Hypothalamus



In the anatomy of mammals, the hypothalamus is a region of the brain located below the thalamus, forming the major portion of the ventral region of the diencephalon and functioning to regulate certain metabolic processes and other autonomic activities. The hypothalamus links the nervous system to the endocrine system by synthesizing and secreting neurohormones often called releasing hormones because they function by stimulating the secretion of hormones from the anterior pituitary gland &mdash; among them, gonadotropin-releasing hormone (GnRH). The neurons that secrete GnRH are linked to the limbic system, which is very involved in the control of emotions and sexual activity. The hypothalamus is also the area of the brain that controls body temperature, hunger and thirst, and circadian cycles.

The hypothalamus connects to the pituitary gland via the tuberoinfundibular pathway.

The anatomic boundaries of the hypothalamus are: rostral, the lamina terminalis; caudal, the posterior margin of the mamillary bodies; dorsal, the hypothalamic sulcus; medial, the third ventricle; lateral, the subthalamus and internal capsule; and ventral, the optic chiasm, tuber cinereum, mammillary bodies, and posterior pituitary.

Hypothalamic nuclei

 * See also: ventrolateral preoptic nucleus

Projections
Most fiber systems of the hypothalamus run in two ways (bidirectional). Projections to areas caudal to the hypothalamus go through the medial forebrain bundle, the mammillotegmental tract and the dorsal longitudinal fasciculus. Projections to areas rostral to the hypothalamus are carried by the mammillothalamic tract, the fornix and stria terminalis.

There are two exceptions on this bidirectional rule: Projections to the pituitary gland are one-way only (from the hypothalamus to the pituitary), and the hypothalamus receives connections from the retina.

Sexual dimorphism
Research has shown that at least one nucleus in the hypothalamus is sexually dimorphic - the medial preoptic area or nucleus, which is, in general, bigger in men than in women. This dimorphism may help explain why men are more easily sexually aroused by visual stimuli than women.

External links:

 * High-Resolution Cytoarchitectural Primate Brain Atlases
 * The Hypothalamus and Pituitary at endotexts.org