The Nervous System

The nervous system is the master controller of the body. Each thought, each emotion, each action—all result from the activity of this system. Through its many parts, the nervous system monitors conditions both within and outside the body. It then processes that information and decides how the body should respond, if at all. Finally, if a response is needed, the system sends out electrical signals that spur the body into immediate action. Although one of the smallest of the body's systems in terms of weight, the nervous system is the most complex and versatile.

DESIGN: PARTS OF THE NERVOUS SYSTEM

The nervous system is a collection of cells, tissues, and organs. It can be split into two separate divisions: the central nervous system and the peripheral nervous system.

The central nervous system (CNS) acts as the command center of the body. It interprets incoming sensory information, then sends out instructions on how the body should react. The CNS consists of two major parts: the brain and the spinal cord.

The peripheral nervous system (PNS) is the part of the nervous system outside of the CNS. It consists mainly of nerves that extend from the brain and spinal cord to areas in the rest of the body. Cranial nerves carry impulses to and from the brain while spinal nerves carry impulses to and from the spinal cord. The PNS can be divided into two systems: the somatic nervous system and the autonomic nervous system. The somatic nervous system controls the voluntary movements of the skeletal muscles. The autonomic nervous system control activities in the body that are involuntary or automatic. These include the actions of the heart, glands, and digestive organs and associated parts.

The autonomic nervous system can be divided further into two subdivisions: the parasympathetic and sympathetic nervous systems. These two subdivisions work against each other. The parasympathetic nervous system regulates involuntary activities that keep the body running smoothly under normal, everyday conditions. The sympathetic nervous system controls involuntary activities that help the body respond to stressful situations.

The Nervous System: Words to Know

Arachnoid (ah-RAK-noid):

Weblike middle layer of the three meninges covering the brain and spinal cord.

Autonomic nervous system (aw-toh-NOM-ik NERV-us SIS-tem):

Part of the peripheral nervous system that controls involuntary actions, such as the heartbeat, gland secretions, and digestion.

Axon (AK-son):

Taillike projection extending out a neuron that carries impulses away from the cell body.

Basal ganglia (BAY-zul GANG-lee-ah):

Paired masses of gray matter within the white matter of the cerebrum that help coordinate subconscious skeletal muscular movement.

Brain:

Central controlling and coordinating organ of the nervous system.

Cauda equina (KAW-da ee-KWHY-nah):

Spinal nerves that hang below the end of the spinal cord.

Central nervous system:

Part of the nervous system consisting of the brain and spinal cord.

Cerebral cortex (se-REE-bral KOR-tex):

Outermost layer of the cerebrum made entirely of gray matter.

Cerebrum (se-REE-brum):

Largest part of the brain, involved with conscious perception, voluntary actions, memory, thought, and personality.

Corpus callosum (KOR-pus ka-LOW-sum):

Large band of neurons connecting the two cerebral hemispheres.

Dendrites (DEN-drites):

Branchlike extensions of neurons that carry impulses toward the cell body.

Diencephalon (die-en-SEF-ah-lon):

Rear part of the forebrain that connects the midbrain to the cerebrum and that contains the thalamus and hypothalamus.

Dura mater (DUR-ah MAY-tur):

Outermost and toughest of the three meninges covering the brain and spinal cord.

Ganglion (GANG-glee-on):

Any collection of nerve cell bodies forming a nerve center in the peripheral nervous system.

Gray matter:

Grayish nerve tissue of the central nervous system containing neuron cell bodies, neuroglia, and unmyelinated axons.

Gyri (JYE-rye):

Outward folds on the surface of the cerebral cortex.

Hippocampus (hip-ah-CAM-pes):

Structure in the limbic system necessary for the formation of long-term memory.

Hypothalamus (hi-po-THAL-ah-mus):

Region of the brain containing many control centers for body functions and emotions; also regulates the pituitary gland's secretions.

Limbic system (LIM-bik SIS-tem):

Group of structures in the cerebrum and diencephalon that are involved with emotional states and memory.

Medulla oblongata (mi-DUL-ah ob-long-GAH-tah):

Part of the brain located at the top end of the spinal cord that controls breathing and other involuntary functions.

Meninges (meh-NIN-jeez):

Membranes that cover the brain and spinal cord.

Midbrain:

Part of the brain between the hypothalamus and the pons that regulates visual, auditory, and rightening reflexes.

Myelin (MY-ah-lin):

Soft, white, fatty material that forms a sheath around the axons of most neurons.

Nerve:

Bundle of axons in the peripheral nervous system.

Neuroglia (new-ROGUE-lee-ah):

Also known as glial cells, cells that support and protect neurons in the central nervous system.

Neuron (NUR-on):

Nerve cell.

Neurotransmitter (nur-oh-TRANS-mi-ter):

Chemical released by the axon of a neuron that travels across a synapse and binds to receptors on the dendrites of other neurons or body cells.

Node of Ranvier (NODE OF rahn-VEEAY):

Small area between Schwann cells on an axon that is unmyelinated or uncovered.

Oligodendrocyte (o-li-go-DEN-dro-site):

Cell that produces the myelin sheath around the axons of neurons in the central nervous system.

Parasympathetic nervous system (pair-ah-simpuh-THET-ik NERV-us SIS-tem):

Division of the autonomic nervous system that controls involuntary activities that keep the body running smoothly under normal, everyday conditions.

Peripheral nervous system (peh-RIFF-uh-ruhl NERV-us SIS-tem):

Part of the nervous system consisting of the cranial and spinal nerves.

Pia mater (PIE-ah MAY-tur):

Delicate innermost layer of the three meninges covering the brain and spinal cord.

Pons:

Part of the brain connecting the medulla oblongata with the midbrain.

Reflex (REE-flex):

Involuntary and rapid response to a stimulus.

Schwann cell (SHWAHN SELL):

Cell that forms the myelin sheath around axons of neurons in the peripheral nervous system.

Somatic nervous system (so-MAT-ik NERV-us SIS-tem):

Part of the peripheral nervous system that controls the voluntary movements of the skeletal muscles.

Spinal cord:

Long cord of nerve tissue running through the spine or backbone that transmits impulses to and from the brain and controls some reflex actions.

Sulci (SUL-sye):

Shallow grooves on the surface of the cerebral cortex.

Sympathetic nervous system (sim-puh-THET-ik NERV-us SIS-tem):

Division of the autonomic nervous system that controls involuntary activities that help the body respond to stressful situations.

Synapse (SIN-aps):

Small space or gap where a nerve impulse passes between the axon of one neuron and a dendrite of the next neuron.

Thalamus (THAL-ah-mus):

Part of the brain behind the hypothalamus that acts as the brain's main relay station, sending information to the cerebral cortex and other parts of the brain.

White matter:

Whitish nerve tissue of the central nervous system containing bundles of myelinated axons.

Neurons

The cells making up the brain, spinal cord, and nerves are called neurons. They are special cells capable of receiving a stimulus (nerve or electrical impulse), transmitting that stimulus throughout their length, and then delivering that stimulus to other cells next to them. The human body contains about 200 billion neurons. Almost half of them are located in the brain.

A neuron consists of three main parts: the cell body, dendrites, and an axon (dendrites and axons are both referred to as nerve fibers). The cell body has most of the same structures found in typical body cells, such as a nucleus (the part of the cell that controls its activities). It is ball shaped, about 0.001 inch (0.002 centimeter) in diameter.

Dendrite comes from the Greek word dendron, meaning "tree." Dendrites are hairlike threads branching off of the cell body like branches of a tree. Extensions of the cell body, they contain the same cytoplasm or cellular fluid found in the cell body. Dendrites are the points through which signals from adjacent neurons enter a particular neuron (the signal is then transmitted to the cell body). Since each neuron contains many dendrites, a neuron can receive signals from many other surrounding neurons.

An axon is a taillike projection extending out of one end of the cell body. It ends in a cluster of branches called terminal branches or axon terminals. Axons have the opposite function of dendrites: they carry nerve impulses away from the cell body. Axons vary in length and diameter. Some (such as those in the central nervous system) are very short, no longer than 0.01 inch (0.02 centimeter). Others (such as those in the peripheral nervous system) can be 3 feet (1 meter) long.

Most long axons are surrounded by a white, fatty material called myelin. The tubelike covering formed is known as a myelin sheath. It serves the same kind of function as the wrapping on a telephone line or an electrical cable. It protects the axon and prevents electrical impulses traveling through it from becoming lost.

Special cells form the myelin sheath by wrapping themselves around the axons of neurons. In the CNS, the cells forming the myelin sheath are called oligodendrocytes. In the PNS, special cells known as Schwann cells form the myelin sheath. The gap or indentation on an axon where one Schwann cell ends and another begins is known as a node of Ranvier. The nodes are unmyelinated (lack a myelin sheath), and the nerve or electrical impulse jumps from node to node as it passes along an axon (in unmyelinated axons, the impulse travels continuously along the axon).

Scientists believe Schwann cells produce a chemical that helps regenerate or restore damaged neurons in the peripheral nervous system. For example, if surgeons are able to reattach a person's severed hand, that person may regain some sensation and movement in that hand as neurons grow and make connections. Conversely, oligodendrocytes lack this ability. This is why an injury to the brain or spinal cord often results in some permanent loss of function.

TYPES OF NEURONS. Neurons in the body may be divided into three groups: sensory neurons, motor neurons, and interneurons. As their name implies, sensory neurons carry impulses or sensations from receptors to the brain or spinal cord (central nervous system). Receptors, which are located in the skin, skeletal muscles, joints, and internal organs, detect changes both inside and outside the body. Motor neurons work in the opposite direction. They carry impulses from the brain or spinal cord to muscles and glands, causing muscles to contract and glands to secrete. Both sensory and motor neurons make up the peripheral nervous system. Interneurons work entirely within the central nervous system. They conduct impulses from sensory to motor neurons.

LEEMAN AND SUBSTANCE P

American neuroendocrinologist Susan E. Leeman (1930– ) is known for her work with substance P, a peptide that helps govern the functioning of the nervous and lymphatic systems. (A peptide is a compound containing two or more amino acids, the building blocks of proteins.) While doing research on protein-hormones, Leeman found a peptide that could stimulate the secretion of saliva. The chemical turned out to be substance P, which had been discovered in the 1930s but had never been isolated (separated from other substances for individual study).

Leeman and her colleagues, working at Brandeis University in Massachusetts, isolated and characterized the peptide. A nerve transmitter that has many functions in the body, substance P is distributed throughout both the central and peripheral nervous systems. Substance P is important in the interaction between the nervous system and the lymphatic system (which governs body immunity) and seems to play a role in inflammation in the body.

Each neuron carries impulses in only one direction. This prevents impulses from traveling both ways in a neuron and canceling each other when they meet.

SUPPORTING CELLS. Neuroglia, or glial cells, are cells that surround neurons in the central nervous system. They do not conduct impulses, but help to support and protect neurons, combining with them to form what is known as nerve tissue. They also supply neurons with nutrients and remove their wastes. Neuroglia are abundant, accounting for some ten times the number of neurons. An example of neuroglia in the CNS are oligodendrocytes.

In the PNS, neurons are supported by Schwann cells and satellite cells (which form around the cell body to protect and cushion it).

