The Endocrine System - Introduction

Tutorial 35: The Endocrine System

Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test

The Endocrine System is composed of glands that secrete chemicals called hormones into the bloodstream for the control of body functions. Several hormones usually act as messenger links in a multiple-stage, multiple-gland process of controlling a particular aspect of physiology. In general, the following scenario describes this multi-stage process. The brain exerts executive control over the endocrine system via the hypothalamus, one of the limbic system structures. Substances released by the hypothalamus trigger the production and secretion of hormones by the pituitary gland. The pituitary hormones circulate in the blood stream to distant glands. In response to the action of the pituitary hormones, these distant glands produce and secrete other hormones that result in specific physiological changes. These changes include general effects on metabolism, reproductive development, adult reproductive functions, and responses to environmental stress. The interaction of the nervous system and endocrine system is most evident in the sympathetic response of the autonomic nervous system. The hypothalamus responds to stress through two separate pathways, via the pituitary gland and the autonomic nervous system. Each pathway stimulates the adrenal glands to secrete substances that prepare the entire body for an emergency. In general, the response of the nervous system to environmental changes via the production and release of neurotransmitters is quicker than the response of the endocrine system to changes via hormonal production and secretion. The changes affected by the endocrine system and hormones are in general more long-term and widespread, although there is some overlap in between the systems in this regard.

Figure 35 illustrates the location of the major endocrine glands, identifies the hormones they secrete, and describes the basic functions of these hormones.

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Most hormones fall into one of three major classes: protein hormones, peptide hormones, and steroid hormones. Peptide hormones are short chain proteins. Protein hormones consist of longer chains of amino acids. Protein and peptide hormones attach themselves to receptor sites on cell membranes where they activate a second messenger such as cyclic AMP. The second messenger then activates enzymes that affect metabolism of the cell or the ability of other substances to cross the cell membrane. These effects may last from minutes to hours. Steroid hormones have a distinct molecular structure of four carbon rings. Steroids enter the cell and attach themselves to cytoplasmic receptors that in turn transport them to the nucleus. Once at the nucleus, the steroid hormones determine which genes will be expressed and, therefore, the protein to be produced. Steroid hormones can also affect the short-term movement of particles across the cell membrane.

There are several classes of steroid hormones, estrogens, androgens, and corticosteroids. Estrogen levels are generally higher in females and androgens higher in males. Genes that are routinely activated by the sex hormones are called "sex-limited" genes. The distribution of estrogen and androgen receptors in the brain is unique. Each, therefore, modifies a different set of neurons and behaviors, underlying some of the gender differences that have a biological basis.

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Tutorial 35: The Endocrine System

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