The hypothalamus is a neuroendocrine organ that regulates physiological processes for survival such as consumption of fluid and food, temperature control, the sleep–wake cycle, growth, and reproduction. As a result of this vast array of functions, the nuclei of the hypothalamus make connections with several other neural and endocrine structures in the body (eFig. 9.86). In this section we will explore the nuclei of the hypothalamus and their main connections.
Located in the ventralmost aspect of the diencephalon, the hypothalamus is bounded by the lamina terminalis anteriorly, the hypothalamic sulcus superiorly, and the tegmentum of the midbrain posteriorly (eFig. 9.87). Laterally the hypothalamus is bordered by the substantia innominata rostrally and the posterior limb of the internal capsule caudally. It forms the floor in addition to the lower portion of the lateral walls in the third ventricle. When viewing the external surface of the ventral brain, the area containing the hypothalamus is circumscribed by the optic chiasm, optic tract, crus cerebri, and caudal edge of the mammillary bodies (eFig. 9.88).
Continuous with the hypothalamus inferiorly is the pituitary gland. These two structures are connected by the infundibulum and hypophysial stalk just caudal to the optic chiasm (eFig. 9.87). The intimate relationship of the hypothalamus to the portal circulation of the pituitary gland allows the hypothalamus to be an efficient regulator of hormone synthesis and release. Releasing and inhibiting factors synthesized by the hypothalamus pass through these portal vessels via the tuberoinfundibular tract (eFig. 9.89) to reach the anterior pituitary (adenohypophysis) and control release of hormones produced by the anterior pituitary such as adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, thyroid-stimulating hormone, growth hormone, and prolactin (eFig. 9.90).
A second connection exists between the hypothalamus and pituitary through nerve fibers, which originate in the supraoptic region and paraventricular nuclei of the medial hypothalamus and extend through the hypothalamohypophyseal tract to the posterior lobe of the pituitary for release into the circulatory system (eFig. 9.89).
Internally, the hypothalamus is composed of many small nuclei, which are divided by a parasagittal plane into a medial and lateral zone. Landmarks for this dividing point are the columns of the fornix and the mammillothalamic tract as they reside within this sagittal plane (eFig. 9.91). The hypothalamus is also divided by coronal planes into a preoptic area and periventricular zone. Note that the periventricular zone is not to be confused with the paraventricular nucleus, which is a thin region of cell bodies lying medial to the medial zone.
Within the lateral zone of the hypothalamus is a large bundle of axons forming the medial forebrain bundle (MFB) (eFig. 9.90). Theses fibers interconnect the hypothalamus with the septal nuclei rostrally and nuclear complexes within the brainstem. Axons arising from the large lateral nucleus in this zone enter the MFB and function in promoting feeding behavior. The other and smaller nuclear group in this zone is the tuberal group. Axons from the tuberal nuclei make connections with the anterior pituitary through the tuberoinfundibular tract to regulate release of hormones in the hypophysial portal system and to the cerebellum to regulate motor activity (eFig. 9.89).