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The hypothalamus is a small almond shaped organ in the lower part of the brain. However, this small organ has numerous connections that help it play an important role in connecting the nervous system with the endocrine system.
The hypothalamus receives several signals and instructions via nerves that help it to function and stimulate the release of hormones and regulatory chemicals from the pituitary as well as other organs. The outpourings of the hypothalamus can be divided into two categories:
There are nerve fibers that run out of the hypothalamus to various control sites. Most fiber systems of the hypothalamus run in two ways. Thus they bring in information and carry out instructions from the organ and are termed bidirectional.
Some of the projections are to areas posterior or below the hypothalamus. These pass via the medial forebrain bundle, the mammillotegmental tract and the dorsal longitudinal fasciculus. Those projections that move to the front part of the hypothalamus are carried by the mammillothalamic tract, the fornix and terminal stria.
There are projections that carry instructions to the areas of the sympathetic motor system (lateral horn spinal segments T1-L2/L3 of the). These are carried by the hypothalamospinal tract and they activate the sympathetic motor pathway.
The hypothalamus releases several hormones from its neurons. These include:
These are released into the blood in the capillaries. From there they reach the portal veins to another capillary bed in the anterior lobe of the pituitary. The releasing hormones stimulate release of hormones from the pituitary gland.
These hormones are all released in periodic spurts. In patients with deficiencies of these hormones the replacement therapy with external hormones needs to be similar spurts to achieve physiological effects. There are two other hormones Antidiuretic hormone (ADH) and Oxytocin.
|Secreted hormone||Abbreviation||Produced by||Effect|
|TRH, TRF, or PRH||Parvocellular neurosecretory neurons||Stimulate thyroid-stimulating hormone (TSH) release from anterior pituitary (primarily)
Stimulate prolactin release from anterior pituitary
|DA or PIH||Dopamine neurons of the arcuate nucleus||Inhibit prolactin release from anterior pituitary|
|Growth hormone-releasing hormone||GHRH||Neuroendocrine neurons of the Arcuate nucleus||Stimulate Growth hormone (GH) release from anterior pituitary|
(growth hormone-inhibiting hormone)
|SS, GHIH, or SRIF||Neuroendocrine cells of the Periventricular nucleus||Inhibit Growth hormone (GH) release from anterior pituitary
Inhibit thyroid-stimulating hormone (TSH) release from anterior pituitary
|Gonadotropin-releasing hormone||GnRH or LHRH||Neuroendocrine cells of the Preoptic area||Stimulate follicle-stimulating hormone (FSH) release from anterior pituitary
Stimulate luteinizing hormone (LH) release from anterior pituitary
|Corticotropin-releasing hormone||CRH or CRF||Parvocellular neurosecretory neurons||Stimulate adrenocorticotropic hormone (ACTH) release from anterior pituitary|
|Oxytocin||Magnocellular neurosecretory cells||Uterine contraction
Lactation (letdown reflex)
|ADH or AVP||Magnocellular neurosecretory neurons||Increases water permeability in the distal convoluted tubule and collecting duct of nephrons, thus promoting water reabsorption and increasing blood volume|