Hypothalamus

The hypothalamus is a tiny part of the brain of vertebrate animals; in humans it weighs about four grams in a brain that weighs on average 1,400 grams (49 ounces). Despite its small size, the hypothalamus plays a pivotal role in an astounding number of functional and behavioral activities that are essential for day-to-day survival of the individual animal (or person) and for continuing survival of its species. Its overall role is to collect and integrate a huge variety of information from the body and to organize neural and endocrine responses that maintain homeostasis (constant internal environment).

Carrying out this single overriding task requires coordinating the activity of the autonomic nervous system and the endocrine system, and ultimately influences several important behaviors. Thus energy metabolism is regulated by control of feeding, drinking, and digestion. Body temperature is monitored and maintained at a constant level (37 to 38°C [98.6 to 100.4°F] in humans) by a complex interplay of behavior and activity in several body systems, and reproductive behavior is coordinated with endocrine regulation of the reproductive organs. Blood pressure and composition of the blood plasma are regulated by hypothalamic mechanisms. The expression of emotions such as fear, rage, and anger are partly controlled by the hypothalamus, and it even helps regulate sleep and levels of consciousness.

Location/Anatomy

The hypothalamus is a thin (3 to 4 millimeters [.118 to .157 inches] in thickness) plate of neural tissue found along either side of the front end of the third ventricle (one of the fluid-filled cavities inside the brain). Deeply buried in the brain, near the center of the cranial cavity, it lies just below the thalamus (a relay center for sensory and motor pathways in the brain). It is almost completely hidden by the overlying cerebral hemisphere, although when a brain is removed for study, the hypothalamus is visible on the basal surface.

The hypothalamus has a special structural and functional relationship with the pituitary gland, which dangles below it, attached by a thin stalk of nerve fibers. Important information passes along both the nerve fibers and the blood vessels of this stalk.

Working Principles

About ten or eleven small, indistinct nuclei (nerve cell groups) are packed into the hypothalamus. Reflecting their complex and highly specialized functions, the cells here use several unusual means of cell-to-cell communication.

Some hypothalamic cells are specialized to detect the presence and the concentration of large molecules such as hormones circulating in the blood and tissue fluids. They are able to do this because even the capillaries here are specialized. Unlike other brain vessels, they permit large molecules like hormones to leak into the tissues and carry signals to the neurons .

Hypothalamic neurons also receive information from other body and brain areas by way of electrical impulses conducted from many sensory sources (signaling pain, vision, and blood pressure, for example) scattered through the body. Other hypothalamic neurons respond by changing their firing pattern when there are changes in the desired values of variables such as blood (body) temperature, glucose concentration, or salt concentrations in the body fluids.

When the hypothalamus, using signals like those just described, establishes a need for response, hypothalamic cells influence other cells in two ways. Like other neurons, they send electrical signals (action potentials) to stimulate or inhibit cells in other regions of the brain and body. In addition, some release chemicals (hormones), usually small proteins called peptides, into the bloodstream so they can act on target cells at a considerable distance.

Localized Hypothalamic Functions

Two of the most prominent hypothalamic nuclei (because their neurons are large) are the paraventricular nucleus and supraoptic nucleus. Upon appropriate stimulation, cells in these nuclei secrete (release) two hormones into the bloodstream. Oxytocin causes uterine contraction during birth and induces milk release in females with young. Antidiuretic hormone (ADH) travels to the kidneys to help the body retain water by decreasing urinary output.

Several other hypothalamic nuclei, mostly located in the anterior area, respond to several different hormones circulating in the body. When hormone levels change, cells in these nuclei release peptide signaling molecules into a special system of blood vessels that carry them to the anterior lobe of the pituitary. These peptides cause pituitary cells to either increase or decrease the secretion of one of about eight specific hormones into the bloodstream. This basic mechanism regulates blood levels of growth hormone, adrenocorticotropic hormone (for response to stress), thyrotropin (regulating basal metabolism), and the several hormones that regulate the reproductive organs and sexual behavior.

Also in the anterior hypothalamus, the tiny suprachiasmatic nuclei sit atop the optic chiasm. A few optic nerve fibers from the eyes end here, informing these cells about cycles of light and darkness. Through their expansive projections to other brain areas, especially the pineal organ, these cells evoke release of the hormone melatonin into the bloodstream and thus help to regulate the body's circadian rhythms. Circadian rhythms are the cyclic, often subtle, fluctuations in many body functions that reoccur at intervals of about twenty-four hours.

