Hormonal production is often controlled through feedback mechanisms from the target organ. In general, feedback occurs when the response to a stimulus (action of a hormone) has an effect on the original stimulus (hormone-secreting cell). The nature of this response determines the type of feedback. Two types of feedback are recognized; a negative feedback occurs when the response diminishes the original stimulus. It is much more common than a positive feedback, which occurs when the response enhances the original stimulus.
To better understand the function of feedback mechanisms, one can point to an air-conditioning system, which also uses a simple negative feedback system. When the compressor produces enough cold air to lower the temperature below the set point of the thermostat, the thermostat is triggered and shuts off the compressor. In this negative feedback system, the lower temperature is then fed back to the compressor and diminishes its response (it shuts off its production of cold air). When the temperature rises back above the set point, the negative feedback is abolished and the compressor comes back on (for more information on negative feedback, see Folder 21.1, Functional Considerations: Regulation of Pituitary Gland Secretion).
Activities of hormones are constantly monitored on many levels beginning with molecular biosynthetic processes to the final end points of hormonal action. Several examples of feedback mechanisms are discussed in the sections on the pituitary, hypothalamus, and thyroid gland.
Hormone-secreting cells are present in many organs to regulate their activity.
This chapter primarily describes the discrete endocrine glands that release their hormones for delivery to the bloodstream for transport to target cells and organs. In other chapters, the endocrine function of adipose tissue as well as individual cells within the gonads, liver, pancreas, kidney, and gastrointestinal system is discussed. The cells of the diffuse neuroendocrine (DNES) system (see page 581) constitute a collection of endocrine cells in the body. In addition to their endocrine function, cells of DNES system exercise autocrine and paracrine control of the activity of their own and adjacent epithelial cells by diffusion of peptide secretions through the extracellular spaces.
The pituitary gland and the hypothalamus, the portion of the brain to which the pituitary gland is attached, are morphologically and functionally linked in the endocrine and neuroendocrine control of other endocrine glands. Because they play central roles in a number of regulatory feedback systems, they are often called the “master organs” of the endocrine system. In the past, the control of pituitary hormone secretion by the hypothalamus was classically regarded as the major function of the neuroendocrine system. However, the field of neuroendocrinology today has expanded to encompass multiple reciprocal interactions between the central nervous system (CNS), autonomic nervous system (ANS), endocrine system, and immune system in the regulation of homeostasis and behavioral responses to environmental stimuli. For example, the neuroendocrine axes in maintaining energy homeostasis were discussed in Chapter 9 (Adipose Tissue).
 FOLDER 21.1 Functional Considerations: Regulation of Pituitary Gland Secretion