term memory loss, while damage of the inferior and anteromedian (front-middle) regions may cause long-term memory loss. Lesions in the prefrontal cortex (far anterior portions of the frontal lobe) may produce problem-solving deficits, inability to make informed decisions, unpredictable emotional states, and bizarre, socially unacceptable behaviors.

Epilepsy is associated with unregulated electrical activity in the cerebrum.

"Left Brain" and "Right Brain"

The two cerebral hemispheres are neither anatomically nor functionally identical. Cortical functions are said to be lateralized when one hemisphere is dominant over the other for a particular function. The side containing the speech centers is called the dominant hemisphere, and is usually the left hemisphere. Most people are highly lateralized for language skills, and lesions in the dominant cortex can cause complete loss of specific language functions. The posterior, superior part of the dominant temporal lobe is important for understanding spoken and written language. Lesions in the language centers produce various forms of aphasia, difficulty understanding or using written or spoken language. The language-dominant hemisphere is also a site of mathematical skills, and intellectual decision making and problem solving using rational, symbolic thought processes.

The nondominant hemisphere is more adept at recognition of complex, three-dimensional structures and patterns of both visual and tactile kinds. It is also the site for recognition of faces and other images, and for non-verbal, intuitive thought processes. Creative and artistic abilities reside in the nondominant hemisphere. Thus, the dominant hemisphere tends to be the more analytical one, and the nondominant hemisphere more intuitive.

The Basal Nuclei

The basal nuclei, or basal ganglia, are four masses of gray matter deep in the cerebrum: the caudate nucleus , putamen, globus pallidus, and amygdala. Functionally related nuclei of the midbrain, such as the substantia nigra, are sometimes considered to belong to the basal nuclei as well. The basal nuclei receive nerve fibers from all areas of the cerebral cortex and are important in motor skills and processing a broad range of cortical information. Skilled motor tasks such as tying one's shoes—things learned and now done with little thought—are controlled by the basal nuclei.

Parkinson's disease, due to degeneration of the substantia nigra, causes slowed movements and tremor.

The Brainstem

The brainstem occupies the base of the brain and includes the diencephalon, midbrain, pons, and medulla.

Diencephalon. The diencephalon is a paired structure with right and left halves. The largest component is the egg-shaped thalamus, which relays incoming information from lower levels of the brain to the cerebral cortex. Little information reaches the cerebral cortex without passing through synapses (neural junctions) in the thalamus. Some information processing occurs here, but the thalamus functions more as a dynamic filter for incoming information.

Immediately ventral to the thalamus is the smaller hypothalamus, the control center for the endocrine system and involuntary visceral motor system. The hypothalamus regulates diverse functions ranging from body temperature to gastrointestinal motility. All functions of the autonomic nervous system are regulated by the hypothalamus, although the hypothalamus can be overridden by input from the cerebrum; for example, in rage, fright, or sexual arousal. The hypothalamus also synthesizes the hormones released by the posterior lobe of the pituitary gland and produces other hormones that control the anterior lobe of the pituitary. The small epithalamus, containing the pineal gland, is posterior to the thalamus. One cranial nerve, the optic nerve (cranial nerve II), is associated with the diencephalon.

Midbrain. The midbrain is the smallest division of the brainstem. Four small humps, the two inferior and two superior colliculi, form the roof of the midbrain. They are involved in auditory and visual reflexes, respectively. Ventral to the cerebral aqueduct is a region of midbrain called the tegmentum. The floor of the midbrain is formed by two massive cerebral peduncles, stalks that attach the cerebrum and lower brainstem. The midbrain gives rise to two cranial nerves associated with eye movements: the oculomotor nerve (III) and trochlear nerve (IV).

Pons. The most striking feature of the pons is a large, rounded, ventral mass, the basal pons, which relays information from the cerebrum to the cerebellum. The tegmentum of the pons lies between the basal pons and the fourth ventricle. It contains nuclei for several cranial nerves, although only cranial nerve (V), the trigeminal nerve, exits and enters the pons itself.

LEVI-MONTALCINI, RITA (1909–)

Biologist with dual U.S. and Italian citizenship who received, with Stanley Cohen, the 1986 Nobel Prize in physiology for her discovery of a substance ("nerve growth factor") that stimulates and guides the growth of nerve cells. During World War II, the Jewish Levi-Montalcini continued her research on the nervous system of chick embryos while hiding from the Germans.