Cells in the anterior and posterior hypothalamic areas detect blood temperature and have connections that allow them to adjust abnormal body temperature. Neural activity in the anterior area activates systems for heat loss, dilating blood vessels of the skin and causing sweating and panting. Neurons in the posterior hypothalamus help to preserve heat by constricting blood vessels of the skin, causing shivering and slowed breathing. Still other hypothalamic nuclei work together to balance food intake. Activity in the lateral hypothalamic area encourages eating while the ventromedial nucleus (VMN) suppresses food intake. Damage to the VMN results in animals (and humans) that overeat to excess and become obese.

In the preoptic area at the front end of the hypothalamus are cells that use several of the hormonal mechanisms already described to drive and regulate the menstrual cycles and other aspects of reproductive organ function and behavior. Finally, a range of behaviors characterized as rage or aggression represent physiological responses to stress; these can be seen following experimental stimulation of the dorsomedial nucleus of animals. Blood pressure and heart rate are elevated, muscles are tensed, the animals show signs of strong internal, emotional feeling.

see also Brain; Central Nervous System; Endocrine System; Female Reproductive System; Homeostasis; Hormones; Male Reproductive System; Pituitary Gland; Temperature Regulation; Thyroid Gland

James L. Culberson

Bibliography

Beardsley, Tim. "Waking Up." Scientific American 275, no. 1 (2000): 14–16.

Delcomyn, Fred. Foundations of Neurobiology. New York: W. H. Freeman and Company, 1998.

Goldstein, Irwin. "Male Sexual Circuitry." Scientific American 282, no. 8 (2000): 70–75.

Kandel, Eric R., James H. Schwartz, and Thomas M. Jessell, eds. Essentials of Neural Science and Behavior. Norwalk, CT: Appleton and Lange, 1995.

Moore, Robert Y. "A Clock for the Ages." Science 284, no. 5423 (1999): 2102–2103.

hypothalamus Although small, this is one of the most important parts of the grey matter of the brain, for it participates in a number of vital activities. It regulates a variety of hormonal functions by action on the pituitary gland, and it exerts magisterial control over the blood vessels and glands of the body via the autonomic nervous system. It is an integral part of the limbic system, which influences important aspects of our behaviour and even our very survival, regulating such functions as emotion, sexual and nutritional appetites, rhythms, and sleep cycles. Some cells of the hypothalamus detect changes in body temperature and chemistry, and participate directly in the control of our temperature and chemical balance.

The hypothalamus, as its name implies, is situated below the thalamus — a huge collection of nuclei in the centre of the cerebral hemispheres. It forms part of the walls and floor of the central chamber of the cerebral ventricles, called the third ventricle. Hanging on a stalk underneath the hypothalamus is the pituitary gland.

The hypothalamus receives many important sensory inputs, which include information from all the major senses, but especially from the taste and smell receptors and from the viscera. It consists of a number of distinct nerve cell clusters or nuclei. The tiny suprachiasmatic nucleus receives axons directly from the optic nerve, carrying information from the eye, which is used to regulate sleep and other bodily rhythms. This nucleus controls a sympathetic pathway to the pineal gland, which plays its part in the ‘biological clock’ by secreting melatonin in amounts that vary with the time of day. This in turn affects a variety of body processes.

Our internal body clock plays a large part in determining our cycles of sleeping and waking. The connection from the eyes to the suprachiasmatic nucleus is thought to reset the clock each day and hence to keep it locked to the periodicity of the world. If the clock could not be altered (albeit with some difficulty and delay) it would be impossible to adapt to night work or to overcome ‘jet lag’, which afflicts us when we fly to other time zones. Visual input to the hypothalamus also seems to play a part in determining mood. The continuous absence of natural light during the winter months at extreme latitudes can precipitate depression. This condition, which is called Seasonal Affective Disorder, can sometimes be reversed simply by exposing the sufferer to a high-intensity, full-spectrum light for a period of time each day.

Parts of the thalamus, and the frontal lobes of the cerebral cortex that are important in controlling mood, also connect to the hypothalamus. Disturbances in these pathways are thought to result in abnormal affective (emotional) behaviour; some of the symptoms of schizophrenia may be related to this system. Axons of neurons in the hippocampus (a specialized part of the cerebral cortex involved in conscious memory) run in a tract called the fornix, which ends on neurons in the mammillary bodies of the hypothalamus. They then send axons to the thalamus. This circuit, crucially important for linking emotions to events in the outside world, is part of the limbic system.

