Most vital endocrine functions in humans are directly influenced by a relatively small brain region—the hypothalamus (roughly 0.003% of the entire brain mass). The neurosecretory cells that regulate the garden pea-sized pituitary (master) gland at the base of the brain are located in the middle third of the hypothalamus. The important neuroendocrine cell groups in this region consist of the para-ventricular and arcuate nuclei. Different groups of cells in these nuclei are responsible for the direct release of some hormones in the bloodstream. These hypothalamic influences are mediated by direct axonal projections through the infundibular stalk to the posterior lobe of the pituitary. They control functions such as water balance through the release of antidiuretic hormone (vasopressin), and control uterine contraction and milk-production/ejection in pregnant and lactating women, respectively, through the release of the hormone oxytocin.
However, the majority of tropic hormones are synthesized and released by the anterior lobe of the pituitary, through the secretion of special hypothalamic peptides termed releasing factors. Upon their secretion, these releasing hormones enter capillaries at the level of the median eminence that coalesce to form portal vessels that run through the infundibular stalk and terminate in vascular sinuses in the anterior lobe of the pituitary. The range of functions associated with anterior pituitary hormones includes maturation and growth, immunity, stress respon-sivity reproduction, energy, and metabolism, broadly defined. Thus, although the pituitary gland is responsible for the release of many hormones acting on body tissues and end-organs, it is admirably enslaved by the hypothalamus, and by negative feedback effects of the released hormones. Negative feedback inhibition is an important regulatory mechanism whereby hormone release acts at several levels, including the pituitary, hypothalamus, and even brain areas that project to the hypothalamus, to reduce its own further release and thus help reduce deviations from optimal set points that are detrimental to organisms. The hypothalamus and the brain circuits associated with its activity play a critical role in endocrine functions, and, in turn, en docrine status has a significant impact on brain processes subserving affect and cognition.
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