Hypophyseal portal system

A further totally separate DA pathway arises from A12 in the arcuate nucleus and forms the tuberoinfundibular tract in the median eminence to the pituitary gland for controlling prolactin release. This is partly achieved by DA being released into capillaries of the hypothalamic-hypophyseal portal system and then inhibiting the prolactin releasing cells (lactotrophs) of the anterior pituitary. 

Arterial blood reaches the pituitary gland via the superior hypophyseal artery, a branch of the internal carotid artery. Venous blood is supplied through a venous portal system that originates in the median eminence of the hypothalamus and ends in sinusoidal capillaries of the pituitary gland. This venous system is known as the hypothalamic-hypophyseal portal system. This system carries neurosecretory hormones from the hypothalamus to the adenohypophysis. These hypothalamic factors stimulate or inhibit the release of hormones from the adenohypophysis. Retrograde flow from the adenohypophysis to the median eminence of the hypothalamus is also believed to occur. With upstream flow, pituitary hormones can reach the hypothalamus and influence hypothalamic function through a short feedback loop. 

Corticotropin-releasing hormone (CRH) is a 41-amino-acid polypeptide produced in the parvicellular neurons of the paraventricular nucleus and released into the hypophyseal portal system in the median eminence. Antidiuretic hormone (ADH, AVP) is also synthesized and released by the same neurons, but in humans, unlike the situation in rats, the amount of ADH released at the median eminence is too small to influence the function of the... 

The manner in which hormones are delivered to the target cells and the presence of specific receptors on the target cells determines the selectivity of hormonal activity. Special delivery systems, like the hypophyseal-portal system which links the hypothalamus to the pituitary, ensure the delivery of sufficient amounts of hormones to the target tissues. Beyond the delivery system, the concentration of specific receptors on the target tissue remains the primary determinant of the sensitivity of the tissue to a hormone [2]. 

The anterior hypothalamus secretes a neurohormone (TRH), that is transported from the brain center to the hypophysis by way of the hypophyseal-portal system. 

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