: hypothalami; from Ancient Greek ὑπό (hupó) 'under' and θάλαμος (thálamos) 'chamber') is a small part of the vertebrate brain that contains a number of nuclei with a variety of functions.

[citation needed] The hypothalamic nuclei include the following:[11][12] The hypothalamus is highly interconnected with other parts of the central nervous system, in particular the brainstem and its reticular formation.

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.

Also, the pattern of secretion of growth hormone is sexually dimorphic;[20] this is why in many species, adult males are visibly distinct sizes from females.

[citation needed] Estrogen and progesterone can influence gene expression in particular neurons or induce changes in cell membrane potential and kinase activation, leading to diverse non-genomic cellular functions.

In general, ERs and progesterone receptors (PRs) are gene activators, with increased mRNA and subsequent protein synthesis following hormone exposure.

[23] Sex steroids are not the only important influences upon hypothalamic development; in particular, pre-pubertal stress in early life (of rats) determines the capacity of the adult hypothalamus to respond to an acute stressor.

The hypothalamus has a central neuroendocrine function, most notably by its control of the anterior pituitary, which in turn regulates various endocrine glands and organs.

Delta wave signalling arising either in the thalamus or in the cortex influences the secretion of releasing hormones; GHRH and prolactin are stimulated whilst TRH is inhibited.

For instance, if a pregnant mouse is exposed to the urine of a 'strange' male during a critical period after coitus then the pregnancy fails (the Bruce effect).

The hypothalamus is bounded in part by specialized brain regions that lack an effective blood–brain barrier; the capillary endothelium at these sites is fenestrated to allow free passage of even large proteins and other molecules.

They also contain neurons with receptors for angiotensin, atrial natriuretic factor, endothelin and relaxin, each of which important in the regulation of fluid and electrolyte balance.

The circumventricular organs may also be the site of action of interleukins to elicit both fever and ACTH secretion, via effects on paraventricular neurons.

In the case of prolactin and leptin, there is evidence of active uptake at the choroid plexus from the blood into the cerebrospinal fluid (CSF).

[citation needed] Findings have suggested that thyroid hormone (T4) is taken up by the hypothalamic glial cells in the infundibular nucleus/ median eminence, and that it is here converted into T3 by the type 2 deiodinase (D2).

All fevers result from a raised setting in the hypothalamus; elevated body temperatures due to any other cause are classified as hyperthermia.

In the sheep, cervical stimulation in the presence of high levels of estrogen can induce maternal behavior in a virgin ewe.

In addition, hypothalamic function is responsive to—and regulated by—levels of all three classical monoamine neurotransmitters, noradrenaline, dopamine, and serotonin (5-hydroxytryptamine), in those tracts from which it receives innervation.

For example, noradrenergic inputs arising from the locus coeruleus have important regulatory effects upon corticotropin-releasing hormone (CRH) levels.

[42][43] The PMD does not modulate defensive behavior in other situations, as lesions of this nucleus had minimal effects on post-shock freezing scores.

Fos-labeled cell analysis showed that the PMDvl is the most activated structure in the hypothalamus, and inactivation with muscimol prior to exposure to the context abolishes the defensive behavior.

Likewise, the hypothalamus has a role in social defeat: nuclei in medial zone are also mobilized during an encounter with an aggressive conspecific.

[6] Recent research has questioned whether the lateral hypothalamus's role is only restricted to initiating and stopping innate behaviors and argued it learns about food-related cues.

According this view, the lateral hypothalamus is "a unique arbitrator of learning capable of shifting behavior toward or away from important events".