HYPOTHALAMIC-HYPOPHYSEAL CIRCULATION.
The hypothalamus is located at the base of brain just above the junction of optic nerves. The blood supply of anterior pituitary originates in the capillaries that richly lace the median eminence area of the hypothalamus. The superior hypophyseal arteries form a dense network of capillaries within the median eminence, which then drain into the portal vessels that descent along the pituitary stalk to the anterior pituitary. The direction of the blood flow is from brain to the pituitary and thus hypothalamus controls the anterior pituitary gland by means of neuro hormones released into the portal circulation. There is also retrograde flow so that pituitary hormones can be directly delivered to the hypothalamus thus having feedback upon the hypothalamus. An additional blood supply is provided by short vessels, which originates in the posterior pituitary that in turn receives its arterial supply from the inferior hypophyseal arteries. .
Neuro endocrine agents from the hypothalamus have positive stimulatory effects on gonadotrophs, thyroid stimulating hormone (TSH), adrenocorticotropic hormone, (ACTH)and represent the individual neuro hormones of the hypothalamus. The neuro hormone that controls gonadotropins is called gonadotropin-releasing hormone (GnRH). The neuro hormone that controls Prolactin is called Prolactin inhibiting hormone and is probably dopamine. The human corticotropin-releasing hormone (CRH) besides being the regulator of ACTH secretion also activates sympathetic nervous system. CRH suppress gonadotropin secretion, an action mediated by endorphin inhibition of GnRH. .
The neural input to gonadotrophs is in the form of the decapeptide gonadotropin-releasing hormone (GnRH), which is secreted in a burst like manner by specialized neurons within the hypothalamus. GnRH stimulates both FSH and LH secretion. The frequency with which pulses of GnRH are released (and possibly the amount of GnRH per neuronal discharge) appears to be regulated by a number of hypothalamic neurotransmitters, which are in turn influenced by multiple factors, including ambient concentration of the gonadal hormones.
