At the core of any well-oiled machine, there is a busy engine that keeps it going. The hypothalamus is at the center of a plethora of physiological processes including, but not limited to, thermoregulation, osmoregulation and hormonal regulation. It also works closely with the pituitary gland to modulate endocrine activity based on the body’s physiological demands. The purpose of this article will be to evaluate the gross anatomical organization and vascular supply of the hypothalamus. Additionally, the article will look at key anatomical relationships to other structures, some associated neuronal pathways and the function of different nuclei of the gland.
The left and right lateral walls of the third ventricle are divided by an anteroposterior depression known as the hypothalamic sulcus. It runs from the anterior tip of the fornix – between the interventricular foramen of Monro superiorly and the anterior commissure inferiorly – to the posterior commissure (superior to the beginning of the cerebral aqueduct of Sylvius). This depression separates the thalamus (superiorly) from the hypothalamus (inferiorly).
The hypothalamus is composed mainly of different nuclei (discrete masses of grey matter in the central nervous system) that synthesize different hormones in response to physiological changes. These nuclei have been grouped into four regions. The preoptic region (at the level of the subcallosal gyrus) houses the preoptic nucleus. In the supraoptic region, there is the suprachiasmatic, supraoptic, paraventricular, and anterior nuclei. The dorsomedial, ventromedial, arcuate, premammillary and lateral tuberal nuclei belong to the tuberal region. Finally, the mammillary and posterior nuclei are residents of the mammillary region.
The first of several major hypothalamic nuclei to be discussed is the paraventricular nuclei. It is located anterior to the pathway of the fornix (arching C-shaped collection of nerve fibers) and posteroinferior to the anterior commissure. The superior part of the lamina terminalis is directly anterior to it, while the supraoptic and dorsomedial nuclei are at inferiorly and posteriorly related, respectively.
The supraoptic nucleus is directly posterior to the inferior part of the lamina terminalis and anteroinferior to the dorsomedial and ventromedial nuclei. It also rests directly above the supraoptic crest, also known as the organum vasculosum laminae terminalis (OVLT), just above the supraoptic recess and the optic chiasm.
The arcuate (infundibular) nucleus rests along the proximal part of the tuber cinereum. It is inferior to the ventromedial, anterior to the mammillary, and posteroinferior to the supraoptic nuclei. The dorsomedial and ventromedial nuclei are found around the mid-medial section of the hypothalamus. The former nucleus is located inferiorly to the latter nucleus.
The posterior nucleus is directly superior to the mammillary nucleus, which is found in the mammillary bodies. The posterior nucleus also lies between the fornical nerve fibers (anteriorly) and the mammillothalamic tract (posteriorly).
The hypothalamus has an almost geometrically central location in the brain. As a result, it is structurally related to several other important parts of the forebrain and midbrain.
- Immediately anteriorly, there is the anterior commissure and the lamina terminalis. The subcallosal area with its gyrus is located anteriorly, just beyond the anterior commissure and lamina terminalis.
- Posteriorly, the mammillary bodies, posterior perforated substance, cerebral peduncle and the cerebral aqueduct of Sylvius can be found.
- Inferiorly (from anterior to posterior), there is the supraoptic recess and crest, the pituitary stalk and gland, and the tuber cinereum.
- Superiorly there is the hypothalamic sulcus, the thalamus, the choroid plexus of the third ventricle and the fornix.
Bilateral to the optic chiasm, and inferior to the anterior perforating substance, the left and right cavernous internal carotid arteries each give an anterior cerebral artery and a posterior communicating cerebral artery. The left and right anterior cerebral arteries anastomose along the ventral part of the longitudinal cerebral fissure (near the lamina terminalis) by way of the anterior communicating artery. The left and right posterior communicating arteries each anastomose with the left and right posterior cerebral arteries (branches of the basilar artery) to unite the vertebral and internal carotid circulatory systems of the brain. The anterior and posterior anastomoses form a circular arterial system known as the cerebral arterial circle of Willis. Anterior and posterior branches of the circle of Willis provide arterial blood to the hypothalamus. The hypothalamus also receives arterial supply from the hypothalamic branches of the superior hypophyseal artery.
The hypothalamic arteries have anastomotic connections to the primary and secondary capillary plexuses of the pituitary gland. The plexuses drain venous blood to the cavernous sinus, which then drains to the superior and inferior petrosal sinuses. The superior petrosal sinus joins the transverse sinus to form the sigmoid sinus. The sigmoid sinus is later joined by the inferior petrosal sinus and becomes the internal jugular vein.
Unmyelinated axons of the paraventricular and supraoptic nuclei travel together as the supraopticohypophyseal tract along the anterior region of the hypothalamus caudally, towards the infundibulum (pituitary stalk). They meet up with the axons of the arcuate nucleus that travels along the tuber cinereum (tuberohypophyseal tract) and travels through the infundibular stem of the neurohypophysis to enter the pars nervosa. These two neuronal tracts form the hypothalamohypophyseal tract. It conveys neurons produced in the cell bodies in the hypothalamus to the pars nervosa, where they are stored.
Two sets of nerve bundles travel to the mammillary body. One journeys from the fornix, anterior to the posterior nucleus and the other from the thalamus, posterior to the posterior nucleus. The latter is known as the mammillothalamic tract.
The different nuclei described earlier each have specific functions. Together, they allow the very small hypothalamus to be the regulator of a wide variety of functions. The preoptic nucleus of the preoptic region modulates the secretion of gonadotropin releasing hormone, which is necessary for sexual reproduction. In the supraoptic region, the paraventricular and supraoptic nuclei regulate osmolality via the production of antidiuretic hormone, and aids in parturition by the action of oxytocin. The anterior nucleus (also in the supraoptic region) is involved with thermoregulation by the stimulation of the parasympathetic nervous system (heat loss-cooling of the body. Damage or destruction of this nucleus causes hyperthermia). The suprachiasmatic nucleus is another supraoptic nucleus that regulates the body’s circadian rhythm.
In the tuberal part of the hypothalamus, the dorsomedial and ventromedial nuclei are involved in controlling the feeding impulse. The former regulates the urge to eat while the latter regulates the sense of fullness. The lateral nucleus is also important in regulation of feeding. Its absence or destruction has been implicated in extremes of starvation such as anorexia nervosa. The arcuate nucleus of this region inhibits lactation by inhibiting the release of prolactin.
Finally, the mammillary region contains the mammillary and posterior nuclei. The posterior nucleus, like its anterior counterpart, is involved in thermoregulation. The differences are that the posterior nucleus operates by way of the sympathetic nervous system (heat conservation -heating of the body. Damage or destruction of this nucleus causes hypothermia). The mammillary nucleus is a prominent site for haemorrhagic lesions in Wernicke’s encephalopathy.