Introduction

The pituitary gland has often been called "the master gland" because so many important functions are governed by the hormones produced there. The hypothalamic releasing hormones function to control the secretions of the pituitary hormones, mainly the anterior pituitary hormones, either positively or negatively. The releasing hormones can be thought of as the main stimuli to generate specific release of pituitary hormones, although we will learn that there are many other factors that are being found to contribute to this control. The function of the releasing hormones can be put into context by briefly considering the overall hormonal or humoral cascade system, starting with the central nervous system. This cascade is seen in overview in Figure 3-1. There are many signals originating either outside or inside the body that are mediated by the central nervous system. Thus, many changes in the environment ultimately can stimulate the secretion of releasing hormones, which produce effects in the body to adapt to the change. Likewise, signals from within the organism, such as those from internal cycling systems, also will trigger this cascade system so that end product hormones from distant target glands can be released. Thus, in Figure 3-1 it is seen that the cascade starts with signals, either from the outside environment or from within the body. These signals may be filtered and transmitted through central nervous system neurons, whose nerve endings may activate other neurons in the hypothalamus. External or internal signals are often mediated by the limbic system or some other system of the brain and may result in the generation of electrical or chemical signals to the hypothalamus. Such signals may produce depolarization of nerve endings, affecting the secretion of releasing hormones. These hormones gain access to local blood circulation through fenestrations (thin walls) in small vessels and are carried to the adenohy-pophysis (anterior pituitary) by the hypothalamic-hypophyseal portal system shown in Figure 3-2. The releasing hormone binds to a specific receptor on a particular cell membrane in the anterior pituitary. The binding sets off a chain of events, culminating in an elevation of intracellular calcium ions. A release of preformed anterior pituitary hormone follows. This

HORMONES, SECOND EDITION

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