Hypothalamus
The hypothalamus lies on either
side of the third ventricle, below the
thalamus and between the optic chiasm and the midbrain. It receives a
significant input from limbic system structures (see Chapter 45) as well as
from the retina. It contains a large number of neurones that are sensitive to
changes in hormone levels, electrolytes and temperature.
It has an efferent output to the
autonomic nervous system (ANS), as well as a critical role in the control of
pituitary endocrine function (a detailed discussion on the endocrinology of the
hypothalamic–pituitary system is beyond the scope of this book). Thus, the
hypothalamus, while being important in the control of the ANS, has a much
greater role in the homoeostasis of many physiological systems (e.g. thirst,
hunger, sodium and water balance, temperature regulation), the control of
circadian and endocrine functions, the ability to form anterograde memories (in
conjunction with the limbic system; see
Chapters 45 and 46) and the
translation of the response to emotional stimuli into endocrinological and
autonomic responses.
The hypothalamus performs a number
of other functions, all of which can be lost or deranged in the disease state.
The most common cause is as a side-effect of surgical removal of pituitary
tumours.
Functions of the hypothalamus
•
The
hypothalamus controls the ANS and damage to it can cause autonomic instability.
The ventromedial part of the hypothalamus has a major role in
controlling the sympathetic nervous system while the lateral hypothalamic
area controls the parasympathetic nervous system (see Chapter 3).
•
It
controls the endocrine functions of the pituitary by the production of
releasing and inhibiting hormones as well as producing antidiuretic hormone
(ADH, also known as vasopressin) and oxytocin. Hypothalamic damage can have
profound systemic effects because of the endocrinological disturbances
associated with it, of which perhaps the most common example is neurogenic diabetes
insipidus, in which there is a loss of the production of ADH from the
hypothalamus. In this condition the patient passes many litres of urine each
day, which needs to be compensated for by increased fluid intake. This is to be
distinguished from nephrogenic diabetes insipidus, where the problem lies
within the ADH receptor in the kidney.
•
It has a
major role in coordinating autonomic and endocrinological responses, both under
physiological conditions, and in the expression of emotional states as coded
for by the limbic system. In cases of hypovolaemia or extreme anxiety, for
example, the hypothalamus mediates not only increased sympathetic activity, but
also enhanced cortisol production via the stimulated release of
adrenocorticotrophic hormone (ACTH) from the anterior pituitary. This is termed
the stress response, which is defined by the rise in cortisol.
•
It has an
important role in thermoregulation. Lesions to the anterior hypothalamic
area cause hyperthermia, while stimulation of this same area lowers body
temperature via the ANS, in contrast to the posterior hypothalamic area,
which behaves in an opposite fashion. It may also mediate some of the more
long-term responses seen with prolonged changes in ambient temperature, such as
increased thyrotrophin-releasing hormone (TRH) production in patients exposed
to a chronically cold environment. Damage to the hypothalamus can lead to
profound changes in the central control of temperature. In septic states, the
production of some cytokines (e.g.
interleukin-1) may reset the thermostat in the hypothalamus to a higher than
normal temperature, accounting for the paradoxical situation of a fever with
physiological evidence of mechanisms designed to conserve or generate heat
(e.g. shivering).
•
It has a
role in the control of feeding. In simple terms, the ventromedial hypothalamus
is often called the satiety centre, in that damage to it causes excessive eating (hyperphagia) and weight gain, while damage to the lateral hypothalamic
(or hunger) area produces aphagia (no eating at all). The control of these
centres involves a number of hormones, including insulin and the more recently
described leptins.
•
It has a
role in the control of thirst and water balance by virtue of its osmoreceptors;
the afferent input from a host of peripheral sensory receptors (e.g. atrial
stretch receptors in the heart, arterial baroreceptors); the activation of
hypothalamic hormone receptors (e.g.
angiotensin II receptors); and its efferent output via the ANS to the heart and
kidney as well as the production of ADH.
•
It has a
role in the control of circadian rhythms via the retinal input to the suprachiasmatic
nucleus. This nucleus appears to be critical in setting the circadian
rhythm as lesion and transplant experiments have shown. Although the exact
mechanism by which these rhythms are mediated is not known, it may involve the production
of melatonin by the pineal gland.
•
It has a
role with the limbic system in memory. Damage to the mammillary bodies,
which receive a significant input from the hippocampal complex as occurs in
chronic alcoholism with thiamine deficiency, produces a profound amnesia (Korsakoff’s
syndrome) of both an anterograde (inability to lay down new memories)
and retrograde (inability to recover old memories) nature. The latter feature
distinguishes these patients from those who have hippoc- ampal damage (see
Chapters 45 and 46) and may explain why patients with Korsakoff’s syndrome tend
to invent missing information
(confabulation).
•
The
hypothalamus may also have a role in sexual and emotional behaviour independent of its endocrinological
influences.