Medulla. The medulla oblongata forms a transition from brain to spinal cord. Many columns of nerve fibers pass vertically through the medulla, going between the spinal cord and higher levels of the brain. The ventral surface of the medulla has a pair of ridges, the medullary pyramids, that contain motor nerve fibers carrying signals down to the spinal cord. Lateral to each pyramid is a mound, the inferior olive, containing neurons that relay information to the cerebellum. A central core of neurons, the reticular formation, contains control centers for the heartbeat and respiration. Three cranial nerves enter or leave the brainstem at the junction between the pons and medulla: the abducens nerve (VI), involved in eye movements; the facial nerve (VII), which controls the muscles of facial expression; and the vestibulocochlear nerve (VIII), which carries signals for hearing and balance. Motor rootlets of the hypoglossal nerve (XII) leave the ventrolateral surface of the medulla and supply muscles of the tongue. Dorsal to the olive are rootlets of the glossopharyngeal nerve (IX) and vagus nerve (X). The glossopharyngeal nerve is involved in taste, salivation, swallowing, and other functions. The vagus nerve supplies many organs of the thoracic and abdominal cavities. Inferior to the rootlets of the vagus nerve are those of the spinal accessory nerve (XI), which innervates several neck and shoulder muscles.

The Cerebellum

The cerebellum, located beneath the occipital lobe and posterior to the medulla and pons, is an important regulator of motor function. It connects to the brainstem by three paired bundles of nerve fibers called the superior, middle, and inferior cerebellar peduncles. Integrity of the cerebellum is necessary to perform smooth, accurate, coordinated movements; to maintain posture; and to learn and regulate complicated motor patterns. Damage to the cerebellum does not produce muscle paralysis or paresis (weakness), but rather a loss of muscle coordination called ataxia.

Comparative Anatomy of the Brain

During the course of vertebrate evolution, the control of body functions other than simple reflexes has become concentrated in the brain. Neurons with related functions have become clustered in specific regions, and axons with similar functions have become bundled into discrete tracts. However, the primitive reticular formation of the brainstem is retained in even the most complex brains. More recently evolved centers and tracts have been added to this primitive core.

Lateral views of four brains illustrate this evolutionary trend in vertebrates. The frog has a relatively simple brain. Its cerebrum and cerebellum are small, but its olfactory and visual centers are well developed. These centers trigger reflexive activity needed for survival. The alligator brain shows a growth of both the cerebrum and cerebellum without significant reduction of the visual or olfactory centers. The cerebrum and cerebellum are more developed in the goose, and the visual and olfactory centers remain well developed. These differences reflect higher levels of cortical function and more complex, coordinated motor functions.

RAMON Y CAJAL, SANTIAGO (1852–1934)

Spanish biologist who received, with Camillo Golgi, the 1906 Nobel Prize in physiology for showing that the nerve cell is the basic unit of the nervous system, a discovery that suggested how nerves could send signals to one another. Ramon y Cajal improved Golgi's silver stain, which revealed how the long threads (dendrites) of nerve cells connect to form a network.

There is extensive enlargement of the cerebrum in the horse. Extensive cortical enlargement throws the cortex into gyri and sulci, accommodating a greater cortical area within the cranial vault. The cerebellum also is larger and more convoluted. Human brains have the most extensive cerebral and cerebellar development. The vertebrate brains have the same twelve pairs of cranial nerves, with the same functions.

see also Central Nervous System; Hearing; Hypothalamus; Nervous Systems; Neurologic diseases; Neuron; Pain; Peripheral Nervous System; Pituitary Gland; Synaptic Transmission; Touch

Alvin M. Burt

Bibliography

Burt, Alvin M. "Organization and Development of the Nervous System," "Brain Stem and Cerebellum," "Telencephalon," and "Cerebral Cortex." In Textbook of Neuroanatomy. Philadelphia, PA: W. B. Saunders, Co., 1993.

Saladin, Kenneth S. "The Central Nervous System." In Anatomy and Physiology: The Unity of Form and Function. New York: McGraw-Hill, 2001.

