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
See also
autonomic nervous system;
body clock;
brain;
thalamus.
nervous system.