Nerves

A nerve is a bundle of axons in the PNS. Each axon or nerve fiber is wrapped in delicate connective tissue. Groups of axons are then bound in coarser connective tissue to form bundles. Finally, many bundles are bound together (along with blood vessels to nourish the axons and Schwann cells) by even tougher connective tissue to form a nerve.

Nerves are categorized like neurons according to the direction in which they conduct impulses. Sensory nerves, made of the axons of sensory neurons, carry impulses to the brain and spinal cord. Motor nerves, made of the axons of motor neurons, carry impulses to the muscles and glands. Mixed nerves contain axons of both sensory and motor neurons. The most abundant nerves, mixed nerves can conduct impulses both to and from the central nervous system.

The brain

The human brain is a soft, shiny, grayish white, mushroom-shaped structure encased within the skull. At birth, a typical human brain weighs between 12 and 14 ounces (350 and 400 grams). By the time an average person reaches adulthood, the brain weighs about 3 pounds (1.36 kilograms). Because of greater average body size, the brains of male are generally about 10 percent larger than those of females. Although brain size varies considerably among humans, there is no correlation or link between brain size and intelligence.

The human brain is composed of up to one trillion nerve cells. One hundred billion of these are neurons, and the remainder are the supporting neuroglia. The brain consists of gray and white matter. Gray matter is nerve tissue in the CNS composed of neuron cell bodies, neuroglia, and unmyelinated axons; white matter is nerve tissue in the CNS composed chiefly of bundles of myelinated axons.

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 arteries and veins on the surface of the brain penetrate inward. Glucose, oxygen, and certain ions pass easily from the blood into the brain; other substances, such as antibiotics, do not. Scientists believe capillary walls 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 divided in terms of structure and function. The four principal sections of the human brain are the brain stem, 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, sneezing, and other basic functions of life are also located in the medulla. A region within the medulla helps to maintain the conscious state. The pons (from the Latin word 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 hypothalamus to the pons and spinal cord. It also contains visual and audio reflex centers involving the movement of the eyeballs and head.

Twelve pair 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 bring information to the brain from regions in the face, head, and neck. For example, the olfactory nerve transmits messages about smell from the nose and the optic nerve transmits visual information from the eyes. The only exception is the vagus nerve (vagus comes from the Latin word meaning "wandering"). It is the lone cranial nerve that serves other areas of the body. The vagus nerve branches extensively to the larynx, heart, lungs, stomach, and intestines. Among other functions, it helps promote digestive activity and regulate heart activity.

THE DIECEPHALON. 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 outside 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 hypothalamus controls normal body temperature and helps regulate the endocrine system, which produces hormones or chemical messengers that regulate body functions (for a further discussion of these actions, see chapter 3). It informs the body when it is hungry, full, or thirsty. It helps regulate sleep and wakefulness and is involved in the emotions of anger and aggression.

THE CEREBRUM. The cerebrum makes up about 80 percent of the brain's weight. It lies above the diencephalon. The cerebrum's outer layer, the cerebral cortex, is made entirely of gray matter (white matter makes up the inner portion of the cerebrum). The tissue of the cerebral cortex is about 0.08 to 0.16 inch (2 to 4 millimeters) thick. The cerebral cortex is folded extensively. The folds are called convolutions or gyri, and the shallow grooves between the folds are sulci. Deeper grooves, which are less numerous, are called fissures. The folds greatly increase the surface area of the cerebral cortex—it would have a surface area of about 5 square feet (1.5 square meters) if spread out—and thus the total number of nerve cell bodies it contains.

Located deep within the white matter of the cerebrum just above the diencephalon are two paired masses of gray matter known as basal ganglia. They are important in coordinating subconscious skeletal muscular movement, such as swinging of the arms while walking.

A deep fissure separates the cerebrum into a left and right hemisphere or half. The corpus callosum, a bundle of more than 200 million neurons, connects the two cerebral hemispheres and carries vast amounts of information between them—an estimated 4 billion nerve impulses per second. By studying patients whose corpus callosum had been destroyed, scientists have learned that differences exist between the left and right hemispheres. 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.

AVERAGE BRAIN WEIGHTS OF DIFFERENT SPECIES

Sperm whale: 17 pounds (7.8 kilograms)

Elephant: 13.2 pounds (6 kilograms)

Bottle-nosed dolphin: 3.3 pounds (1.5 kilograms)

Human (adult): 3 pounds (1.36 kilograms)

Camel: 1.5 pounds (0.76 kilogram)

Hippopotamus: 1.3 pounds (0.58 kilogram)

Polar bear: 1.1 pounds (0.5 kilogram)

Chimpanzee: 14.7 ounces (420 grams)

Lion: 8.4 ounces (240 grams)

Dog: 2.5 ounces (72 grams)

Cat: 1.1 ounces (30 grams)

Rabbit: 0.4 ounce (11.5 grams)

Squirrel: 0.26 ounce (7.6 grams)

Hamster: 0.05 ounce (1.4 grams)

Bull frog: 0.008 ounce (0.24 gram)

Scientists have further divided each cerebral hemisphere into lobes named after the overlying bones of the skull: frontal (forehead area), temporal (on the sides above the ears), parietal (top part of the head), and occipital (back of the head) lobes.

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 is meant by "being human." It enables humans 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 the various lobes house areas with specific functions. The frontal lobes contain motor areas that generate impulses for voluntary movements. An area usually located in the left frontal lobe is called Broca's area. It coordinates the movements of the mouth involved in speaking. The parietal lobes contain general sensory areas that receive impulses from receptors in the skin. The temporal lobes contain auditory areas (receive impulses from the ears for hearing) and olfactory areas (receive impulses from receptors in the nose for smell). The occipital lobes contain visual areas that receive impulses from the retinas of the eyes. Different areas in the occipital lobes are concerned with judging distance and other spatial relationships.

WHY EINSTEIN WAS A GENIUS

German-born American theoretical physicist Albert Einstein (1879–1955), who formulated the theory of relativity (an approach for studying the nature of the universe), is considered by many to have been one of the greatest physicists of all time.

When Einstein died in 1955, the physician who performed the autopsy removed his brain from his body in order to perform scientific studies on it. For years, however, the brain was kept in a jar (for a while, the jar was even placed in a cardboard box behind a beer cooler). Although the physician took measurements of Einstein's brain and cut it into 240 pieces of varying size, he published none of his findings.

In 1996, the physician allowed Canadian researchers to study the remains of Einstein's brain. What the researchers found might explain the reason Einstein was a genius. They discovered that a region in Einstein's brain was 15 percent larger than the same area in people with average intelligence. That region controls mathematical thought, spatial relationships, and other mental processes. Known as the inferior parietal lobe, it is located about the level of the ear, starting in the front of the brain and extending twothirds of the way back.

The researchers believe the enlarged region created a space for more neurons to make connections between each other and to work together more easily.

Association areas, those not involved with a particular movement or sensation, are located in all the lobes. These areas are concerned with emotions and intellectual processes. In association areas, innumerable impulses are processed that result in memory, emotions, judgment, personality, and intelligence: what truly makes each person an individual.

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 actions of the muscular system needed for movement, balance, and posture. All motor activity in the body depends on the cerebellum.

THE LIMBIC SYSTEM. The limbic system is a horseshoe-shaped area of the brain located along the border between the cerebrum and diencephalon. Key structures of the limbic system include the almond-shaped amygdala and the sea horse-shaped hippocampus. The limbic system is concerned with emotional states (such as rage, fear, and sexual arousal) and memory. The hippocampus, in particular, plays a vital role in learning and long-term memory.

The spinal cord

The spinal cord, a glistening white rope, is a continuation of the brain stem. It transmits impulses to and from the brain and controls some reflex actions. On average, the spinal cord measures about 18 inches (45 centimeters) in length and about 0.5 inch (14 centimeters) in width. It weighs about 1.25 ounces (35 grams).

The vertebral column or backbone encloses the spinal cord. This long channel, made of individual bones called vertebrae, protects the spinal cord from mechanical injury. Like the brain, the spinal cord is also cushioned and protected by meninges. Arteries run along the surface of the spinal cord, supplying it with a nourishing blood supply.

The spinal cord is composed of roughly 13.5 million neurons. It appears white because its thick outer layer is made of white matter. This layer contains many myelinated axons that form bundles—called tracts—that carry either sensory information or motor commands. Tracts that carry sensory information toward the brain are called ascending tracts. Those that carry motor commands from the brain into the spinal cord are called descending tracts.

Within the spinal cord is an H- or butterfly-shaped gray area composed of gray matter. Cell bodies of neurons and supporting neuroglia make up this gray matter. Many of these cell bodies are those of motor neurons. Their axons pass out of the spinal cord to control skeletal muscles or to regulate the actions of smooth muscles, cardiac muscles, and glands.

Extending out from the spinal cord between the vertebrae are thirty-one pair of spinal nerves. All these nerves are mixed nerves, containing thousands of axons of both sensory and motor neurons. Inside the vertebral column, however, each nerve is split into two branches that connect with the spinal cord. The branch that attaches on the rear (posterior) portion of the spinal cord is called the dorsal root. It contains the axons of sensory neurons. On

each dorsal root is an enlarged area called the dorsal root ganglion (a ganglion is any collection of neuron cell bodies in the PNS). This ganglion contains the cell bodies of the sensory neurons. The branch that attaches on the front (anterior) portion of the spinal cord is called the ventral root. It contains the axons of motor neurons.

The thirty-one pairs of spinal nerves exit the vertebral column to serve the areas of the body close by. The first or top eight pairs (located in the neck area) bring impulses to and from the head, neck, shoulders, arms, and diaphragm. The next twelve pairs (located in the chest area) bring impulses to and from the trunk of the body, including internal organs such as the heart and lungs. The remaining eleven pairs bring impulses to and from the lower part of the body—the hips, pelvic cavity, and legs. Damage to a spinal nerve or either of its roots will result in the loss of sensation and in paralysis of the area of the body being served by that nerve.

The spinal cord does not extend all the way down the vertebral column. In order to reach their proper openings to exit the column, the last eleven pair of spinal nerves hang below the end of the spinal cord like long hairs. Because of their appearance, they are collectively called the cauda equina (in Latin, cauda equina means "horse's tail").

WORKINGS: HOW THE NERVOUS SYSTEM FUNCTIONS

Reading a book, walking through a field, playing a musical instrument, digesting a holiday meal, remembering a lost relative—the nervous system regulates all of the body's activities, from the simplest to the most complex. In order to perceive and to respond to the world around us and the changes within us, the body's tissues, organs, and organ systems must function. In order for those body parts to function, they must be stimulated and regulated by nerve impulses.

Neurons have the ability to respond to a stimulus and convert it into a nerve impulse. They also have the ability to transmit that impulse to other neurons or the cells of muscles or glands.

Transmission of nerve impulses

In neurons, information travels in the form of nerve impulses that are conducted along axons. In myelinated axons, a nerve impulse can travel up to about 325 feet (100 meters) per second. In unmyelinated axons, the impulse travels much slower, about 1.5 feet (0.5 meter) per second.

Impulses do not travel in and between neurons like electric currents through telephone wires. For nerve impulses to be transmitted throughout the body, electrochemical reactions must occur in neurons. As stated earlier, dendrites are the points through which signals or impulses from adjacent neurons enter a particular neuron. If a dendrite of a neuron is stimulated, electrical and chemical changes take place throughout the cell.

