The hypothalamus is the part of the diencephalon associated with visceral, autonomic, endocrine, affective, and emotional behaviour. It lies in the walls of the third ventricle, separated from the thalamus by the hypothalamic sulcus. The rostral boundary of the hypothalamus is roughly defined as a line through the optic chiasm, lamina terminalis, and anterior commissure, and an imaginary line extending from the posterior commissure to the caudal limit of the mamillary body represents the caudal boundary. Externally, the hypothalamus is bounded rostrally by the optic chiasm, laterally by the optic tract, and posteriorly by the mamillary bodies. Dorsolaterally, the hypothalamus extends to the medial edge of the internal capsule (Fig. 2.1.1) (1).
The complicated anatomy of this area of the central nervous system (CNS) is the reason why, for a long time, little was known about its anatomical organization and functional significance. Even though the anatomy of the hypothalamus is well established it does not form a well-circumscribed region. On the contrary, it is continuous with the surrounding parts of the CNS: rostrally, with the septal area of the telencephalon and anterior perforating substance; anterolaterally with the substantia innominata; and caudally with the central grey matter and the tegmentum of the mesencephalon. The ventral portion of the hypothalamus and the third ventricular recess form the infundibulum, which represents the most proximal part of the neurohypophysis. A bulging region posterior to the infundibulum is the tuber cinereum, and the zone that forms the floor of the third ventricle is called the median eminence. The median eminence represents the final point of convergence of pathways from the CNS on the peripheral endocrine system and it is supplied by primary capillaries of the hypophyseal portal vessels. The median eminence is the anatomical interface between the brain and the anterior pituitary. Ependymal cells lining the floor of the third ventricle have processes that traverse the width of the median eminence and terminate near the portal perivascular space; these cells, called tanycytes, provide a structural and functional link between the cerebrospinal fluid (CSF) and the perivascular space of the pituitary portal vessels.
The conspicuous landmarks of the ventral surface of the brain can be used to divide the hypothalamus into three parts: anterior (preoptic and supraoptic regions), middle (tuberal region), and caudal (mamillary region). Each half of the hypothalamus is also divided into a medial and lateral zone. The medial zone contains the so-called cell-rich areas with well-defined nuclei. The scattered cells of the lateral hypothalamic area have long overlapping dendrites, similar to the cells of the reticular formation. Some of these neurons send axons directly to the cerebral cortex and others project down into the brainstem and spinal cord.
