In the previous chapters we explored important sensory and regulatory mechanisms involving the nervous system. Our study of the regulation of body function continues in this chapter, in which we discuss the regulatory function of the endocrine system. The endocrine system uses signaling molecules called hormones, which are released by glands into the bloodstream and sent throughout the body. These hormones can thus affect any or all tissues of the body. As you learn concepts of hormone function, try to see them as part of an overall system of regulating the body’s functions. In the next chapter, we continue the story of endocrine regulation by exploring specific endocrine glands and the hormones they produce.

ORGANIZATION OF THE ENDOCRINE SYSTEM

The endocrine system and nervous system both function to achieve and maintain stability of the internal environment. Each system may work alone or in concert with each other as a single neuroendocrine system, performing the same general functions within the body: communication, integration, and control.

Both the endocrine system and the nervous system perform their regulatory functions by means of chemical messengers sent to specific cells. In the nervous system, neurons secrete neurotransmitter molecules to signal nearby cells that have the appropriate receptor molecules. In the endocrine system, secreting cells send hormone (from the Greek hormaein, “to excite”) molecules by way of the bloodstream to signal specific target cells throughout the body. Tissues and organs that contain endocrine target cells are called target tissues and target organs, respectively. As with postsynaptic cells, endocrine target cells must have the appropriate receptor to be influenced by the signaling chemical—a process called signal transduction. Many cells have receptors for neurotransmitters and hormones, so they can be influenced by both types of chemicals.

Whereas neurotransmitters are sent over very short distances across a synapse, hormones diffuse into the blood to be carried to nearly every point in the body. The nervous system can directly control only muscles and glands that are innervated with efferent fibers, whereas the endocrine system can regulate most cells in the body. The effects of neurotransmitters are rapid and short lived compared with the effects of hormones, which appear more slowly and last longer. Table 18-1 compares endocrine structure and function with nervous structure and function (Figure 18-1).

TABLE 18-1

Comparison of the Endocrine System and Nervous System

FEATURE	ENDOCRINE SYSTEM	NERVOUS SYSTEM
*Overall Function*	Regulation of effectors to maintain homeostasis	Regulation of effectors to maintain homeostasis
Control by regulatory feedback loops	Yes (endocrine reflexes)	Yes (nervous reflexes)
Effector tissues	Endocrine effectors: virtually all tissues	Nervous effectors: muscle and glandular tissue only
Effector cells	Target cells (throughout the body)	Postsynaptic cells (in muscle and glandular tissue only)
*Chemical Messenger*	Hormone	Neurotransmitter
Cells that secrete the chemical messenger	Glandular epithelial cells or neurosecretory cells (modified neurons)	Neurons
Distance traveled (and method of travel) by chemical messenger	Long (by way of circulating blood)	Short (across a microscopic synapse)
Location of receptor in effector cell	On the plasma membrane or within the cell	On the plasma membrane
Characteristics of regulatory effects	Slow to appear, long lasting	Appear rapidly, short lived

Endocrine glands secrete their products, hormones, directly into the blood. Because they do not have ducts, they are often called “ductless glands.” This characteristic distinguishes endocrine glands from exocrine glands, which secrete their products into ducts (see Chapter 6, p. 141). Many endocrine glands are made of glandular epithelium, whose cells manufacture and secrete hormones. However, a few endocrine glands are made of neurosecretory tissue. Neurosecretory cells are simply modified neurons that secrete chemical messengers that diffuse into the bloodstream rather than across a synapse. In such cases, the chemical messenger is called a hormone rather than a neurotransmitter. For example, when norepinephrine is released by neurons, diffuses across a synapse, and binds to an adrenergic receptor in a postsynaptic neuron, we call norepinephrine a neurotransmitter. On the other hand, we call norepinephrine a hormone when it diffuses into the blood (because no postsynaptic cell is present) and then binds to an adrenergic receptor in a distant target cell.

Glands of the endocrine system are widely scattered throughout the body. New discoveries in endocrinology continue to add to the long list of hormone-secreting tissues. However, even the most newly discovered endocrine tissues and their hormones operate according to some basic physiological principles.

In this chapter, we focus our discussion primarily on a few major endocrine glands and their principal hormones. Figure 18-2 and Table 18-2 summarize the names and locations of these representative endocrine glands. After you are familiar with the basic principles of endocrinology and the major examples of glands and their hormones, you will be prepared for additional examples that you will encounter as you continue your study of the human body.