Brain

The brain is a mass of nerve tissue located in an animal's head that controls the body's functions. In simple animals, the brain functions like a switchboard picking up signals from sense organs and passing information to muscles. The brain is also responsible for a variety of involuntary behavior , including keeping the heart beating, and maintaining blood pressure and temperature . In more advanced forms, particularly vertebrates , a more analytical brain coordinates complex behaviors. In higher vetebrates, the brain coordinates thinking, memory , learning , and emotions. The brain is part of an animal's central nervous system , which receives and transmits impulses. It works with the peripheral nervous system, which carries impulses to and from the brain and spinal cord via nerves running throughout the body.

Invertebrate brain

Nematodes (roundworms ) have a simple brain and nervous system consisting of approximately 300 nerve cells, or neurons. Sensory neurons located in the head end of the animal detect stimuli from the environment and pass messages to the brain. The brain then sends out impulses through a ventral nerve cord to muscles which respond to the stimulus . The way that the interneurons of the brain process the data determines the response.

The earthworm and other annelids, as well as insects and other arthropods , have more complex nervous systems. In these animals, there are paired ventral nerve cords that run from head to tail on the animal's underside. Cell bodies of neurons in the cords form pairs of ganglia in each body segment. Four of the most anterior ganglia fuse in the head to form a brain. As a result, the brain ganglia are larger than the segmental ganglia, and also contain a larger proportion of sensory motor neurons. The brain ganglia have some dominance over the segment ganglia. The ventral nerve cords, brain, and segmental ganglia comprise the central nervous system. Neuronal fibers in the cords, bundled into nerves that carry communications between ganglia, make up the peripheral nervous system.

The earthworm's brain consists of paired ganglia in the head end. An impulse, such as touch , light , or moisture, is detected by receptor cells in the skin. A pair of nerves in each of the earthworm's segments carries the signal to the brain and smaller ganglia in each segment, where the signals are analyzed. The central nervous system then transmits impulses on nerves that coordinate muscle action, causing the earthworm to move.

In insects, specialized sense organs detect information from the environment and transmit it to the central nervous system. Such sense organs include simple and compound eyes, sound receptors on the thorax or in the legs, and taste receptors. The brain of an insect consists of a ganglion in the head. Some of the segmental ganglia are fused, allowing better communication between the segments. The information that insects use for behaviors such as walking, flying, mating, and stinging is stored in the segmental ganglia. In experiments in which heads are cut off of cockroaches and flies , these insects continue to learn.

Vertebrate brain

The central nervous system of vertebrates consists of a single spinal cord, which runs in a dorsal position along the back, and a highly developed brain. The brain is the dominant structure of the nervous system. It is the master controller of all body functions, and the analyzer and interpreter of complex information and behavior patterns. We can think of the brain as a powerful neural computer. The peripheral nervous system, composed of nerves that run to all parts of the body, transmits information to and from the central nervous system.

The vertebrate brain is divided into three main divisions: the forebrain, the midbrain, and the hindbrain. The hindbrain connects the brain to the spinal cord, and a portion of it, called the medulla oblongata, controls important body functions such as the breathing rate and the heart rate. Also in the hindbrain, the cerebellum controls balance. The forebrain consists of the cerebrum, thalamus, and hypothalamus. The forebrain controls, among other things, the sense of smell in vertebrates.

During the first few weeks of development, the brain of a vertebrate looks like a series of bulges in the neural tube. It is hard to see a difference when we examine the early embryonic brains of fish , amphibians , reptiles , birds , and mammals . As the brain develops, the bulges enlarge, and each type of vertebrate acquires its own specific adult brain that helps it survive in its environment. In the forebrain of fish, the olfactory (smell) sense is well developed, whereas the cerebrum serves merely as a relay station for impulses. In mammals, on the other hand, the olfactory division is included in the limbic system, which also controls emotions, and the cerebrum is highly developed, operating as a complex processing center for information. Optic lobes are well-developed in the midbrain of nonmammalian vertebrates, whereas in mammals the vision centers are mainly in the forebrain. In addition, a bird's cerebellum is large compared to the rest of its brain, since it controls coordination and balance in flying.