Every neuron communicates with other neurons or with other types of cells. Neurons that transmit impulses to other neurons do not actually touch one another. The small space or gap where the impulse passes between the axon of one neuron and a dendrite of the next neuron is known as the synapse. A synapse measures about 0.000001 inch (0.0000025 centimeter).

When a neuron is inactive or resting, the tissue fluid that surrounds it contains more positive ions than are inside the neuron (an ion is an atom or group of atoms that has an electrical charge, either positive or negative). The major positive ions outside the cell are sodium; the major positive ions inside the cell are potassium. Because there are more positive ions outside the neuron, its internal surface is slightly negative. As long as the outside remains positive and the inside negative, the neuron remains at rest.

When a dendrite of a neuron is stimulated, tiny "gates" in the membrane of the dendrite and cell body begin to open and close. According to the laws of diffusion, molecules always move from an area where they exist in greater numbers to an area where they exist in lesser numbers. So, when these gates are open, sodium ions flow into the cell body. This temporary movement of ions changes the electrical charges on the membrane of the cell—positive on the inside, negative on the outside. This results in an electrical charge or nerve impulse on the surface of the cell. The impulse rapidly passes from the dendrite, down the length of the cell body, and along the length of the axon. An all-or-nothing response, the nerve impulse never goes partway along a neuron, but along its entire length.

WAVES IN THE BRAIN

The electrical charges (nerves impulses) created by the billions of neurons in the brain combine to generate an electrical field. That field can be measured by a machine called an electroencephalograph. The machine records the electrical activity as horizontal zigzag patterns on a screen or a page. Those patterns are known as brain waves.

As different regions of the brain are stimulated or are quieted down, the brain wave patterns change. There are four major types of brain waves. Alpha waves occur in normal, healthy adults who are awake but relaxed. The waves produce regular, fast patterns, about 8 to 13 cycles per second. Beta waves result when a person is concentrating or thinking about something. They produce patterns that are small and very fast, about 13 to 30 cycles per second. Theta waves are found in the brains of children and in adults who are under emotional stress. In adults, theta waves may also indicate a brain disorder. The patterns of theta waves are large and slow, about 4 to 8 cycles per second. The largest and slowest-moving waves are delta waves. These regular-patterned waves appear in the brains of infants and in the brains of sleeping adults. They are also found in the brains of individuals who have suffered brain damage.

By comparing the brain waves of a person to those found in normal, healthy individuals, physicians can determine if the brain of that person has been injured or infected by a disease.

As soon as the gates on the membrane of the neuron open, they close and the body restores the correct balance of sodium and potassium ions inside and outside the neuron. Only after the balance has been restored and the neuron is at rest can another impulse be conducted along the neuron.

NEUROTRANSMITTERS. When the impulse or electrical current has reached the terminal branches or end of the axon, it stimulates the branches to release chemicals known as neurotransmitters. As their name suggests, neurotransmitters are the mechanisms by which a nerve impulse travels from one neuron to another or to body cells.

Once released from an axon, a neurotransmitter drifts across the synapse to a second neuron. When it has reached that cell, it attaches itself to specialized parts, called receptors, in the dendrites of the second neuron. This act of attaching creates the stimulus for dendrites in the second neuron. Those

dendrites then respond to this stimulus just as the first neuron responded to its stimulus, and the nerve impulse continues.

Scientists have identified a number of neurotransmitters, including dopamine, serotonin, and acetylcholine. Each neurotransmitter occurs in certain types of neurons and has specific functions. It can either start an action or stop it, such as causing a muscle to contract or a gland to stop secreting. For example, acetylcholine is the neurotransmitter released at the terminal branches of motor neurons that come in close contact with muscle fibers (in this case, the synapse is called the neuromuscular junction). When acetylcholine attaches to receptors on the membrane of the muscle fiber, it triggers an electrical charge that quickly travels from one end of the muscle fiber to the other, causing it to contract.

The transmission of a nerve impulse—first along a neuron, then to another neuron—is an electrochemical event. The impulse traveling along a neuron is electrical, but the transfer of that impulse (the stimulation of the next neuron by neurotransmitters) is chemical. The actions involved in the creation and transfer of a nerve impulse ensure that it can travel in one direction only in a neuron—from dendrite through axon.

Even though nerve impulses travel in one direction only, a neuron can have as many as 100,000 synapses connecting it to other neurons. Thus, a neuron can receive and transmit many impulses over a variety of pathways, connecting with many different neurons at the same time. The path an impulse takes, from a particular neuron to another one, determines the meaning of that impulse and the action it evokes.

Reflexes

It is not always necessary for a nerve impulse traveling along sensory nerves to reach the brain before a response causes the body to react in some way. When a stimulus causes a response that is involuntary, rapid, and predictable, that response is known as a reflex. Reflexes are classified according to the systems of the PNS: autonomic reflexes and somatic reflexes. Autonomic reflexes control the activity of the smooth muscles, heart, and glands. They also regulate complex body functions such as digestion, blood pressure, sweating, and swallowing. Somatic reflexes control skeletal muscles. In general, reflexes control much of what the body must do every day.

The pathway a nerve impulse travels when a reflex is initiated is called the reflex arc. A typical reflex action begins when a sensory receptor (in the skin, sense organ, or other internal organ) is activated by some kind of stimulus. The receptor generates an impulse in a sensory neuron and the impulse then travels along other sensory neurons to the spinal cord. In the gray matter of the spinal cord, the impulse passes from a sensory neuron through an interneuron into a motor neuron. The motor neuron then transmits the impulse through other motor neurons to a muscle or gland, where some type of response occurs.

For example, if a person touches a hot stove, that person immediately pulls his or her hand away. Heat and pain receptors in the skin were stimulated to send impulses to the spinal cord, where they were transferred to motor neurons that eventually connected to muscles in the hand, which were stimulated to contract and pull the hand away. Although it may seem that the pain caused by the hot stove led to the quick withdrawal of the hand, the movement actually occurred milliseconds before the brain became aware of the pain and initiated some response. This is an example of a somatic reflex.

Reflexes such as this one are important in safeguarding the body against potentially harmful changes outside or inside the body. Reflexes are also a good tool in evaluating the condition of the nervous system. If a reflex is exaggerated or even absent, a nervous system disorder may be present. That is why doctors often perform the knee-jerk reflex test during a routine physical examination: a sharp rap on the tendon below the knee should cause the quadriceps muscle on the front part of the thigh to contract, forcing the lower leg to kick outward.

Autonomic nervous system

Much of what occurs in the body every day occurs without an individual being consciously aware. The heart beats, the lungs expand, blood vessels contract and dilate (widen), and the stomach and intestines break down food and move it through the system. These actions and all others that take place without willful control are regulated by the autonomic nervous system (ANS), a part of the peripheral nervous system.

All body systems contribute to homeostasis or the ability of the body to maintain the internal balance of its functions. The minute-to-minute stability of the body, however, is largely dependent on the workings of the ANS.

The ANS is broken down into two subdivisions. The part that keeps body systems running smoothly on a daily basis is called the parasympathetic nervous system. The part that comes into play when emergencies or stressful situations arise is called the sympathetic nervous system. Both subdivisions service the same body organs and use motor nerves to do so (some glands and skin structures receive only sympathetic motor nerves).

The parasympathetic nervous system is in control when the body is at rest. The neurons for this system originate in the brain stem and in the lower region of the spinal cord. The impulses conducted through this system target body organs in an effort to conserve body energy and promote normal digestion and elimination. In the digestive tract, secretions and peristalsis (series of wavelike muscular contractions that move material in one direction through a hollow organ) increase. Heart rate, the force of heart contractions, and blood pressure all decrease. The pupils of the eyes constrict to limit the amount of light entering the body. Kidneys increase their production of urine. Nutrient levels in the blood increase, and cells throughout the body add the extra nutrients to their energy reserves.

The sympathetic nervous system has the opposite effect on the body. It comes into play when an individual is faced with a "fight-or-flight" situation. The neurons for this system originate from the middle of the spinal cord. Their impulses seek to stimulate the body into using its energy. The activity of the digestive and urinary organs decreases. The liver releases glucose (sugar) into the blood for use by the cells as energy. Heart rate and contraction, blood pressure, and blood flow to skeletal muscles all increase. The eyes dilate to let more light in. In short, the sympathetic nervous system prepares the body to respond to some threat, whether that response is to run, to see better, or to think more clearly.

LEVI-MONTALCINI'S NERVE GROWTH FACTOR

Italian-born American neurobiologist Rita Levi-Montalcini (1909– ) is recognized for her groundbreaking research on nerve cell growth.

She discovered a protein in the human nervous system that she named the nerve growth factor (NGF). For her work, which has proven useful in the study of several disorders (including Alzheimer's disease), Levi-Montalcini received the 1986 Nobel Prize for physiology or medicine (she shared the award with biochemist Stanley Cohen).

After graduating from medical school in Italy in 1936, Levi-Montalcini began to research the nervous system. She conducted experiments on chicken embryos (organisms in their earliest stages of development) in order to study how neurons are differentiated, or how they are formed and assigned a particular function in the developing body. Levi-Montalcini believed that a specific nutrient was essential for neuron growth. In the 1950s, she finally isolated or obtained a sample of the substance that caused neurons to grow and labeled it NGF.

The work that Levi-Montalcini began in the late 1930s has been carried on by researchers who realize the important role that NGF can possibly play in treating degenerative diseases (those in which organs or tissue deteriorate and stop functioning).

The parasympathetic and sympathetic nervous systems rarely work independently of each other. Instead, they often work together, especially in affecting vital organs. Their opposing effects help to maintain the dynamic balance of the internal body.

AILMENTS: WHAT CAN GO WRONG WITH THE NERVOUS SYSTEM?

Because of its role as the master controller of the body, when the nervous system becomes disabled, so does the rest of the body. Injuries to the brain and spinal cord can easily occur in contact sports (such as football, hockey, and boxing) and as a result of falls or collisions in other activities (such as bicycling, horseback riding, skiing, and soccer). They may range from mild concussions where the brain is jarred against the skull, resulting in the temporary slight loss of higher mental functions, to severe spinal injuries where the spinal cord is pinch or severed, resulting in permanent paralysis or even death.

The nervous system can be also adversely affected by diseases or disorders. Some may be genetic (hereditary), others caused by an illness or disease. The following are just a few of the many diseases and disorders that can affect this system or its parts.

NERVOUS SYSTEM DISORDERS

Alzheimer's disease (ALTS-hi-merz): Disease of the nervous system marked by a deterioration of memory, thinking, and reasoning.

Amyotrophic lateral sclerosis (a-me-o-TROW-fik LA-ter-al skle-ROW-sis): Also known as Lou Gehrig's disease, a disease that breaks down motor neurons, resulting in the loss of the ability to move any of the muscles in the body.

Carpal tunnel syndrome (CAR-pal TUN-nel SIN-drome): Disorder caused by the compression at the wrist of the median nerve supplying the hand, causing numbness and tingling.

Epilepsy (EP-eh-lep-see): Disorder of the nervous system marked by seizures that often involve convulsions or the loss of consciousness.

Huntington's disease: Inherited, progressive disease causing uncontrollable physical movements and mental deterioration.

Migraine (MY-grain): A particularly intense form of headache.

Parkinson's disease: Progressive disease in which cells in one of the movement-control centers of the brain begin to die, resulting in a loss of control over speech and head and body movements.

