The endocrine system consists of a group of glands that produces regulatory chemicals called hormones. The endocrine system and the nervous system work together to control and coordinate all other systems of the body. The nervous system controls such rapid actions as muscle movement and intestinal activity by means of electrical and chemical stimuli. The effects of the endocrine system occur more slowly and over a longer period. They involve chemical stimuli only, and these chemical messengers have widespread effects on the body. Although the nervous and endocrine systems differ in some respects, the two systems are closely related. For example, the activity of the pituitary gland, which in turn regulates other glands, is controlled by the brain’s hypothalamus. The connections between the nervous system and the endocrine system enable endocrine function to adjust to the demands of a changing environment.
Hormones are chemical messengers that have specific regulatory effects on certain cells or organs. Hormones from the endocrine glands are released directly into the bloodstream, which carries them to all parts of the body. They regulate growth, metabolism, reproduction, and behavior. Some hormones affect many tissues, for example, growth hormone, thyroid hormone, and insulin. Others affect only specific tissues. For example, one pituitary hormone, thyroid-stimulating hormone (TSH), acts only on the thyroid gland; another, adrenocorticotropic hormone (ACTH), stimulates only the outer portion of the adrenal gland. The specific tissue acted on by each hormone is the target tissue. The cells that make up these tissues have receptors in the plasma membrane or within the cytoplasm to which the hormone attaches. Once a hormone binds to a receptor on or in a target cell, it affects cell activities, regulating the manufacture of proteins, changing the permeability of the membrane, or affecting metabolic reactions.
Chemically, hormones fall into two main categories:
* Amino acid compounds. These hormones are proteins or related compounds also made of amino acids. All hormones except those of the adrenal cortex and the sex glands fall into this category.
* Steroids. These hormones are types of lipids derived from the steroid cholesterol. Steroid hormones are produced by the adrenal cortex and the sex glands. Steroid hormones can be recognized by the ending -one, as in progesterone, testosterone.
The amount of each hormone that is secreted is normally kept within a specific range. Negative feedback, is the method most commonly used to regulate these levels.
In negative feedback, the hormone itself (or the result of its action) controls further hormone secretion. Each endocrine gland tends to oversecrete its hormone, exerting more effect on the target tissue. When the target tissue becomes too active, there is a negative effect on the endocrine gland, which then decreases its secretory action. We can use as an example the secretion of thyroid hormones (Fig. 8-1). As described in more detail later, a pituitary hormone, called thyroid-stimulating hormone (TSH), triggers secretion of hormones from the thyroid gland located in the neck. As blood levels of these hormones rise under the effects of TSH, they act as negative feedback messengers to inhibit TSH release from the pituitary. With less TSH, the thyroid releases less hormone and blood levels drop. When hormone levels fall below the normal range, the pituitary can again begin to release TSH. This is a typical example of the kind of selfregulating system that keeps hormone levels within a set normal range. Less commonly, some hormones are produced in response to positive feedback. In this case, response to a hormone promotes further hormone release. Examples are the action of oxytocin during labor, as described in Chapter 1, and the release of some hormones in the menstrual cycle. The release of hormones may fall into a rhythmic pattern. Hormones of the adrenal cortex follow a 24-hour cycle related to a person’s sleeping pattern, with the level of secretion greatest just before arising and least at bedtime. Hormones of the female menstrual cycle follow a monthly pattern.
Figure 8-1 Negative feedback control of thyroid hormones. The anterior pituitary releases thyroid stimulating hormone (TSH) when the blood level of thyroid hormones is low. A high level of thyroid hormones inhibits release of TSH and thyroid hormone levels fall.