Study guidelines
- 1.
Hypothalamic neuroendocrine cells fulfil the basic criteria both for neurons and for endocrine cells. Small neuroendocrine cells control release of hormones by the purely endocrine cells of the anterior pituitary gland. Large ones have their terminals in the posterior pituitary, where they release hormones directly.
- 2.
Some neurons confined to the hypothalamus are involved in control of body temperature, food and fluid intake, and sleep. Others, involved in attack and defence responses, and memory are controlled by the limbic system.
The hypothalamus develops as part of the limbic system, which is concerned with preservation of the individual and of the species. Therefore it is logical that the hypothalamus should have significant controls over basic survival strategies, including reproduction, growth and metabolism, food and fluid intake, attack and defence, temperature control, the sleep–wake cycle, and aspects of memory.
Most of its functions are expressed through its control of the pituitary gland and of both divisions of the autonomic nervous system.
Gross anatomy
The hypothalamus occupies the side walls and floor of the third ventricle. It is a bilateral, paired structure. Despite its small size—it weighs only 4 g—it has major functions in homeostasis and survival. Its homeostatic functions include control of body temperature and circulation of blood. Its survival functions include regulation of food and water intake, the sleep–wake cycle, sexual behaviour patterns, and defence mechanisms against attack.
Boundaries
The boundaries of the hypothalamus are as follows ( Figures 26.1 and 26.2 ):
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Superior : the hypothalamic sulcus separating it from the thalamus.
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Inferior : the optic chiasm , tuber cinereum , and mammillary bodies . The tuber cinereum shows a small swelling, the median eminence , immediately behind the infundibulum (‘funnel’) atop the pituitary stalk.
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Anterior : the lamina terminalis.
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Posterior : the tegmentum of the midbrain.
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Medial : the third ventricle.
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Lateral : the internal capsule.
Subdivisions and nuclei
In the sagittal plane, it is customary to divide the hypothalamus into three regions: anterior (supraoptic), middle (tuberal), and posterior (mammillary). These areas are small even in large mammals, and the descriptive use of ‘regions’ has been convenient for animal experiments involving placement of lesions and often serves us well in the clinical setting with humans. Named nuclei in the three regions are listed in Table 26.1 .
| Posterior | Middle | Anterior |
|---|---|---|
| Posterior | Paraventricular | Preoptic |
| Mammillary | Dorsomedial | Supraoptic |
| Tuberomammillary | Lateral | Suprachiasmatic |
| Dorsal | Ventromedial Arcuate |
In the coronal plane the hypothalamus can be divided into lateral , medial , and periventricular regions. The full length of the lateral region is occupied by the lateral hypothalamic nucleus . Merging with the lateral nucleus is the medial forebrain bundle , carrying aminergic fibres to the hypothalamus and to the cerebral cortex.
Functions
Hypothalamic control of the pituitary gland
The arterial supply of the pituitary gland comes from hypophyseal branches of the internal carotid artery ( Figure 26.3 ). One set of branches supplies a capillary bed in the wall of the infundibulum. These capillaries drain into portal vessels , which pass into the adenohypophysis (anterior lobe). There they break up to form a second capillary bed, which bathes the endocrine cells and drains into the cavernous sinus.
The neurohypophysis receives a direct supply from the inferior hypophyseal arteries. The capillaries drain into the cavernous sinus, which delivers the secretions of the anterior and posterior lobes into the general circulation.
Secretions of the pituitary gland are controlled by two sets of neuroendocrine cells . Neuroendocrine cells are true neurons in having dendrites and axons and in conducting nerve impulses. They are also true endocrine cells because they liberate their secretions into capillary beds ( Figure 26.4 ). With one exception (mentioned below), the secretions are peptides, synthesised in clumps of granular endoplasmic reticulum and packaged in Golgi complexes. The peptides are attached to long-chain polypeptides called neurophysins . The capillaries concerned are outside the blood–brain barrier and are fenestrated.
The somas of the neuroendocrine cells occupy the hypophysiotropic area in the lower half of the preoptic and tuberal regions. Contributory nuclei include the preoptic , supraoptic , paraventricular , ventromedial , and arcuate (infundibular). Two classes of neurons can be identified: parvocellular (small) neurons reaching the median eminence and magnocellular (large) neurons reaching the posterior lobe of the pituitary gland.
The parvocellular neuroendocrine system
Parvocellular neurons of the hypophysiotropic area give rise to the tuberoinfundibular tract , which reaches the infundibular capillary bed. Action potentials travelling along these neurons result in calcium-dependent exocytosis of releasing hormones from some and inhibiting hormones from others, for transport to the adenohypophysis via the portal vessels. The cell types of the adenohypophysis are stimulated/inhibited in accordance with Table 26.2 . In the left-hand column, the only nonpeptide parvocellular hormone is the prolactin-inhibiting hormone, which is dopamine , secreted from the arcuate (infundibular) nucleus.
| RH/IH | Anterior Lobe Hormone |
|---|---|
| Corticotropin RH | ACTH |
| Thyrotropin RH | Thyrotropin |
| Growth hormone RH | Growth hormone |
| Growth hormone IH (Somatostatin) | Growth hormone |
| Prolactin RH | Prolactin |
| Prolactin IH (Dopamine) | Prolactin |
| Gonadotropic hormone RH | FSH/LH |
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