Functions of the Endocrine System
Objective 1
Define the general functions of the endocrine system. Describe hormones, target cells, and describe the ability of target cells to up- or down-regulate receptors. Classify endocrine organs, those whose sole function is to secrete hormones. Compare and contrast the mechanisms of action of neurotransmitters and hormones. Explain the difference between circulating, paracrine, and autocrine hormones. Define and give examples of interactions between hormones including permissive, synergistic, and antagonistic effects.
Function and General Features of the Endocrine System
For the purpose of regulating homeostasis, the body is continually receiving, interpreting, and responding to stimuli. The two main systems of the body that are responsible for this task are the nervous and endocrine systems.
With mediator molecules, homeostatic mechanisms are strictly controlled. In the nervous system, these molecules are neurotransmitters; in the endocrine system, they are hormones.
Molecules that regulate homeostasis can be both neurotransmitters and hormones; it just depends on how they are used.
Neurotransmitters are released from neurons to act locally and quickly in response to a nerve impulse. They bind to receptors in a postsynaptic membrane near their site of release. Neurotransmitters act very quickly and have a short duration. Their effectors are usually glandular cells, various types of muscle cells, and other neurons. Hormones on the other hand can be delivered to cells throughout the body, and bind to receptors in or on the target cell. In contrast to neurotransmitters, hormones take anywhere from seconds to hours to days to begin working, and their duration tends to be much longer.
A hormone is a mediator molecule secreted from one part of the body that circulates via the body fluids to cells in another part of the body. Initially, hormones are secreted into the interstitial fluid, then typically the bloodstream, thereby having access to the entire body.
A specific cell will respond to the hormone due to the presence of peripheral or intra-cellular receptors. Because these cells are the specific targets of the hormones, they are referred to as target cells. Hormones function to regulate: chemical composition and volume of blood, metabolism and energy balance, contraction of smooth and cardiac muscle fibers, glandular secretion, growth and development, reproduction, sleep-wake cycles, and some immune functions.
Endocrine signaling can occur in several ways. Most hormones are secreted into the interstitial fluid and then into the blood, thereby giving them access to the entire body, they are referred to as circulating hormones. In paracrine hormone signaling, hormones diffuse from their source and act locally, causing changes in nearby cells. In autocrine hormone signaling, cells have receptors for the hormones they secrete, so they can stimulate themselves, or (more often) turn off their secretion after a qu