Human brain

The living human brain is a soft, shiny, grayish white, mushroom-shaped structure. Encased within the skull, it is a 3 lb (1.4 kg) mass of nerve tissue that keeps us alive and functioning. On average, the brain weighs 13.7 oz (390 g) at birth , and by age 15 grows to approximately 46 oz (1,315 g). The human brain is composed of up to one trillion nerve cells—100 billion of them are neurons, and the remainder are supporting (glial) cells. Neurons receive, process, and transmit impulses, while glial cells (neuroglia) protect, support, and assist neurons. The brain is protected by the skull and by three membranes called the meninges—the outermost the dura mater, the middle the arachnoid, and the innermost the pia mater. Also protecting the brain is cerebrospinal fluid, a liquid that circulates between the arachnoid and pia mater in the subarachnoid space. Many bright red arteries and bluish veins on the surface of the brain penetrate inward. Glucose, oxygen , and certain ions pass easily from the blood into the brain, whereas other substances, such as antibiotics , do not. The capillary walls are believed to create a blood-brain barrier that protects the brain from a number of biochemicals circulating in the blood.

The parts of the brain can be studied in terms of structure and function. Four principal sections of the human brain are the brain stem (the hindbrain and midbrain), the diencephalon, the cerebrum, and the cerebellum.

The brain stem

The brain stem is the stalk of the brain, and is continuous with the spinal cord. It consists of the medulla oblongata, pons, and midbrain. A part of the brain stem, the medulla oblongata is a continuation of the spinal cord. All the messages that are transmitted between the brain and spinal cord pass through the medulla via fibers in the white matter. The fibers on the right side of the medulla cross to the left and those on the left cross to the right. The result is that each side of the brain controls the opposite side of the body. There are three vital centers in the medulla which control the heartbeat, the rate of breathing, and the diameter of the blood vessels. Centers that help coordinate swallowing, vomiting, hiccoughing, coughing, and sneezing are also located in the medulla. The reticular formation occurs partially in the medulla and in other parts of the central nervous system. The reticular formation operates in maintaining our conscious state. The pons (meaning bridge) conducts messages between the spinal cord and the rest of the brain, and between the different parts of the brain. The midbrain conveys impulses from the cerebral cortex to the pons and spinal cord. It also contains visual and audio reflex centers involving the movement of eyeballs and head.

Twelve pairs of cranial nerves originate in the underside of the brain, mostly from the brain stem. They leave the skull through openings and extend as peripheral nerves to their destinations. Cranial nerves include the olfactory nerve that brings messages about smell from the nose and the optic nerve that conducts visual information from the eyes.

The diencephalon

The diencephalon lies above the brain stem, and embodies the thalamus and hypothalamus. In the diencephalon, the thalamus is an important relay station for sensory information for the cerebral cortex from other parts of the brain. The thalamus also interprets sensations of pain , pressure , temperature, and touch, and is concerned with some of our emotions and memory. It receives information from the environment in the form of sound, smell, and taste. The hypothalamus performs numerous important functions. These include the control of the autonomic nervous system (a branch of the nervous system involved with control of a number of body functions, such as heartbeat rate and digestion). The hypothalamus helps regulate the endocrine system and controls normal body temperature. It tells us when we are hungry, full, and thirsty. It helps regulate sleep and wakefulness, and is involved when we feel angry and aggressive.

The cerebrum

The cerebrum, constituting about 87.5% of the brain weight, spreads over the diencephalon. The cerebral cortex is the outer layer of the brain and is composed of gray matter made up of nerve cell bodies. It is about 0.08 in (2 mm) thick and its surface area is about 5 sq ft (1.5 sq m)—around half the size of an office desk. White matter, composed of nerve fibers covered with myelin sheaths, lies beneath the gray matter. With the rapid growth of the brain during embryonic development, the gray matter grows faster than the white matter and folds on itself. The folds are called convolutions or gyri, and the grooves between them are known as sulci. A deep longitudinal fissure separates the cerebrum into a left and right hemisphere. Each cerebral hemisphere is divided into frontal, temporal, parietal, and occipital lobes. The corpus callosum, a large bundle of fibers, connects the two cerebral hemispheres. The thalamus and subcortical nuclei, or basal ganglia, are areas of gray matter that exist below the white matter.

