Complex animals have evolved two separate systems for the control of body tissues. One is the nervous system, which makes direct connections with specific muscles and glands and regulates their activity by the focal release of neurotransmitters. The other system is the endocrine system, where hormones, secreted into the circulation, can exert effects on remote tissues in many different locations simultaneously. The classical distinction between the two systems is, however, blurred. Some hormones, such as antidiuretic hormone and oxytocin, are released into the bloodstream by neurones, rather than by typical endocrine cells. In other situations, hormones are released only to act locally, not all over the body, as with paracrine cells. Occasionally, the hormone feeds back on to the cell that secreted it, as in autocrine regulation. The interface between neural and endocrine control lies in the hypothalamus and related areas of the brain. This region also helps integrate the output of the autonomic nervous system, which controls visceral function. Hypothalamic areas also regulate appetite behaviours for food, water, sex, etc. Autonomic nervous system, appetites, and hormones all contribute to homeostasis — the regulation of the internal environment of the body. The hypothalamus and the pituitary gland form the ‘hypothalamic–pituitary endocrine axis’. This axis regulates much of the body’s endocrine activity through a system of hypothalamic factors. These factors, which are hormones in their own right, regulate the release of individual pituitary hormones. Each pituitary ‘trophic’ hormone then controls a part of the overall endocrine system. Thus, pituitary hormones control hormone production by thyroid, adrenal cortex, liver, and gonads. This complex cascade of hormonal control is regulated by various types of negative feedback based on plasma hormone concentrations. The hypothalamus and pituitary are also controlled by higher centres in the brain. Other endocrine tissues also use negative feedback control, but rather than the level of the hormone itself, it is the level of stimulus that regulates hormone secretion. Thus, rising plasma osmolarity (or decreasing blood volume) stimulates antidiuretic hormone secretion, and rising plasma glucose stimulates insulin secretion. Combinations of hormones are sometimes used to regulate an aspect of the internal environment. The control of plasma calcium by calcitonin, parathormone, and calcitriol (1,25-dihydroxycholecalciferol), and of plasma glucose by insulin and glucagon, are examples.
Central Nervous System Sites of Action of an Enkephalin Analogue, (D-Met2, Pro5)-Enkephalinamide, and Naloxone on the Secretion of Five Anterior Pituitary Hormones of Male Rats
