Understanding the Hypothalamus

The following information is taken from a paper written by Laurence Garey, a well known neuroscientist and anatomist, for “World Of The Body” The Oxford Companion to the Body. Copyright © 2001, 2003 by Oxford University Press. It contains introductory information about the important functions of the hypothalamus. There is much more detailed and technical information available on the internet, however, as a parent or patient trying to understand the complex nature of a hypothalamic hamartoma, we recommend you start here and then refer to the other articles and papers listed at the end of this section.

The Hypothalamus by Laurence Garey:

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

Additional recommended articles/sources:

For an excellent but more technical article on the hypothalamus: www.scholarpedia.org by Dr Clifford Saper (2009) Hypothalamus. Scholarpedia, 4(1):2791