Sensory areas of the cerebrum interpret sensory impulses. Spoken and written language are transmitted to a part of the cerebrum called Wernicke's area where meaning is extracted, and sent to Broca's area, one of the motor areas of the cerebrum. Motor areas of the cerebrum control muscle movements. Within Broca's area, thoughts are translated into speech , and muscles are coordinated for speaking. Impulses from other motor areas direct our hand muscles when we write, and our eye muscles when we scan the page for information.

Association areas of the cerebrum are concerned with emotions and intellectual processes, by connecting sensory and motor functions. In our association areas, innumerable impulses are processed that result in memory, emotions, judgment, personality, and intelligence.

Certain structures in the cerebrum and diencephalon make up the limbic system. These regions function in memory and emotions, and are associated with pain and pleasure.

By studying patients whose corpus callosum were destroyed, scientists realized that differences existed between the left and right sides of the cerebral cortex. The left side of the brain functions mainly in speech, logic, writing, and arithmetic . The right side of the brain, on the other hand, is more concerned with imagination, art, symbols, and spatial relations.

The cerebellum

The cerebellum is located below the cerebrum and behind the brain stem, and is shaped like a butterfly. The "wings" are the cerebellar hemispheres, and each consists of lobes that have distinct grooves or fissures. The cerebellum controls the movements of our muscular system needed for balance, posture, and maintaining posture.

Studying the brain

At the end of the nineteenth century, Santiago Ramon y Cajal, a Spanish scientist, studied neurons using stain developed by Camillo Golgi. Cajal realized that the brain was made up of individual units and not a continuous net as was believed at the time. His studies uncovered a large variety of neurons that differed in size and shape. He explained that neurons received signals on dendrites and transmitted impulses on axons. Since his work, researchers have learned that neurons carry information in the form of brief electrical impulses called action potentials that result when positively charged sodium ions travel across the axon membrane from the fluid outside to the cytoplasm inside. When a nerve impulse reaches the end of an axon, neurotransmitters are released at junctions called synapses. The neurotransmitters are chemicals that bind to receptors on the receiving neurons, triggering the continuation of the impulse. Fifty different neurotransmitters have been discovered since the first one was identified in 1920. By studying the chemical effects of neurotransmitters in the brain, scientists have made advances in finding medicines for the treatment of mental disorders, and determining the actions of drugs on the brain.

Researchers today are able to trace various molecules that are transported along axons during action potentials. Microelectrodes are used to detect the currents that cross synapses. Using this information, wiring diagrams are created that model the patterns of information flow within the brain.

Considerable knowledge about the human brain has been obtained during brain surgery by stimulating specific areas with a mild electric current , and from the observation of patients with brain damage. In the 1920s, a Canadian neurosurgeon named Wilder Penfield electrically stimulated different parts of the brains of some of his patients. He found this caused them to remember specific events from the past. For example, one patient heard someone from the past singing a particular song. From this and other studies, scientists realized that specific functions are localized in specific parts of the brain. Recently, scientists observed the behavior of a woman whose amygdala (an almond-shaped group of cells in the cerebrum) was destroyed. The amygdala plays a role in emotions and social relationships. The researcher realized that without an amygdala, the patient could not read facial expressions. As a result, she couldn't judge the intentions of others, and often made poor social decisions.

Until recently, scientists believed that brain cells do not regenerate, thereby making brain injuries and brain diseases untreatable. Researchers are now trying to help such patients with neuron transplants, introducing nerve tissue into the brain. They are also studying substances, such as nerve growth factor (NGF), that someday may be used to help regrow nerve tissue.

Since the 1950s, scientists have begun to understand the process of sleep. They find that sleep occurs in different stages. One stage is called rapid eye movement (REM) sleep. We dream during REM sleep, a period when there is a lot of brain activity and eye movements and the body is inactive. The pons, an area of the brainstem, sets off REM sleep and dreaming. During REM sleep, the brain emits characteristic brain waves. Non-REM sleep usually comes first, takes up about 75% of our sleep, and is much quieter. Its stages get deeper and deeper. Non-REM sleep also has its own particular brain waves. The two types of sleep alternate during the night. Scientists are beginning to understand the factors that control sleep and wakefulness. These include a biological clock, a group of about 10,000 neurons in the hypothalamus that trigger off waking up; homeostasis , the body's tendency to maintain equilibrium in physiological systems; and changes in the level of norepinephrine and serotonin, neurotransmitters in the brain.