Poliomyelitis (po-lee-o-my-eh-LIE-tis): Contagious viral disease that can cause damage to the central nervous system, resulting in paralysis and loss of muscle tissue.

Alzheimer's disease

Alzheimer's disease (AD) is a progressive (tending to grow worse) neurological disorder that results in dementia—impaired memory, thinking, and reasoning. AD usually occurs in old age. It affects approximately 4 million people in the United States and is the fourth leading cause of death among adults (after heart disease, cancer, and stroke). Scientists believe that 5 to 10 percent of people over the age of sixty-five suffer from some form of the disease. AD affects men and women almost equally.

The primary symptoms of AD are the gradual loss of memory, lessened ability to perform routine tasks, disorientation, difficulty in learning, loss of language skills, impairment of judgment and planning, and mood or behavioral changes. Depression, paranoia, and delusions may also arise. The disease begins slowly and gradually. Most people die within eight years after being diagnosed with AD. Some die within a year, while others may live as long as twenty years.

Scientists do not know the exact cause or causes of AD. Research has shown that in people with AD, the centers of the brain concerned with learning, reasoning, and memory become clogged with abnormal tissue. What triggers this twisted mass of tissue to form is, again, unknown.

There is currently no cure for AD. Treatments, however, are available to alleviate or lessen some of the symptoms. Two drugs that have been approved by the U.S. Food and Drug Administration both increase the levels of the neurotransmitter acetylcholine in the brain. This helps increase the communication ability of the remaining neurons, thereby modestly increasing a person's ability to perform normal daily activities. Research into this disabling disease continues.

Amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a disease that breaks down tissues in the nervous system, affecting the nerves that control body movement. ALS is also known as motor neuron disease and Lou Gehrig's disease, after the American baseball player (1903–1941) whose career it ended.

The disease affects approximately 30,000 people in the United States. It usually begins between the ages of forty and seventy, although it can begin at a younger age. Men are slightly more likely to develop ALS than women. Eighty percent of those people affected with the disease die within five years. However, noted English theoretical physicist Stephen Hawking (1942– ) has lived with ALS for over thirty years.

Technically, ALS causes the destruction of motor neurons that control voluntary muscle movement. For yet unknown reasons, the motor neurons in the spinal cord and brain die. As they die, the muscles they normally control cannot be moved as effectively and begin to waste away. The ability to move virtually any of the muscles in the body is soon lost. ALS principally affects the muscles controlled by conscious thought, such as those in the arms, legs, and trunk.

Weakness in the arms and legs, usually more pronounced on one side than the other, is the earliest symptom of ALS. Later symptoms include the loss of the ability to walk, to use the arms and hands, to speak clearly or at all, to swallow, and to hold the head up. Lung infection, brought on by poor swallowing and the weakness of respiratory muscles, is often the cause of death.

There is currently no cure for ALS, and no treatment that can significantly alter its course.

Carpal tunnel syndrome

Carpal tunnel syndrome is a condition in which the squeezing or compression of the median nerve that passes through the wrist results in numbness,

tingling, weakness, or pain in one or both hands. The hands may become so weakened that opening jars or grasping objects becomes difficult and painful. Women between the ages of thirty and sixty have the highest rates of carpal tunnel syndrome.

The carpal tunnel is a space formed by the carpal (wrist) bones and the carpal ligament (a connective tissue that attaches bone to bone). Through this space pass the median nerve and tendons of the fingers and thumb (the median nerve runs from the neck through the middle of the arm to the fingers). When the tendons within the carpal tunnel become inflamed, they swell and press on the median nerve.

Most often, performing a job that requires repeated bending or twisting of the wrists increases the likelihood of developing the disorder. Continuous flexing of the wrist, as when typing on a keyboard or playing a piano, can cause compression of the median nerve. A number of other conditions can also cause swelling of the carpal tunnel and pressure on the median nerve. Such conditions include pregnancy, arthritis, hypothyroidism (reduced function of the thyroid gland), diabetes, menopause (the point in a woman's life when menstruation ceases and childbearing is no longer possible), and pituitary gland abnormalities.

Carpal tunnel syndrome is treated initially by applying a brace or splint, to prevent the wrist from bending and to relieve pressure on the median nerve. If a person's job is causing the disorder, performing other work may be necessary. Treatment of a related medical condition may relieve the symptoms of carpal tunnel syndrome. Severe cases may require surgery to decrease compression of the median nerve.

Epilepsy

Epilepsy comes from the Greek word for seizure. A seizure is a sudden disruption in the brain's normal electrical activity accompanied by a state of altered consciousness. Epilepsy is a condition characterized by recurrent seizures during which a person may lose consciousness and experience convulsions, or violent repetitive muscle contractions. Scientists believe epilepsy affects 1 to 2 percent of the population of the United States. One in every two cases develops before the age of twenty-five.

The repeated symptoms associated with epilepsy are the result of unusually large electrical charges from neurons in a particular area of the brain. However, the reason this occurs in 50 to 70 percent of all cases of epilepsy is unknown. Epilepsy is sometimes the result of trauma at the time of birth, such as insufficient oxygen to the brain or head injury. Other causes of the disorder may be head injury resulting from a car accident, alcoholism, inflammation of the meninges, infectious diseases such as measles or mumps, or lead or mercury poisoning.

The best known examples of epilepsy are grand mal and petit mal. The term mal comes from the French word meaning "illness," while grand and petit refer respectively to "large" and "small" episodes of the illness.

In the case of grand mal, an epileptic (person suffering from epilepsy) is likely to have some indication (such as a distinctive smell, taste, or other unusual sensation) that a seizure is imminent. 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. The thrashing movements of the limbs that follow are caused by opposing sets of muscles alternately contracting. The person may also lose control of the bladder and or bowels. When the seizures stop, 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 grand mal seizure, the petit mal may seem insignificant. The person interrupts whatever he or she is doing and for up to about 20 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 that an interruption has taken place. Petit mal seizures generally begin at the age of four and stop by the time a child has become an adolescent. Untreated, petit mal seizures can recur as many as 100 times a day and may progress to grand mal seizures.

Epilepsy is a recurrent, lifelong condition that must be managed on a long-term basis. A number of drugs are available to help eliminate seizures or make the symptoms less frequent and less severe. About 85 percent of all seizure disorders can be partially or completely controlled if an epileptic person takes anti-seizure medications according to directions, gets enough sleep, and eats balanced meals.

Huntington's disease

Huntington's disease (HD) is an inherited, progressive (tending to grow worse) disease that causes uncontrollable physical movements and mental deterioration. It is occasionally referred to as Woody Guthrie's disease, after the American folk singer (1912–1967) who died from it. Approximately 30,000 people in the United States are affected by HD.

BENJAMIN CARSON: CELEBRATED NEUROSURGEON

American pediatric neurosurgeon Benjamin Carson (1951– ) has undertaken many high-risk operations on children involving complex and delicate neurosurgical procedures. In 1987 he gained international acclaim for leading a team of seventy medical personnel that separated a pair of Siamese twins (children who are born physically connected) who were joined at the backs of their heads.

One of Carson's most difficult operations took place in 1985 when he operated on a four-yearold boy with a malignant tumor of the brain stem (a malignant tumor is one that spreads and causes physical harm). Other physicians had stated that the cancer could not be removed with surgery, but Carson disagreed and went ahead with the operation. The boy eventually made a complete recovery.

Another notable operation involved a four-year-old girl who suffered from multiple seizures—up to 120 a day. Her right side was paralyzed, and she had a rare brain disease that, if unchecked, would have left her with serious neurological damage. Carson removed the diseased left hemisphere of her brain during a ten-hour operation. Six months later she had regained nearly complete use of her right arm and leg and was free of seizures. The right side of her brain had taken over functions of the left.

Carson's most famous medical operation occurred in 1987, when he led a team of doctors, nurses, and technicians to separate a pair of German Siamese twins who shared a blood vessel in the back of their heads. Carson devised a plan to separate the twins by completely shutting down their blood flow, severing their common blood vessel, and then restoring their individual vessel systems. While the entire procedure lasted twenty-two hours, Carson and another surgeon had only one hour to conduct the actual surgery and restoration. The operation went smoothly until Carson noticed the vessels that carried blood from the brain of each child were more tangled than had been expected. Twenty minutes after stopping the twins' circulation, Carson made the final cut. He then had forty minutes to reconstruct the severed blood vessels and close. Just a few minutes before the hour limit, the twins were separated and the operating tables were wheeled apart.

The symptoms of HD usually appear in people between the ages of thirty and fifty. They fall into three categories: motor or movement symptoms, personality and behavioral changes, and mental decline. Early motor symptoms include restlessness, twitching, and a desire to move about. As the disease progresses, a person may experience involuntary jerking or twisting, difficulty in speaking and swallowing, impaired balance, depression, anxiety, the inability to plan and carry out routine tasks, slowed thought, and impaired or inappropriate judgment.

In 1993, scientists discovered the exact gene that causes HD. The gene makes a protein. When the gene is defective, the protein it creates interacts with other proteins in brain cells where it occurs, and this interaction ultimately leads to cell death. A parent, either the mother or father, who has the defective gene has a 50 percent chance of passing it on to his or her children. Male and females are affected equally.

There is no cure for HD and no treatment that can slow its rate of progression. Drugs and physical therapy can help reduce the effects of involuntary muscular movements and mental deterioration. Death usually occurs fifteen to twenty years after a person has been diagnosed with the disease.

Migraine

Migraine is a particularly intense form of headache lasting several hours or more. 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.

Migraines affect as many as 24 million people in the United States. Approximately 18 percent of women and 6 percent of men experience at least one migraine attack per year. Migraines often begin in adolescence, but are rare after the age of sixty.

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

The second phase, known as the aura, is marked by visual disturbances. These are described as flashing lights, shimmering zigzag lines, spotty vision, and other disturbances in one or both eyes. Other symptoms such as tingling or numbness in the hands may occur as well. All these symptoms can be acutely distressing to the patient. This phase usually precedes the actual headache by one hour or less.

Phase three consists of the headache itself, usually described as a severe throbbing or pulsating pain on one or both sides of the head. It may be accompanied by nausea and vomiting. Light, noise, and movement may worsen the condition. This phase may last from four to seventy-two hours.

During the final phase, called the postdrome, the person often feels drained and washed-out. This feeling generally subsides within twenty-four hours.

Migraine appears 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 to other brain centers, including the visual cortex. The second step occurs when blood vessels in the brain dilate (expand), increasing blood flow. The third step occurs when nerve cells that control the sensation of pain in the head and face become activated. The pain of migraine is thought to result from this combination of increased pain sensitivity, tissue and blood vessel swelling, and inflammation.

Several drugs may be used to reduce the pain and severity of a migraine attack. Research has shown that a combination of acetaminophen (Tylenol), aspirin, and caffeine can effectively relieve symptoms for many migraine sufferers. Research has also shown a connection between migraine and low levels of serotonin, a neurotransmitter found in the brain. Drugs that chemically resemble serotonin have proven effective in treating migraine.

Parkinson's disease

Parkinson's disease (PD) is a progressive (tending to grow worse) disease in which cells in one of the movement-control centers of the brain begin to die. Nerves and muscles become weak, and the control over speech and head and body movements is lost. Individuals suffering from PD move around very slowly and their hands may tremble. Approximately 500,000 people in the United States, both men and women, are affected by PD.

