Hypothalamic–anterior pituitary axes

Acting via the anterior lobe of the pituitary gland the hypothalamus is the hub of five neuroendocrine axes which regulate aspects of reproduction, metabolism, development, and growth. Neurons in numerous hypothalamic nuclei send their axons to the external zone of the median eminence and the tuberoinfundibular tract. Such axons secrete hypophysiotropic hormones into the hypothalamic–pituitary portal system that carries them into the anterior lobe. Each hypophysiotropic hormone act on a specific population of cells in the anterior lobe, either exciting or restraining their secretion of a particular stimulating hormone. Hypophysiotropic hormones which excite secretion are known as releasing hormones, those which inhibit are known as release-inhibiting hormones.

Trophic hormones of the anterior pituitary are secreted into the systemic circulation and have endocrine effects on target tissues, mainly endocrine glands as shown in table below. Secretion from the neuroendocrine axes is modulated by negative feedback, defined as a method which acts to hold few variables at a set point. In endocrinology the set point is normally the blood concentration of a hormone and the negative feedback operates at numerous levels of the neuroendocrine axis.

The figure below, when the concentration of the end product hormone surpasses the set point, more receptors are activated in the hypothalamus and anterior pituitary that accordingly decrease their output of hormones. The effect is that, after little delay, the concentration of end product hormone falls. When it falls below the set point the hypothalamus and pituitary secrete more of their hypophysiotropic and trophic hormones, annoying a raise in concentration of the end product hormone. Autofeedback inhibition is a extraordinary case of negative feedback in which a substance directly restrains its own synthesis. Numerous methods exist to modify the set points of physiological systems therefore hormone concentrations can be differing as situational change. Therefore, secretion of hypothalamic hormones is pulsatile with a period of 60–180 min. This drives pulsatile discharge of anterior pituitary hormones. The amplitude and period of the pulses differs on a circadian (around a day) basis and in few illustrations on longer time scales.