Figure 109

Inputs that act directly on endocrine-gland cells to stimulate or inhibit hormone secretion.

during the early period of sleep and is quite low or absent during the rest of the day and night. The mechanisms underlying these cycles are ultimately traceable to cyclical variations in the activity of neural pathways involved in the hormone's release.

Hormone secretion is controlled mainly by three types of inputs to endocrine cells (Figure 10-9): (1) changes in the plasma concentrations of mineral ions or organic nutrients; (2) neurotransmitters released from neurons impinging on the endocrine cell; and (3) another hormone (or, in some cases, a paracrine/au-tocrine agent) acting on the endocrine cell. There is actually a fourth type of input—chemical and physical factors in the lumen of the gastrointestinal tract—but it applies only to the hormones secreted by the gastrointestinal tract and will be described in Chapter 17.

Before we look more closely at each category, it must be stressed that, in many cases, hormone secretion is influenced by more than one input. For example, insulin secretion is controlled by the extracellular concentrations of glucose and other nutrients, by both sympathetic and parasympathetic neurons to the insulin-secreting endocrine cells, and by several hormones acting on these cells. Thus, endocrine cells, like neurons, may be subject to multiple, simultaneous, often opposing inputs, and the resulting output—the rate of hormone secretion—reflects the integration of all these inputs.

One more point should be made to avoid misunderstanding. The term "secretion" applied to a hormone denotes its synthesis and release from the cell. Some inputs to endocrine cells specifically stimulate or inhibit only synthesis, for example, by altering the expression of the gene for that hormone, with changes in release occurring as a secondary result. In contrast, other inputs directly influence only the actual release of the hormone from the cell, and some inputs influence both synthesis and release. For

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

10. Principles of Hormonal Control Systems

© The McGraw-Hill Companies, 2001

PART TWO Biological Control Systems simplicity in this chapter and the rest of the book, we will generally not distinguish between these possibilities when we refer to stimulation or inhibition of hormone "secretion."

Control by Plasma Concentrations of Mineral Ions or Organic Nutrients

There are multiple hormones whose secretion is directly controlled, at least in part, by the plasma concentrations of specific mineral ions or organic nutrients. In each case, a major function of the hormone is to regulate, in a negative-feedback manner, the plasma concentration of the ion or nutrient controlling its secretion. For example, insulin secretion is stimulated by an elevated plasma glucose concentration, and the additional insulin then causes, by several actions, the plasma glucose concentration to decrease (Figure 10-10).

Control by Neurons

The adrenal medulla behaves like a sympathetic ganglion and thus is stimulated by sympathetic pregan-glionic fibers. In addition to its control of the adrenal medulla, the autonomic nervous system has influences on other endocrine glands (Figure 10-11b). Both parasympathetic and sympathetic inputs to these other glands may occur, some inhibitory and some stimulatory. Examples are the secretion of insulin and the gastrointestinal hormones.

t Plasma glucose concentration

I

Insulin-secreting cells t Insulin secretion

Insulin-secreting cells t Insulin secretion

f Plasma insulin concentration

I

Insulin's target cells t Actions of insulin

Insulin's target cells t Actions of insulin

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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  • Harriet Wallace
    How are plasma concentrations of specific hormones controlled?
    6 years ago

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