Scientists have not yet been able to determine exactly what causes the brain cells to die. Certain chemical toxins in illegal drugs have been found to bring on symptoms of PD, but these toxins cannot be linked to the numerous cases of PD that arise each year. A deficiency of the neurotransmitter dopamine has been suggested as a possible cause for PD. In this case, scientists believe that when too little dopamine is produced in the body, neurons are not sufficiently stimulated, which can lead to symptoms characteristic of PD.

Symptoms of PD include tremors (usually beginning in the hands), slow movements, muscle rigidity or stiffness, balance problems, decreased eye-blinking, depression, speech changes, sleep problems, constipation, and irritability.

There is currently no cure for or a way to prevent PD. Most drugs prescribed treat the symptoms of the disease only. Regular, moderate exercise has been shown to improve certain motor functions in people afflicted with PD. Over time, the symptoms of PD worsen and become less responsive to drug treatments.

Poliomyelitis

Poliomyelitis (often referred to simply as polio) is a serious infectious disease that attacks muscle-controlling nerves and can eventually cause paralysis. Poliomyelitis is caused by one of three related viruses, and it primarily affects children. However, adults can also be infected. There is no drug that can cure polio once a person has been infected.

Poliomyelitis is infectious, meaning it is spread primarily through contact with the saliva or feces of someone who already has the disease. The virus enters the body through the mouth and multiplies rapidly in the intestines. Eventually it enters the bloodstream, then gains access to the central nervous system where it travels along nerve pathways.

Symptoms usually begin to show one to three weeks after the virus is contracted. In some cases, the attack may be so mild that it goes unnoticed. The body quickly develops immunity and the virus is eliminated. A more severe attack gives rise to symptoms that resemble those of influenza (fever, sore throat, vomiting, diarrhea, stiff neck and back, and muscle pain). About two-thirds of people infected in such a way recover without suffering any paralysis.

A serious attack occurs if the virus reaches the central nervous system and invades motor nerves. Inflammation and the destruction of those nerves can result. Without stimulation from those nerves, muscle tissue then weakens and paralysis develops. Usually the paralysis is only temporary, and about 50 percent of people infected recover without permanent disability. However, if any of the cells attacked by the virus are destroyed, they cannot be replaced and muscle function is permanently impaired. About 25 percent of people who recover after being seriously infected have severe permanent disability. If the nerve cells of the brain stem are attacked, the muscles controlling swallowing, heartbeat, and breathing are paralyzed. The result is death.

In the 1950s, two types of vaccines were developed to provide the body immunity to the poliomyelitis virus. The first contains components or parts of the virus while the second contains live but weakened virus. Both vaccines prompt the body's lymphatic system to produce antibodies that will attack any future invading forms of the disease. Poliomyelitis is now a rare disease in the United States.

TAKING CARE: KEEPING THE NERVOUS SYSTEM HEALTHY

The growth of the brain stops once an individual reaches young adulthood. As aging takes place, neurons become damaged and die. Since they cannot reproduce themselves, their number in the brain continually decreases. However, the number lost under normal conditions is only a small percentage of the total.

As in all body systems, though, aging brings about structural changes in the brain and nervous system. With the loss of neurons, brain size and weight decrease. Blood flow to the brain also decreases. Again, these changes are normal and usually are not the cause of mental disabilities in elderly people.

However, certain activities in life can accelerate the aging process of the brain, leading to mental impairment. Boxers and alcoholics are prime candidates to suffer from slurred speech, tremors, and dementia (impaired memory, thinking, and reasoning) long before others. Certain drugs, high blood pressure, arteriosclerosis (diseased condition in which the walls of arteries become thickened and hard, interfering with the circulation of blood), poor nutrition, depression, and stress can also lead to premature loss of mental abilities.

Learning can continue throughout life as neural pathways are always available and ready to be developed. Like the muscles of the body, the mind can remain strong if used regularly. The following activities, if maintained on a life-long basis, can help keep the nervous system operating at peak efficiency: eating a proper diet low in fat and high in fiber, maintaining a healthy weight, consuming proper amounts of good-quality drinking water, getting adequate rest, engaging in regular exercise, not smoking or taking illegal drugs, drinking only moderate amounts of alcohol (if at all), and reducing stress levels.

FOR MORE INFORMATION

Books

Barmeier, Jim. The Brain. San Diego, CA: Lucent Books, 1996.

Edelson, Edward. The Nervous System. New York: Chelsea House, 1991.

Greenfield, Susan A. The Human Mind Explained. New York: Henry Holt, 1996.

Parker, Steve. The Brain and Nervous System. Austin, TX: Raintree/Steck-Vaughn, 1997.

Simon, Seymour. The Brain: Our Nervous System. New York: Morrow, 1997.

Silverstein, Alvin, Virginia Silverstein, and Robert Silverstein. The Nervous System. New York: Twenty-First Century Books, 1994.

Turkington, Carol. The Brain Encyclopedia. New York: Facts on File, 1996.

Wade, Nicholas, ed. The Science Times Book of the Brain. New York: Lyons Press, 1998.

WWW Sites

American Academy of Neurology
http://www.aan.com/
Homepage of the American Academy of Neurology, which includes information about neurology and the academy.

A Brief Tour of the Brain
http://suhep.phy.syr.edu/courses/modules/MM/Biology/biology.html
Site provides information about the brain, beginning with a brief history of the study of the brain, then describing the brain's large-scale features, building blocks, and organization.

Cyber Anatomy: Nervous System
http://tqd.advanced.org/11965/html/cyber-anatomy_nervous.html
Site provides detailed information on the nervous system that is divided into two levels: the first geared for students in grades 6-9, the other for students in grades 10-12.

Mind Over Matter: The Brain's Response to Drugs
http://www.nida.nih.gov/MOM/TG/momtg-index.html
From the National Institute on Drug Abuse, a site that explains the effects of legal and illegal drugs, such as nicotine and marijuana, on the brain. Also includes a section on brain anatomy.

Nervous System
http://www.fm.cnyric.org/Wellwood/humansystems/Nervous%20System/nervsyst.html
Site provides links to other sites that provide information such as a description of the nervous system and facts on the brain.

Neuroscience for Kids
http://faculty.washington.edu/chudler/neurok.html
Highly recommended site provides an extensive amount of information on the nervous system and the brain. Includes experiments and activities, a list of on-line and off-line books and articles, current events and new discoveries in brain research, and links to other sites.

Central Nervous System

Definition

The central nervous system (CNS) consists of the brain and spinal cord.

Description

Comprised of the brain and spinal cord, the CNS is central to the body as opposed to peripheral. It is responsible for unconscious and conscious body functions as well as intellectual functions that allow humans to think. The neuron is the basic nerve cell of the nervous system, however, the nervous system contains many types of cells other than nerve cells. The nerve cell consists of a nucleus in a cell body, dendrites, and axons. The CNS is heavily guarded with protective features, however, this does not make it immune to disease and disorder. The CNS is cushioned with cerebrospinal fluid (CFS), protected by three layers of meninges, and protected from the body by the blood-brain barrier. The blood-brain barrier prevents harmful substances and disease-causing organisms from contaminating the bloodstream entering the CNS.

The skull (cranium) encloses the brain. Three main areas of the brain, the cerebrum, cerebellum, and brainstem, contain myelinated nerve fibers and white matter, in contrast to the cortex of the brain, the layer of gray matter. The cerebrum, the largest part of the brain containing the left and right hemispheres, controls conscious activities like motor functions. The cerebellum, located above the brainstem, communicates with other regions of the brain and spinal cord to control balance and coordination. The brainstem, the lowest part of the brain extending to the spinal cord, controls unconscious activities necessary for survival like breathing and blood pressure. The spinal cord is protected by the vertebral column from the cervical area to the sacrum. The spinal cord nerves relay sensory information to the brain and motor information to the body.

Function

The central nervous system (CNS) is a processing center that integrates sensory and motor activities via the brain and spinal cord. Nerves are like an electrical wire with an insulating sheath, called myelin, that facilitates the smooth, high-speed transmission of messages. Information in the nervous system is carried by brief electrical impulses that are conducted away from the body of the nerve cell along the axons. When impulses reach the tips of axons, information is transmitted to the next nerve cell in line, or to a muscle or organ. At the point of contact, or synapse, the information is carried across the gap between cells by neurotransmitters. The CNS has two-way communication. Nerve fibers either relay messages to the brain to communicate sensory stimuli, or they relay messages away from the brain to the body's tissues and organs. It is through the spinal cord that messages are sent back and forth to the brain. Spinal nerves in the spinal cord connect the message relay system to rest of the body

Role in human health

Many health care providers, neurologists, neurosurgeons, physiatrists, neuropsychologists, nurses, occupational therapists, speech language pathologists, physical therapists, and vocational rehabilitation counselors, play a role on the health care team and share in the responsibility of the neurologic patient. A combination of methods for assessing patients with neurologic diseases and disorders enables health care professionals to make informed treatment recommendations. These include neurologic and physical examination, laboratory tests such as lumbar puncture, brain-imaging scans such as MRI, neuropsychological testing, and nuclear medicine tests. Treating neurologic diseases and disorders requires active patient participation along with education and support from health care professionals. While patient education may be time-consuming, it is extremely important to the process of maintaining health and preventing disease.

Common diseases and disorders

There are hundreds of disabling neurological diseases and disorders that affect every age, race, and ethnicity. A few examples include developmental disorders (cerebral palsy ), degenerative diseases (Parkinson's and Alzheimer's disease ), metabolic diseases (Tay-Sachs disease), cerebrovascular diseases (stroke), autoimmune diseases (multiple sclerosis ), and tumors (glioblastoma).

Alzheimer's disease

Alzheimer's disease (AD), a progressive, neurodegenerative disease, is the most common cause of dementia in later life. The course of the disease varies from person to person. Although the cause of AD is not yet known, researchers have found a familial tendency for AD. They have also implicated several possibilities including genetics, environmental factors, and biochemical changes in the brain caused by low levels of certain neurotransmitters. AD usually begins after age 65, however, it may have an early onset as early as age 40.

SYMPTOMS. The severity and progression of symptoms vary from person to person. Some of the early symptoms of AD, such as forgetfulness or loss of concentration, can easily be overlooked because they resemble signs of aging or could result from fatigue, depression, or the use of certain medications. Symptoms of AD include:

• memory loss, confusion, loss of concentration
• difficulty recognizing family and friends
• poor judgment, indifference
• changes in behavior and personality
• disorientation, wandering, sleep disturbance
• agitation, anxiety, depression

There is no specific test to confirm a diagnosis of AD. A combination of tools including brain imaging technologies, patient and family history, physical and neurological examination, and neuropsychological testing are utilized to definitively diagnose AD. There is no cure for AD and no clinically proven way to slow the progression of the disease. For some patients, medication may alleviate some of the symptoms. Treatments are intended to make the patient more comfortable.

Stroke

Stroke, also called cerebral vascular accident (CVA), occurs when the blood flow to part of the brain is disrupted. There are primarily two categories of stroke, ischemic and hemorrhagic. The most frequent cause of stroke is a blockage (ischemic) of a blood vessel in the brain. The blockage can have several causes but all with the same result, brain cell damage or death. Brain cells cannot survive without a blood supply of oxygen and nutrients. Blockage of blood flow in the brain can be caused by a clot in a blood vessel (thrombosis) of the brain, the movement of a clot from another part of the body (embolism) to the brain, or a severe narrowing of an artery in the brain (stenosis). In a hemorrhagic stroke, a blood vessel in the brain bursts, bleeding into the brain (intracerebral hemorrhage) or into the spaces surrounding the brain.

