The Endocrine System

Receptors

Chemical signals bind to proteins or glycoproteins called receptor molecules to produce a response. The shape and chemical characteristics of each receptor site allow only certain chemical signals to bind to it. This tendency is called specificity.

There are two major types of receptor molecules that respond to an intercellular chemical signal:

1. Intracellular receptors: These receptors are located in either the cytoplasm or the nucleus of the cell. Signals diffuse across the cell membrane and bind to the receptor sites on intracellular receptors.
2. Membrane-bound receptors: These receptors extend across the cell membrane, with their receptor sites on the outer surface of the cell membrane. They respond to intercellular chemical signals that are large, water-soluble molecules that do not diffuse across the cell membrane.

When an intercellular signal binds to a membrane-bound receptor, three general types of responses are possible:

1. Receptors that directly alter membrane permeability: For example, acetylcholine (adrenaline) from nerve fiber endings binds to receptors that are part of the membrane channels for sodium ions.
2. Receptors and G proteins: A G proteins (guanine nucleotide-binding proteins) are found on the inner surface of the plasma membrane and function as receptors of hormones. For example, chemical signals include cyclic adenosine monophosphate glycerol and inositol triphosphate that bind to receptor molecules in the cell and alter their activity to produce a response.
3. Receptors that alter the activity of enzymes: For example, increasing the activity of an enzyme responsible for the breakdown of glycogen into glucose makes glucose available as an energy source for muscle contractions.

Some intercellular chemical signals diffuse across cell membranes and bind to intracellular receptors. Because these intracellular chemical signals are relatively small and soluble in lipids, they can diffuse through the cell membrane. The chemical signal and the receptor bind to DNA in the nucleus and increase specific messenger RNA synthesis in the nucleus of the cell. The messenger RNA then moves to the ribosomes, then to the cytoplasm, where new proteins are produced.

In contrast, intercellular chemical signals that bind to membrane-bound receptors produce rapid responses. For example, a few intercellular chemical signals can bind to their membrane-bound receptors, and each activated receptor can produce many intracellular chemical signal molecules that rapidly activate many specific enzymes inside the cell. This pattern of response is called the cascade effect.

Hormones

The term endocrine implies that intercellular chemical signals are produced within and secreted from endocrine glands, but the chemical signals have effects at locations that are away from, or separate from, the endocrine glands that secrete them. The intercellular chemical signals, or hormones, are transported in the blood to tissues some distance from the glands. Hormones are produced in minute amounts by a collection of cells to influence the activity of those tissues in a specific way. For example, neurohormones are hormones secreted from cells of the nervous system.

Hormones are distributed in the blood to all parts of the body, but only certain tissues, called target tissues, respond to each type of hormone. Target tissue is made up of cells that have receptor molecules for a specific hormone. Each hormone can only bind to its receptor molecules and cannot influence the function of cells that do not have receptor molecules for the hormone.

Regulation of Hormone Secretion

The secretion of hormones is controlled by negative-feedback mechanisms. Negative-feedback mechanisms keep the body functioning within a narrow range of values consistent with life. Hormone secretion is regulated in three ways:

1. Blood levels of chemicals: The secretion of some hormones is directly controlled by the blood levels of certain chemicals. For example, blood glucose levels control insulin secretion.
2. Hormones: The secretion of some hormones is controlled by other hormones. For example, hormones from the pituitary gland act on the ovaries and the testes, causing those organs to secrete sex hormones.
3. Nervous system: These hormones are controlled by the nervous system. For example, epinephrine is released from the adrenal medulla as a result of nervous system stimulation.

Endocrine Glands and their Secretions

The endocrine system consists of ductless glands that secrete hormones directly into the blood. An extensive network of blood vessels supplies the endocrine glands. The following is a table of hormones secreted by the anterior pituitary gland.

The following is a table of hormones secreted by the anterior pituitary gland.

Additional common hormones are listed in the chart below.

The Effects of Aging

The aging process affects hormone activity in one of three ways: their secretion can decrease, remain unchanged, or increase.

In women, the decline in estrogen levels leads to menopause. In men, testosterone levels usually decrease gradually. Decreased levels of growth hormone may lead to decreased muscle mass and strength. Decreased melatonin levels may play an important role in the loss of normal sleep-wake cycles (circadian rhythms) with aging.

Use the following chart as a reference for the production levels of certain hormones within the body throughout the aging process.

Let’s Review

• The endocrine system functions with the nervous system to regulate the many activities critical to the maintenance of homeostasis.
• Chemical signals, or ligands, are molecules released from one location that move to another location to produce a response.
• Chemical signals bind to proteins or glycoproteins called receptor molecules to produce a response.
• A hormone is an intercellular chemical signal that is produced in minute amounts by collections of cells to influence the activity of those tissues in a specific way.
• Hormones are distributed in the blood to all parts of the body, but only certain tissues, called target tissues, respond to each type of hormone.
• Negative-feedback mechanisms control the secretion of hormones.
• The endocrine system consists of ductless glands that secrete hormones directly into the blood.
• The aging process affects hormone activity.

Endocrine System Flashcard Review