Where and how does memory occur? This is another question that has puzzled scientists for decades. Recent information suggests that memory is not stored in a single brain center, but instead is part of numerous processing systems in the cerebral cortex. Scientists believe that memory involves chemical and structural changes in neurons, as well as changes in the strength of synapses.

Technology provides useful tools for researching the brain and helping patients with brain disorders. An electroencephalogram (EEG) is a record of brain waves, electrical activity generated in the brain. An EEG is obtained by positioning electrodes on the head and amplifying the waves with an electroencephalograph, and is valuable in diagnosing brain diseases such as epilepsy and tumors.

Scientists use three different techniques that involve scans to study and understand the brain and diagnose disorders:

1. Magnetic resonance imaging (MRI) depends on the use of a magnetic field to display the living brain at various depths as if in slices. Not blocked by bone, MRI allows the viewer to zoom in on any region and obtain reliable pictures of brain tissue.
2. Positron emission tomography (PET) results in color images of the brain displayed on the screen of a monitor. During this test, a technician injects a small amount of a substance, such as glucose, that is marked with a radioactive tag. The marked substance shows where glucose is consumed in the brain. PET is used to study the chemistry and activity of the normal brain and to diagnose abnormalities such as tumors.
3. Magnetoencephalography (MEG) measures the electromagnetic fields created between neurons as electrochemical information is passed along. When under the machine, if the subject is told, "wiggle your toes," the readout is an instant picture of the brain at work. Concentric colored rings appear on the computer screen that pinpoint the brain signals even before the toes are actually wiggled.

Using an MRI along with MEG, physicians and scientists can look into the brain without using surgery. They hope to use these techniques for the early diagnosis of disorders such as Alzheimer and Parkinson disease . They foresee that these techniques could help paralysis victims move by supplying information on how to stimulate their muscles, or indicating the signals needed to control an artificial limb. Furthermore, by understanding what areas of the brain are active while performing particular tasks, and by mapping information regarding increased or decreased metabolism in particular regions of the brain, a variety of questions regarding various disease states, as well as a variety of questions regarding the phenomenonal potential of the human brain, may be answered

Researchers are studying any number of issues regarding brain functioning. Fascinating research revealing the presence of tiny magnetic bits within the brain has suggested that humans (like some insects and birds) have the potential to navigate via interactions with the earth's magnetic field . Technological research into computers and artifical intelligence are furthered by a good understanding of the amazing, computer-like intelligence of the human brain; conversely, studies of the human brain have been furthered by efforts to create artifical intelligence. Research into the human brain is actually thought to be in its infancy; myriad topics of investigation must be explored in order to grasp the complexities and intricacies of the human brain.

See also Electrocardiogram (ECG); Neurotransmitter; Nuclear medicine.

Resources

books

Jackson, Carolyn, ed. How Things Work: The Brain. Alexandria, VA: Time-Life Books, 1990.

Rumbaugh, Duane M., and D.A. Shaw. Intelligence of Apes and Other Rational Beings (Current Perspectives in Psychology). New Haven, CT: Yale University Press, 2003.

periodicals

Carey, Joseph, ed. Brain Facts Washington, DC: Society for Neuroscience, 1993.

Friedrich, M.J. "A Bit of Culture for Children: Probiotics May Improve Health and Fight Disease." Journal of the American Medical Association no. 284 (September 2000): 1365-1366. Golden, Frederic. "Mental Illness: Probing the Chemistry of the Brain." Time 157 (January 2001). "Mind and Brain." Scientific American (September, 1992).

other

The Nature of the Nerve Impulse. Films for the Humanities and Sciences, 1994-95. Videocassette.

Bernice Essenfeld

KEY TERMS

Broca's area

—Area in the cerebrum that organizes thought and coordinates muscles for speech.

Ganglion

—A cluster of nerve cell bodies, usually found outside of the central nervous system.

Glial cells

—Nerve cells (other than neurons) located in the brain that protect, support, and assist neurons.

REM sleep

—Rapid eye movement sleep that is characterized by dreaming, active brain activity, and numerous eye movements.

Wernicke's area

—The portion of the left side of the brain that stores and retrieves words and word patterns.