SYMPTOMS. The signs and symptoms of stroke depend on the areas of the brain affected and the functions they control. The right cerebral hemisphere controls the left side of the body and the left cerebral hemisphere controls the right side of the body. The symptoms of stroke may be:

• sudden numbness or weakness, especially on one side of the body
• sudden confusion, difficulty speaking, or understanding speech
• sudden difficulty seeing in one or both eyes
• sudden trouble walking
• sudden dizziness, loss of balance or coordination
• sudden severe headache
• paralysis, pain

Risk factors for stroke are either changeable or not. For example, a person cannot change their age or family history, but they can change behaviors, like smoking, that put them at risk for a stroke. Risk factors for stroke include:

• high blood pressure
• heart disease
• diabetes
• smoking
• heavy alcohol consumption
• drug abuse
• high blood cholesterol levels
• family history of stroke or TIA
• age over 55

According to the National Center for Health Statistics, stroke is the third leading cause of death in the United States. In order to improve this statistic, patients need prompt interventions. The public education campaign, "Know Stroke: Know the Signs. Act in Time," promoted by the National Institute of Neurological Disorders and Stroke (NINDS) teaches people how to recognize the signs and symptoms of stroke and the importance of prompt medical treatment to improve recovery. Tissue plasminogen activator (t-PA), the first Food and Drug Administration (FDA) approved acute ischemic stroke treatment, needs to be given within a three-hour window of the onset of symptoms to dissolve the clot. Stroke is diagnosed by a neurological examination, blood tests, brain imaging scans, Doppler ultrasound, or arteriography.

Multiple sclerosis

Multiple sclerosis (MS) is a common neurological disease that occurs mainly in young adults. The course of the disease varies from person to person and is categorized by type. Relapsing-remitting MS (RRMS) has a course of acute attacks with full or partial recovery during remissions. Secondary progressive MS (SPMS) is initially relapsing-remitting and then becomes progressive. Primary progressive MS (PPMS) has a progressive course from the beginning of the disease with no remissions. Progressive-relapsing MS (PRMS) has a progressive course from the beginning with acute relapses, with or without full recovery.

MS is believed to be an autoimmune disease, where the immune system targets the myelin in the central nervous system. MS is the most common demyelinating disease. During an attack (exacerbation), inflammation occurs in the white matter of the central nervous system. This process is followed by destruction of myelin causing areas called plaques. Not only is the myelin damaged, but the attack may also damage or sever the nerve fibers underneath the myelin. The nerve cannot conduct or send a signal properly without the myelin sheath.

MS can be difficult to diagnose because there are other diseases with similar symptoms, and there is no specific test to confirm the diagnosis. A process of elimination may be done along with a combination of imaging technologies such as MRI, CSF analysis, evoked potentials (EVP), medical history, and clinical examination.

SYMPTOMS. The symptoms of MS are unpredictable, can vary greatly from person to person, and can be intermittent. The neurological symptoms of MS are the result of demyelination. Symptoms include:

• blurred, double vision, and blind spots
• numbness, tingling, and paresthesias (pins and needles)
• fatigue, dizziness, and vertigo
• difficulties with memory loss, concentration, and attention
• difficulty with coordination, balance, and gait
• muscle and nerve pain, muscle weakness, and tremors
• bladder and bowel problems

There are many conventional and alternative treatment therapies to alleviate the symptoms of MS, but there is no cure. However, four medications have been shown to slow down disease progression. The FDA approved Interferon beta 1b (Betaseron) in 1993, Interferon beta 1a (Avonex) in 1996, and glatiramer acetate (Copaxone) in 1996 for the treatment of RRMS. They were clinically shown to decrease the frequency and severity or attacks by approximately 30%. Mitoxantrone (Novantrone), a chemotherapy agent that suppresses immune function, was FDA approved in 2000 for the treatment of worsening RRMS and SPMS.

Research

Research in neuroscience and the development of new research techniques and technologies offer hope to the millions of patients and families affected by neurological diseases and disorders. Research into new treatments does not solve the problem, therefore, scientists are researching areas such as neurogenetics to discover how to prevent certain disorders. New treatment options and the possibility of cures can bring a sense of optimism to patients. However, the research process takes years before a treatment is available to the public. Only controlled clinical trials with human participants can determine if a treatment is safe and effective for patients.

KEY TERMS

Antigen— A substance that stimulates the immune system to produce antibodies.

Autoimmune— The immune system sees self as a foreign antigen and attacks a part of the body.

Cerebrospinal fluid— Fluid surrounding and cushioning the brain and spinal cord.

Cognitive— Information processing functions carried out by the brain that include comprehension, memory, attention, planning, problem-solving, etc.

Dura mater— The outermost layer of meninges.

Exacerbation— The appearance of new symptoms or the aggravation of old symptoms.

Gene— Basic units of heredity in every cell that tell the cell how to put together different proteins.

Interferon— A naturally occuring immune system protein.

Lumbar puncture— A diagnostic procedure that uses a hollow needle to penetrate the spinal canal between the vertebrae to remove cerebrospinal fluid for analysis.

Meninges— Three layers of protective connective tissue covering the brain and spinal consisting of the pia mater, arachnoid, and dura mater.

Myelin— A protective coating made of fat and protein that insulates the nerve and enhances efficient nerve fiber conduction.

Neurotransmitter— A chemical that transmits a nerve impulse at the synapse.

Resources

BOOKS

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision: DSM-IV-TR. Washington, DC: American Psychiatric Association, 2000.

Burks, J. S., and K. P. Johnson. Multiple Sclerosis: Diagnosis, Medical Management and Rehabilitation. New York: Demos, 2000.

Caplan, L.R. Caplan's Stroke: A Clinical Approach. Boston: Butterworth-Heinenmann, 2000.

PERIODICALS

"Recommendations for the Establishment of Stroke Centers: A Consensus Statement from the Brain Attack Coalition." Journal of American Medical Association (June 21, 2000): 3102-3109.

Stroke Council of the American Heart Association. "Primary Prevention of Ischemic Stroke." Circulation (Jan. 2, 2001): 163-182.

ORGANIZATIONS

Alzheimer's Association. 919 N. Michigan Ave., Ste. 1100, Chicago, IL 60611-1676. (800) 272-3900. 〈http://www.alz.org〉.

Alzheimer's Disease Education and Referral Center. PO Box 8250. Silver Spring, MD 20907-8250. (800) 438-4380. 〈http://www.alzheimers.org〉.

American Academy of Neurology. 1080 Montreal Ave., St. Paul, MN 55116. (651) 695-1940.

American Neurological Association. 5841 Cedar Lake Rd., Ste. 204, Minneapolis, MN 55416. 〈http://www.ana.org〉.

American Stroke Association. 〈http://www.strokeassociation.org/Professional/pro.html〉.

Multiple Sclerosis Association of America. 706 Haddonfield Rd. Cherry Hill, NJ 08002. (800) 532-7667. 〈http://www.msaa.com〉.

National Institute of Neurological Disorders and Stroke. National Institutes of Health. Bethesda, MD 20892. 〈http://www.ninds.nih.gov〉.

National Multiple Sclerosis Society. 733 Third Ave., 6th Fl., New York, NY 10017-3288. (800) 344-4867. 〈http://www.nmss.org〉.

Central nervous system

Definition

The central nervous system (CNS) is composed of the brain and spinal cord. The brain receives sensory information from the nerves that pass through the spinal cord, as well as other nerves such as those from sensory organs involved in sight and smell. Once received, the brain processes the sensory signals and initiates responses. The spinal cord is the principle route for the passage of sensory information to and from the brain.

Information flows to the central nervous system from the peripheral nervous system , which senses signals from the environment outside the body (sensory-somatic nervous system) and from the internal environment (autonomic nervous system). The brain's responses to incoming information flow through the spinal cord nerve network to the various effector organs and tissue regions where the target responsive action will take place.

Description

Brain

The brain is divided into three major anatomical regions, the prosencephalon (forebrain), mesencephalon (midbrain), and the rhombencephalon (hindbrain). The brain also contains a ventricular system , which consists of four ventricles (internal cavities): two lateral ventricles, a third ventricle, and a fourth ventricle. The ventricles are filled with cerebrospinal fluid and are continuous with the spinal canal. The ventricles are connected via two interventricular foramen (connecting the two lateral ventricles to the third venticle), and a cerebral aqueduct (connecting the third ventricle to the fourth ventricle).

The brain and spinal cord are covered by three layers of meninges (dura matter, arachnoid matter, and pia mater) that dip into the many folds and fissures. The meninges are three sheets or layers of connective tissue that cover all of the spinal cord and the brain. Infections of the meninges are called meningitis. Bacterial, viral, and protozoan meningitis are serious and require prompt medical attention. Between the arachnoid and the pia matter is a fluid called the cerebrospinal fluid. Bacterial infections of the cerebrospinal fluid can occur and are life-threatening.

GROSS ANATOMY OF THE BRAIN The prosencephalon is divided into the diencephalon and the telencephalon (also known as the cerebrum). The cerebrum contains the two large bilateral hemispherical cerebral cortex that are responsible for the intellectual functions and house the neural connections that integrate, personality, speech, and the interpretation of sensory data related to vision and hearing.

The midbrain, or mesencephalon region, serves as a connection between higher and lower brain functions, and contains a number of centers associated with regions that create strong drives to certain behaviors. The midbrain is involved in body movement. The so-called pleasure center is located here, which has been implicated in the development of addictive behaviors.

The rhombencephalon, consisting of the medulla oblongata, pons, and cerebellum , is an area largely devoted to lower brain functions, including autonomic functions involved in the regulation of breathing and general body coordination. The medulla oblongata is a cone-like knot of tissue that lies between the spinal cord and the pons. A median fissure (deep, convoluted fold) separates swellings (pyramids) on the surface of the medulla. The pons (also known as the metencephalon) is located on the anterior surface of the cerebellum and is continuous with the superior portion of the medulla oblongata. The pons contains large tracts of transverse fibers that serve to connect the left and right cerebral hemispheres.

The cerebellum lies superior and posterior to the pons at the back base of the head. The cerebellum consists of left and right hemispheres connected by the vermis. Specialized tracts (peduncles) of neural tissue also connect the

cerebellum with the midbrain, pons, and medulla. The surface of the cerebral hemispheres (the cortex) is highly convoluted into many folds and fissures.

The midbrain serves to connect the forebrain region to the hindbrain region. Within the midbrain a narrow aqueduct connects ventricles in the forebrain to the hindbrain. There are four distinguishable surface swellings (colliculi) on the midbrain. The midbrain also contains a highly vascularized mass of neural tissue called the red nucleus that is reddish in color (a result of the vascularization) compared to other brain structures and landmarks.

Although not visible from an exterior inspection of the brain, the diencephalon contains a dorsal thalamus (with a large posterior swelling termed the pulvinar) and a ventral hypothalamus that forms a border of the third ventricle of the brain. In this third ventral region lies a number of important structures, including the optic chiasma (the region where the ophthalmic nerves cross) and infundibulum.

