The hypothalamus is located below the thalamus, just above the brainstem. In the terminology of neuroanatomy, it forms the ventral part of the diencephalon. All vertebrate brains contain a hypothalamus. In humans, it is roughly the size of an almond.

They contain a number of small nuclei.

Anterior Hypothalamus

Tuberal Hypothalamus

Posterior Hypothalamus

One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland.

The hypothalamus is responsible for certain metabolic processes and other activities of the autonomic nervous system. It synthesizes and secretes certain neurohormones, often called releasing hormones or hypothalamic hormones, and these in turn stimulate or inhibit the secretion of pituitary hormones. The hypothalamus controls body temperature, hunger, important aspects of parenting and attachment behaviors, thirst, fatigue, sleep, and circadian rhythms.

The hypothalamus is a brain structure made up of distinct nuclei as well as less anatomically distinct areas. It is found in all vertebrate nervous systems. In mammals, the axons of magnocellular neurosecretory cells in the paraventricular nucleus and the supraoptic nucleus (both located in the hypothalamus) contain oxytocin and vasopressin (antidiuretic hormone), and project into the posterior pituitary.

Much smaller parvocellular neurosecretory cells, neurons of the paraventricular nucleus, release corticotropin-releasing hormone and other hormones into the hypophyseal portal system, where these hormones diffuse to the anterior pituitary.

Neural connections

The hypothalamus is highly interconnected with other parts of the central nervous system, in particular the brainstem and its reticular formation. As part of the limbic system, it has connections to other limbic structures including the amygdala and septum, and is also connected with areas of the autonomous nervous system.

The hypothalamus receives many inputs from the brainstem, the most notable from the nucleus of the solitary tract, the locus coeruleus, and the ventrolateral medulla.

Most nerve fibres within 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 terminal stria.

Projections to areas of the sympathetic motor system (lateral horn spinal segments T1-L2/L3) are carried by the hypothalamospinal tract and they activate the sympathetic motor pathway.

In 2008, Hamani et al reported an unexpected effect of hypothalamic stimulation in a 50-year-old man undergoing treatment for refractory obesity. When stimulation was initiated intraoperatively at the most ventral contact, the patient reported experiencing a sudden sense of déjà vu, recalling a scene from 20 years earlier that became increasingly vivid as stimulation intensity increased. When electrode coordinates were plotted into stereotactic space, the ventral-most contacts were found to be in close association with the fornix 1).

Hamani C, McAndrews MP, Cohn M, et al.. Memory enhancement induced by hypothalamic/fornix deep brain stimulation. Ann Neurol. 2008;63(1):119–123.
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