FIGURE 18-2 Locations of some major endocrine glands.

TABLE 18-2

Some of the Major Endocrine Glands

NAME	LOCATION
Hypothalamus	Cranial cavity (brain)
Pituitary gland	Cranial cavity
Pineal gland	Cranial cavity (brain)
Thyroid gland	Neck
Parathyroid glands	Neck
Thymus	Mediastinum
Adrenal glands	Abdominal cavity (retroperitoneal)
Pancreatic islets	Abdominal cavity (pancreas)
Ovaries	Pelvic cavity
Testes	Scrotum
Placenta	Pregnant uterus

QUICK CHECK

1. What is meant by the term target cell?

2. Describe how the nervous system and the endocrine system differ in the way they control effectors.

HORMONES

Classification of Hormones

Hormone molecules can be classified in various useful ways. For example, when classified by general function, hormones can be identified as tropic hormones (hormones that target other endocrine glands and stimulate their growth and secretion), sex hormones (hormones that target reproductive tissues), anabolic hormones (hormones that stimulate anabolism in their target cells), and many other functional names. Another useful way to classify hormones is by their chemical structure. Because this method of classifying hormones is so widely used, we will briefly describe it in the following paragraphs.

STEROID HORMONES

All of the many hormones secreted by endocrine tissues can be classified simply as steroid or nonsteroid (Figure 18-3). Steroid hormone molecules are manufactured by endocrine cells from cholesterol, an important type of lipid in the human body (see Chapter 2, p. 50). As Figure 18-4 shows, because all steroid hormones are derived from a common molecule, cholesterol, they have a characteristic chemical group at the core of each molecule. Because steroids are lipid soluble, they can easily pass through the phospholipid plasma membrane of target cells. Figure 18-5 outlines the metabolic pathways used by endocrine cells to convert cholesterol into steroid hormones. Examples of important steroid hormones include cortisol, aldosterone, estrogen, progesterone, and testosterone.

FIGURE 18-3 Chemical classification of some major hormones.

NONSTEROID HORMONES

Most nonsteroid hormones are synthesized primarily from amino acids rather than from cholesterol (Figure 18-6). Some nonsteroid hormones are protein hormones. These hormones are long, folded chains of amino acids, a structure typical of protein molecules of any sort (see Chapter 2, p. 52). Included among the protein hormones are insulin, parathyroid hormone, and others listed in Figure 18-3. Protein hormones that have carbohydrate groups attached to their amino acid chains are often classified separately as glycoprotein hormones (Figure 18-6, D).

Another major category of nonsteroid hormones consists of the peptide hormones. Peptide hormones such as oxytocin and antidiuretic hormone are smaller than the protein hormones. They are each made of a short chain of amino acids, as Figure 18-6, B, shows. Figure 18-3 lists examples of peptide hormones.

Yet another category of nonsteroid hormones consists of the amino acid derivative hormones. Each of these hormones is derived from only a single amino acid molecule. There are two major subgroups within this category. One subgroup, the amine hormones, is synthesized by modifying a single molecule of an amino acid—either tyrosine or tryptophan. Amine hormones such as epinephrine and norepinephrine are produced by neurosecretory cells (secreted as hormones) and by neurons (secreted as neurotransmitters). Another subgroup of amino acid derivatives produced by the thyroid gland are all synthesized by adding iodine (I) atoms to a tyrosine molecule (Figure 18-6, C). Figure 18-3 lists examples of hormones derived from single amino acids.

QUICK CHECK

3. How are steroid hormones able to pass through a cell’s plasma membrane easily?

4. Name some of the different general types of nonsteroid hormones. Give an example of each.

How Hormones Work

GENERAL PRINCIPLES OF HORMONE ACTION

As previously stated, hormones signal a cell by binding to specific receptors on or in the cell. In a “lock-and-key” mechanism, hormones will bind only to receptor molecules that “fit” them exactly. Any cell with one or more receptors for a particular hormone is said to be a target of that hormone (Figure 18-7). Cells usually have many different types of receptors; therefore they are target cells of many different hormones.

FIGURE 18-7 **The target cell concept.**

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May 25, 2016 | Posted by admin in ANATOMY | Comments Off

ORGANIZATION OF THE ENDOCRINE SYSTEM