Obscuring the diencephalon are the two large, well-developed, and highly convoluted cerebral hemispheres that comprise the cerebrum. The cerebrum is the largest of the regions of the brain. The corpus callosum is connected to the two large cerebral hemispheres. Within each cerebral hemisphere lies a lateral ventricle. The cerebral hemispheres run under the frontal, parietal, and occipital bones of the skull. The gray matter cortex is highly convoluted into folds (gyri) and the covering meninges dip deeply into the narrow gaps between the folds (sulci). The divisions of the superficial anatomy of the brain use the gyri and sulcias anatomical landmarks to define particular lobes of the cerebral hemispheres. As a rule, the lobes are named according to the particular bone of the skull that covers them. Accordingly, there are left and right frontal lobes, parietal lobes, an occipital lobe, and temporal lobes.

In a reversal of the pattern found within the spinal cord, the cerebral hemispheres have white matter tracts on the inside of the hemispheres and gray matter on the outside or cortex regions. Masses of gray matter that are present within the interior white matter are called basal ganglia or basal nuclei.

Spinal cord

The spinal cord is a long column of neural tissue that extends from the base of the brain, downward (inferiorly) through a canal created by the spinal vertebral foramina. The spinal cord is between 16.9 and 17.7 inches (43 and 45 centimeters) long in the average woman and man, respectively. The spinal cord usually terminates at the level of the first lumbar vertebra.

The spinal cord is enclosed and protected by the vertebra of the spinal column. There are four regions of vertebrae. Beginning at the skull and moving downward, there are the eight cervical vertebrae, 12 thoracic vertebrae, five lumbar vertebrae, five sacral vertebrae, and one set of fused coccygeal vertebra.

Along the length of the spinal cord are positioned 31 pairs of nerves. These are known as mixed spinal nerves, as they convey sensory information to the brain and response information back from the brain. Spinal nerve roots emerge from the spinal cord that lies within the spinal canal. Both dorsal and ventral roots fuse in the intervertebral foramen to create a spinal nerve.

Although there are only seven cervical vertebra, there are eight cervical nerves. Cervical nerves one through seven (C1–C7) emerge above (superior to) the corresponding cervical vertebrae. The last cervical nerve (C8) emerges below (inferior to) the last cervical vertebrae from that point downward the spinal nerves exit below the corresponding vertebrae for which they are named.

In the spinal cord of humans, the myelin-coated axons are on the surface and the axon-dendrite network is on the inside. In cross-section, the pattern of contrasting color of these regions produces an axon-dendrite shape that is reminiscent of a butterfly.

The nerves of the spinal cord correspond to the arrangement of the vertebrae. There are 31 pairs of nerves, grouped as eight cervical pairs, 12 thoracic pairs, five lumbar pairs, five sacral pairs, and one coccygeal pair. The nerves toward the top of the cord are oriented almost horizontally. Those further down are oriented on a progressively upward slanted angle toward the bottom of the cord.

Toward the bottom of the spinal cord, the spinal nerves connect with cells of the sympathetic nervous system. These cells are called pre-ganglionic and ganglionic cells. One branch of these cells is called the gray ramus communicans and the other branch is the white ramus communicans. Together they are referred to as the rami. Other rami connections lead to the pelvic area.

The bi-directional (two-way) communication network of the spinal cord allows the reflex response to occur. This type of rapid response occurs when a message from one type of nerve fiber, the sensory fiber, stimulates a muscle response directly, rather than the impulse traveling to the brain for interpretation. For example, if a hot stove burner is touched with a finger, the information travels from the finger to the spinal cord and then a response to move muscles away from the burner is sent rapidly and directly back. This response is initiated when speed is important.

Development and histology of the CNS

Both the spinal cord and the brain are made up of structures of nerve cells called neurons. The long main body extension of a neuron is called an axon. Depending on the type of nerve, the axons may be coated with a material called myelin. Both the brain and spinal cord components of the central nervous system contain bundles of cell bodies (out of which axons grow) and branched regions of nerve cells that are called dendrites. Between the axon of one cell body and the dendrite of another nerve cell is an intervening region called the synapse. In the spinal cord of humans, the myelin-coated axons are on the surface and the axon-dendrite network is on the inside. In the brain, this arrangement is reversed.

The brain begins as a swelling at the cephalic end of the neural tube that ultimately will become the spinal cord. The neural tube is continuous and contains primitive cerebrospinal fluids. Enlargements of the central cavity (neural tube lumen) in the region of the brain become the two lateral, third, and forth ventricles of the fully developed brain.

The embryonic brain is differentiated in several anatomical regions. The most cephalic region is the telencephalon. Ultimately, the telencephlon will develop the bilateral cerebral hemispheres, each containing a lateral ventricle, cortex (surface) layer of gray cells, a white matter layer, and basal nuclei. Caudal (inferior) to the telecephalon is the diencephalon that will develop the epithalamus, thalamus, and hypothalamus

Caudal to the diencephalon is the mesencephalon, the midbrain region that includes the cerebellum and pons. Within the myelencephalon region is the medulla oblongata.

Neural development inverts the gray matter and white matter relationship within the brain. The outer cortex is composed of gray matter, while the white matter (myelinated axons) lies on the interior of the developing brain.

The meninges that protect and help nourish neural tissue are formed from embryonic mesoderm that surrounds the axis established by the primitive neural tube and notochord. The cells develop many fine capillaries that supply the highly oxygen-demanding neural tissue.

Diseases and disorders of the CNS

Diseases that affect the nerves of the central nervous system include rabies, polio, and sub-acute sclerosing pan-encephalitis. Such diseases affect movement and can lead to mental incapacitation. The brain is also susceptible to disease, including toxoplasmosis and the development of empty region due to prions. Such diseases cause a wasting away of body function and mental ability. Brain damage can be so compromised as to be lethal.

Resources

BOOKS

Bear, M., et al. Neuroscience: Exploring the Brain. Baltimore: Williams & Wilkins, 1996.

Goetz, C. G., et al. Textbook of Clinical Neurology. Philadelphia: W.B. Saunders Company, 1999.

Goldman, Cecil. Textbook of Medicine, 21st ed. New York: W.B. Saunders Co., 2000.

Guyton & Hall. Textbook of Medical Physiology, 10th ed. New York: W.B. Saunders Company, 2000.

Tortora, G. J., and S. R. Grabowski. Principles of Anatomy and Physiology, 9th ed. New York: John Wiley and Sons Inc., 2000.

Brian Douglas Hoyle, PhD

Paul Arthur

Central nervous system carcinoma

Definition

A central nervous system carcinoma is a malignant tumor arising in the cells of the brain or spinal cord.

Description

The central nervous system (CNS) is comprised of the brain and spinal cord. The CNS takes its name from the crucial role it plays in maintaining physical and mental well-being (homeostasis). The brain controls and monitors the body's activity; the spinal cord conveys information to the body from the brain, and vice versa. Consequently, a tumor in the CNS disrupts motor (e.g., standing, walking, writing) and sensory (e.g., seeing, tasting, hearing) activities.

The two major components of brain tissue are neurons (nerve cells) and glial cells. About half of all malignant CNS tumor growth starts in glial cells. Long thought to be mere space-holders, glial cells have been found to be extremely important. These cells actually protect and nourish the neurons, and may also help them transmit information. There are many different types of glial carcinomas, or gliomas.

The three layers of tissue, meninges, that cover the brain and spinal cord; and the pituitary and pineal parts of the brain, are also common sites for tumor growth. About 40% of benign (noncancerous) CNS tumors occur in the meninges, and the pituitary and pineal glands.

Some cancers that originate in organs, such as the kidneys, spread (metastasize) to the brain and spinal cord. These metastases differ from CNS carcinomas, however.

Demographics

About 35, 000 cases of CNS carcinoma are diagnosed each year. The Central Brain Tumor Registry of the United States (CBTRUS) puts the incidence of CNS tumors at 12.8 per 100, 000 person-years. The rate is slightly higher in males and slightly lower in females. Over a lifetime the chance a man will be diagnosed with and die from a CNS tumor is 1 in 200 and for a woman that rate is 1 in 263.

The older a person is the more likely he or she is to be diagnosed with CNS carcinoma. According to CBTRUS, the pediatric (individuals ages 0-19 years) incidence of CNS tumors is significantly lower, or about 3.8 per 100, 000 person-years. People under the age of 20 years also have a higher survival rate. They are five times more likely to live at least five years with a CNS tumor than are people between the ages of 45 and 64 years.

In addition to the diagnoses of primary CNS tumors (those that originate in the brain and spinal cord) is the diagnoses of metastatic cancer. Metastatic cancers are those that have spread from other primary sites, such as the breast, prostate, lungs, and colon. For every person diagnosed with a primary CNS tumor, at least four other individuals will be diagnosed with cancer that has metasta-sized to the brain and spinal cord. Occasionally, the identification of a metastatic brain cancer leads a physician to discover a cancer in another organ, or the primary site .

Causes and symptoms

The cause of CNS carcinoma is unknown. Important factors might include heredity, genetic make-up, and exposure to radiation and chemicals. Head injury might lead to meningiomas (carcinoma of the meninges). Extra or missing chromosomes or other genetic abnormalities are linked to the development of some CNS tumors. In one study a group of researchers led by T. Ballard showed pilots and flight attendants are at greater risk for CNS carcinoma, perhaps because of their frequent exposure to high levels of cosmic radiation.

Many individuals display no symptoms of CNS carcinoma until the tumor has grown large enough to exert pressure on part of the CNS. Because the skull covers the brain and the vertebral column protects the spinal cord, a growing tumor soon pushes up against a barrier of bone. The bone limits the expansion of the tumor and the cancerous and adjacent parts of the CNS become distorted. The meninges then swell in response to the distortion, producing symptoms.

Symptoms include:

• headache
• muscle weakness
• exhaustion
• nausea and vomiting
• changes in vision
• seizures

When a tumor is in the spinal cord, symptoms include back pain and incontinence (inability to control defecation and urination). Paralysis on one side of the body (hemiparesis), which often indicates a stroke in an elderly person, sometimes occurs because of a brain tumor.

Diagnosis

Seizures and difficulties with walking, speech, sight, or other day-to-day activities usually cause patients with CNS carcinoma to consult a physician. The techniques a physician uses to diagnose CNS carcinoma begins with an examination and medical history. Some combination of blood tests, x ray , computed tomography (CT), and magnetic resonance imaging (MRI) is used. If a tumor is detected with a CT or MRI scan a biopsy is usually done to determine the type of tumor.

Treatment team

A CNS carcinoma requires attention from several different types of physician specialists. A neurologist, a physician specializing in the nervous system, does the initial assessment. A radiologist interprets x rays, CT and MRI images. A hematologist or oncologist evaluates the results of blood tests. A pathologist studies the tissue from a biopsy. The surgery team that removes the tumor typically includes a neurosurgeon and an orthopedic surgeon. The orthopedic surgeon takes part because it is necessary to cut through bone to reach the brain and maneuver around vertebrae to reach the spinal cord. At premier cancer centers teams of physicians work collaboratively with one person, usually an oncologist, taking the lead. Physical and occupational therapists who help with rehabilitation following treatment and surgery, and registered nurses who administer chemotherapy , are also part of the team.

