The pituitary gland has often been called “the master gland” because so many important functions are governed by the hormones it secretes. In this chapter we will see that the pituitary hormones are under the control of both the central nervous system, especially the hypothalamus and the secretions of the distal endocrine glands and other internal signals. The hypo thalamic releasing hormones function to control the secretions of the anterior pituitary hormones, either positively or negatively. The hormones secreted by the distal endocrine glands exert, in general, negative feed back effects in the brain and in the anterior pituitary.
The hormonal cascade systems involving the central nervous system, hypothalamus, anterior pituitary, and distal endocrine glands are outlined in Figure 1. There are many signals originating either outside or inside the body that are mediated by the central nervous system. Thus, changes in the environment can ultimately stimulate the secretion of hypothalamic releasing hormones, which, through their effects on pituitary hormonal secretion allow the body to adapt to the change. Likewise, signals from within the organ ism can trigger this cascade system so that secretion of hormones from distant target glands can be affected.
Fig1. Overview of hypothalamic-pituitary-peripheral systems. An archetypal cascade of hormonal events is shown, beginning with an external or internal environmental signal. These signals (green boxes) are transmitted to the central nervous system and then propagated to the hypothalamus, the anterior pituitary (adenohypophysis), and the peripheral target gland (pink boxes) for generating the appropriate biological response. The order of magnitude of the amount of hormone released in each of the three final steps is given in parentheses. The increasing amounts of hormones secreted at each step provide for amplification and the cascade is kept in check by both short (e.g., anterior pituitary to hypothalamus) and long (peripheral gland to hypothalamus) negative feedback loops.
Note that proceeding down the cascade shown in Figure 1 from the central nervous system to the peripheral hormone, there is an increasing amount (mass) of the hormone released at each stage: the hypo thalamic releasing hormones are secreted in nanogram amounts, the anterior pituitary hormones in microgram amounts, and the terminal hormone in microgram to milligram amounts. In addition, the stability of these hormones increases from a half-life of 3–10 minutes for the releasing hormones to minutes to hours for the anterior pituitary hormones and hours to several days for the distal hormones. These biochemical features of the hormones at different points of the cascade indicate a high degree of amplification stemming from a unique event, the initiating signal from either the external or internal environment. The cascade systems are kept in check by negative feedback loops, long ones from the distal target gland to the hypothalamus, anterior pituitary, or central nervous system and shorter ones from, for example, the pituitary to the hypothalamus.