Many nerve cells in the hypothalamus have a so-called ‘neuroendocrine’ function — instead of producing transmitter substances that simply communicate directly with other neurons, they secrete chemicals that act as hormones, circulating in the blood and affecting other parts of the body. In the front part of the hypothalamus lie the supraoptic and paraventricular nuclei, which send axons down through the stalk of the pituitary gland and into its posterior lobe, called the ‘neurohypophysis’. These nerve fibres end in large swellings that release into the bloodstream the hormones oxytocin (which causes contraction of smooth muscle in the uterus and breast) and vasopression or antidiuretic hormone (which makes blood vessels constrict and controls the salt balance of the body by reducing the loss of water in the urine). The disease diabetes insipidus, in which there is excessive production of urine, is due to damage to the vasopressin system.

Other neuroendocrine parts of the hypothalamus secrete specialized hormones, called ‘releasing factors’, into the blood of small capillary vessels (called the hypophysial portal system), which run down into the anterior lobe of the pituitary gland, where they stimulate specialized cells to secrete other hormones that pass into the general circulation and affect remote organs. These include growth hormone (which regulates growth), prolactin (which controls milk production in the breast), and follicle stimulating hormone (which acts on the ovaries). Two of the hormones of the anterior pituitary act on yet other endocrine glands: adrenocorticotrophic hormone stimulates the adrenal gland and thyrotrophin the thyroid. In these cases, the ‘cascade’ of chemicals (releasing factor, to anterior pituitary hormone, to target endocrine gland) amplifies the effect of the initial signal in the hypothalamus.

The great Oxford neurophysiologist Sir Charles Sherrington called the hypothalamus the ‘head ganglion of the autonomic nervous system’. Anterior parts of the hypothalamus excite parasympathetic functions such as constriction of the pupils of the eye, stimulation of the gastrointestinal tract, salivation, and respiratory and cardiac depression. The posterior hypothalamus brings on sympathetic activity, such as dilatation of the pupils, inhibition of gastrointestinal function and salivation, and increased respiration, heart rate, and blood pressure. These effects are produced by fibres projecting from the hypothalamus to parasympathetic nuclei in the brain stem, and to sympathetic centres in the spinal cord.

Laurence Garey

Hypothalamus

A section of the forebrain, connected to other parts of the forebrain and midbrain, that is involved in many complex behaviors.

The hypothalamus, which together with the thalamus makes up the section of the forebrain called the diencephalon, is involved in such aspects of behavior as motivation , emotion , eating, drinking, and sexuality . Lying under the thalamus, the hypothalamus weighs only a fraction of an ounce and is a little larger than the tip of the thumb. It is connected to the autonomic nervous system , and controls the entire endocrine system using the pituitary gland to direct the work of all the other endocrine glands . If a particular section of the hypothalamus is destroyed, an overwhelming urge to eat results; damage to another section of a male's hypothalamus can reduce the sex drive. Yet another part of the hypothalamus, the suprachiasmatic nuclei (SCN), is the site of a person's "internal clock" that regulates biological rhythms according to a cycle of roughly 24 hours. From the SCN, signals reach areas of the hindbrain that regulate sleep and wakefulness. With neurons firing on a 24-or 25-hour cycle, it determines the periods of greatest alertness—whether one is "morning person" or a "night person." Pathways from the SCN to the eyes connect its circadian rhythms to external cycles of light and dark.

Different roles have been identified for various sections of the hypothalamus in interpreting and acting on hunger signals. The ventromedial nucleus, whose neurons detect blood levels of glucose, signals when it is time to stop eating. Rats in whom this part of the hypothalamus has been destroyed will eat extremely large quantities of food, enough to triple their body weight. Similarly, the lateral hypothalamus signals when it is time to begin eating. Yet another area, the paraventricular nucleus, appears to motivate the desire for particular types of foods, depending on which neurotransmitters are acting on it at a particular time.

See also Brain

hypothalamus Part of the vertebrate brain that is derived from the forebrain and located on the ventral surface below the thalamus and the cerebrum. The hypothalamus regulates a wide variety of physiological processes, including maintenance of body temperature, water balance, sleeping, and feeding, via both the autonomic nervous system (which it controls) and the neuroendocrine system. Its endocrine functions are largely mediated by the pituitary gland. The pituitary responds to releasing hormones produced by the hypothalamus, which thereby indirectly controls hormone production in other glands.

hy·po·thal·a·mus / ˌhīpəˈ[unvoicedth]aləməs/ • n. (pl. -mi / -ˌmī/ ) Anat. a region of the forebrain below the thalamus that coordinates both the autonomic nervous system and the activity of the pituitary, controlling body temperature, thirst, hunger, and other homeostatic systems, and involved in sleep and emotional activity.DERIVATIVES: hy·po·tha·lam·ic / ˌhīpōˌ[unvoicedth]əˈlamik/ adj.