Clinical staging, treatments, and prognosis

By studying tissue from the tumor and surrounding cells the oncologist determines whether the tumor is growing and, if so, how fast. There is an elaborate system for assigning grades to the tumors that depends on things such as which part of the brain was served by the glial cell(s) in which the tumor began.

A plan for treatment is based on the location, size, and rate of growth of the tumor. Surgical removal of the tumor, radiation, and chemotherapy are all used. Method of treatment depends on the type of CNS carcinoma. In some cases the treatment is strictly palliative (provides comfort) and is not expected to halt the course of the cancer. Drugs, such as steroids, are often given to reduce swelling and, correspondingly, reduce pain and other symptoms.

About three-quarters of all individuals diagnosed with CNS carcinoma die before attaining a five-year survival rate.

Alternative and complementary therapies

Relaxation techniques may help to relieve pain from swelling.

Coping with cancer treatment

Being an active participant in the treatment team, something that specialized cancer centers encourage, is one way to cope. Joining a support group also may help.

Clinical trials

The National Cancer Institute at the National Institutes of Health operates an information service that provides the most up-to-date information about clinical trials . The number is (800) 4-CANCER ([800] 422-6237).

Prevention

Limiting exposure to cosmic radiation and chemicals might lower the risk. However, experts have no specific recommendations for prevention.

Special concerns

Psychological changes as simple as mood swings and as severe as major changes in personality are possible. Sensory impairment is also possible. Advance directives , or written instructions for the care a person wants at each juncture of treatment, should be prepared and legalized as early in the therapeutic process as possible. Such directives make the patient's choices clear should he or she become unable to express them as the cancer progresses. Doing so relieves loved ones of the responsibility for making those decisions, which can become extremely difficult.

See Also Brain and central nervous system tumors

Resources

BOOKS

Schold, S. Clifford Jr. et al. Primary Tumors of the Brain and Spinal Cord. Boston: Butterworth-Heinemann, 1997.

PERIODICALS

Ballard, T. et al. "Cancer Incidence and Mortality Among Flight Personnel: A Meta-Analysis" Aviation, Space, and Environmental Medicine. 71 (March 2000): 216-24.

Black, P. M. "Brain Tumors" (part two) New England Journal of Medicine 324 (May 31, 1991): 1555-1564.

Huncharek, M. et al. "Chemotherapy Response Rates in Recurrent/Progressive Pediatric Glioma; Results of a Systematic Review" Anticancer Research 19 (July-Aug. 1999): 3569-74.

ORGANIZATIONS

American Brain Tumor Association. 2720 River Road, DesPlaines, IL 60018 (800) 886-2282 <http://www.abta.org>

The Brain Tumor Society. 124 Watertown Street, Suite 3-H, Watertown, MA 02472. (617) 924-9997 <http://www.tbts.org>

OTHER

"Year 2000 Standard Report" Central Brain Tumor Registry of the United States 28 March 2001 6 July 2001 <http://www.cbtrus.org/2000/y2kstats_report.htm>

"Facts and Statistics from the American Brain Tumor Association" CancerWise.org 28 March 2001. 6 July 2001 <http://www.cancerwise.org/archive/august/facts_figures/ff_brain.html>

Diane M. Calabrese

KEY TERMS

Biopsy

—Tissue sample taken from body for microscopic examination.

Carcinoma

—A cancer that originates in cells that developed from epithelial tissue, a tissue that forms layers and often specializes to cover and protect organs.

Computed tomography (CT)

—X rays aimed at sections of the body (by rotating equipment) and images appear as slices. Results are assembled with a computer to give a three-dimensional image.

Homeostasis

—Self-regulating mechanisms are working, body is in equilibrium, no uncontrolled cell growth.

Magnetic resonance imaging (MRI)

—Magnetic fields and radio frequency waves are used to make images of the inside of the body.

Meninges

—The three layers of tissue that cover the brain and spinal cord.

Pineal

—A very small gland in the center of the brain that is sensitive to light.

Pituitary

—A gland at the base of the brain that produces hormones.

QUESTIONS TO ASK THE DOCTOR

• Which type of CNS carcinoma do I have?
• With this type of carcinoma, what is the five-year survival rate for a person of my age and gender?
• What is the one-year survival rate?
• Is there a center that specializes in treating this type of cancer?
• Are there any clinical trials in which I might be eligible to participate?
• Does this health care institution have a support group for individuals with my type of carcinoma?
• What is your approach to relieving pain? (Do we agree?)

central nervous system The central nervous system (CNS) consists of the brain (inside the skull) and spinal cord (inside the vertebral column), which derive from a single, continuous tube of neural tissue that forms at an early stage in the embryo. The head end of the tube develops into the cerebral hemispheres, the cerebellum, and the brain stem. The lowest part of the brain stem, the medulla oblongata, merges with the spinal cord at the large hole (foramen magnum) in the base of the skull. The spinal cord in an adult human is about 45 cm long, tapering to a cone at its lower end. It has two swellings, the cervical and lumbar enlargements, which are due to accumulations of nerve cells (neurons) responsible for innervating the upper and lower limbs.

Both the brain and the spinal cord are ensheathed by three protective meninges (from the Greek for ‘membranes’). The outer one, the dura mater (‘dura’ because it is relatively hard and strong; ‘mater’ because it protects like a mother) lines the skull and the tunnel that runs through the centre of the vertebrae. The delicate, innermost pia mater (‘pia’ is Latin for soft) envelops the brain and the spinal cord closely down to the level of the upper lumbar vertebrae. Between the pia and dura is a space, particularly voluminous below where the cord itself ends. The dura is lined by the arachnoid mater, which is more fragile than the dura, being likened to a spider's web, the Greek origin of its name. So the space between the pia and dura, which contains cerebrospinal fluid (CSF), is called the subarachnoid space.

Twelve pairs of cranial nerves are attached to the brain, at various levels. Some, such as the olfactory and optic nerves, are purely sensory. Others, such as those supplying the muscles that move the eyeball, are motor. The tenth cranial nerve, called the ‘vagus’ (Latin for wanderer), carries sensory information from some of the viscera and also contains the outflow of parasympathetic fibres (part of the autonomic nervous system) that innervate the heart, the bronchial tree, the smooth muscle of much of the gut, and various glands.

Evolutionarily, the central nervous system is derived from the repetitive, segmented chain of nerve cells found in invertebrates, and this segmental pattern is still clear in the human spinal cord, and even in the lower parts of the brain. It is most evident in the spinal nerves, 31 pairs in all, that sprout from the sides of the spinal cord. Strictly, these spinal nerves are part of the peripheral nervous system, but their organization is best understood in relation to the cord itself. For each vertebra, on each side of the cord is a dorsal root (‘dorsal’ means on the top), containing sensory nerve fibres from the periphery of the body, which are destined to end in the cord or the base of the brain. The neuron cell bodies for these fibres are in swellings on the dorsal roots (dorsal root ganglia). There is a corresponding pair of ventral roots (‘ventral’ literally meaning on the side nearer the stomach), containing axons from motor neurons in the cord, on their way to skeletal muscles. Additional fibres leave the ventral roots in the middle levels of the cord to innervate smooth muscle (e.g. that of blood vessel walls and the gut), glands, and the heart. These are the sympathetic fibres and are also part of the autonomic nervous system. Just outside each vertebra, the dorsal and ventral roots unite to form a spinal nerve on each side.

If you cut a spinal cord transversely, you can see, even with the naked eye, a central, butterfly-shaped core of darker material. If such a section is examined with a microscope it becomes clear that this core consists of grey matter (grey because of a concentration of cell bodies), surrounded by columns of white matter (white because it consists largely of nerve fibres — axons), running up and down the cord. The dorsal part of the grey matter receives fibres of the dorsal root, relaying information about touch, temperature, pain, and also position sense. The ventral part of the grey matter contains motor neurons that send out their axons in the ventral root to reach the skeletal muscles.

Imagine what happens as nervous impulses arrive at the cord through fibres of the dorsal root. This sensory information is of several types. Firstly, it is either somatic (from skin, muscles, and joints) or visceral (from the internal organs) in origin. Secondly it may give rise to conscious sensation, which presupposes that the information is transmitted from the spinal cord to higher brain centres, and ultimately the cerebral cortex. Alternatively it remains unconscious, in which case it may be handled by brain centres such as the cerebellum, or it may simply feed a pathway within the spinal cord, ultimately resulting in signals passing out to cause muscle reactions (a spinal reflex).

Fibres concerned with touch, temperature, and pain end on nerve cell bodies in the dorsal grey matter, which in turn send axons across to the other side of the cord and then up to the brain. Many of them reach the thalamus, projecting thence to, amongst other regions, the somatic sensory cortex — a strip at the front edge of the parietal lobe of the cerebral hemispheres. Fibres that convey conscious position sense and fine discriminative touch also enter the cord by dorsal roots, but they behave differently in that they immediately turn upward on the same side of the cord and run all the way up through a tract of white matter called the dorsal columns, merely sending branches into the dorsal grey matter of the spinal cord along the way. These fibres end on groups of cells in the medulla of the brain stem, whose axons cross to the other side and run up to the thalamus, from where axons run up to the somatic sensory cortex.

The basic function of the central nervous system is to generate appropriate reactions to sensory signals, from inside or outside the body. The simplest form of such a reaction is a ‘reflex’ — an involuntary response to a sensory stimulus. The circuit of nerve cells and axons responsible is called a reflex arc. The simplest form of reflex arc involves an incoming fibre, which traverses the dorsal grey matter of the spinal cord to terminate at a synapse on a motor neuron in the ventral grey matter, whose axon runs out to a muscle. Since this circuit contains only one synaptic connection, it is called a monosynaptic reflex. The best known example is the ‘tendon jerk reflex’: when a muscle is suddenly stretched it reflexly contracts, to oppose the stretching. For instance, when the tendon just below the knee is tapped, stretching the thigh muscles to which this tendon is attached, the same muscles contract, causing the leg to kick. Such tendon jerks are tested as part of a routine neurological examination, to assess the state of synaptic connections. This very simple type of reflex arc is relatively rare, most reflexes being complex or multisynaptic. This implies that the circuit between incoming sensory fibre and motoneuron includes other nerve cells. As these may innervate several levels of the cord, or even cross to the other side, these reflexes can be much more sophisticated than simple ones. For example, burning the tip of a finger may result in reflex withdrawal of the whole upper limb.

Some reflexes, although involuntary, almost certainly involve connections running through the cerebral cortex, or through the cerebellum, which is particularly involved in the learning and execution of motor skills, especially highly automated ones whose operation does not intrude into consciousness.

As well as major ‘ascending’ pathways carrying sensory information up to the corresponding regions of the cerebral cortex, the white matter of the brain stem and spinal cord also contains many tracts of fibres running downwards. The largest of these is the corticospinal, or pyramidal, tract, which originates in large neurons in motor areas of the cerebral cortex, and descends to the lower brain stem, where most of its axons cross over and enter the spinal cord to end, without interruption, on motoneurons in the grey matter.

This brief account leaves the impression that the central nervous system is little more than a set of cables running up and down, with something akin to a telephone switchboard in between. In reality, the human central nervous system is a monstrous biological computing instrument (although many would contest the analogy with a conventional computer), which is somehow capable of capturing the meaning of events in the outside world, representing them in memories and as conscious experiences, and making decisions that go far beyond automatic reactions to immediate events.

Laurence Garey