Neurotransmitters versus Secretions Help
Neurotransmitters versus Secretions
The synapse between two neurons, like the neuromuscular junction between a motor neuron and a muscle fiber, employs a special type of chemical communication (see Figure 15.1, A and B). Specifically, vesicles within the axon terminals of an excited neuron rupture and release neurotransmitter molecules (such as acetylcholine). The neurotransmitter molecules then diffuse and bind to the membrane of either another neuron (in the case of a synapse) or a skeletal muscle fiber (in the case of a neuromuscular junction). In both cases, however, there is a type of chemical communication between cells that occurs with the release of neurotransmitter molecules.
Glands And Secretion
But there is another mode or way of chemical communication between cells. This mode is called secretion (sih- KREE -shun). Secretion is literally the “process of separating” – the separation of certain substances from the bloodstream, followed by their release.
The secreted (see- KREE -ted) substance usually comes from epithelial cells. Remember that epithelial tissue is the body’s main covering and lining tissue. But it also occurs within glands . A gland is a “little acorn” – a rounded, somewhat “acorn”-shaped mass of one or more epithelial cells that have become specialized for the function of secretion.
Exocrine ( EK -suh-krin) glands are glands of “external” ( exo -) secretion that release some useful product into a duct, which then carries the secretion to some body surface (study Figure 15.1, C). A good example of exocrine glands are the sweat glands of the skin, which secrete sweat into numerous sweat ducts. Sweat, as you no doubt remember, is a useful product because it helps cool the body and prevent hyperthermia (excessive body temperature).
Endocrine ( EN -doh-krin) glands , in contrast, are glands of “internal” ( endo -) secretion that release a hormone into the bloodstream, right within the gland itself. A hormone is literally “an arouser” ( hormon ). A hormone is a chemical messenger secreted into the bloodstream by an endocrine gland. It gets its name from the fact that the hormone often “arouses” (stimulates) certain target cells in the body to increase their activity. The target cells of the hormone may be located far downstream, but the blood will eventually circulate to bring the hormone molecules to them (see Figure 15.1, D).
The endocrine glands, like the nervous system, are vitally important in both communication and control of the internal environment within the bodies of vertebrates. In fact, it is sometimes very hard to separate the nervous and endocrine systems at all! In these cases, we use the term neuroendocrine ( NUR -oh- en -doh-krin) system to describe them. The neuroendocrine system is an organ system that contains parts of the nervous system, as well as parts of the endocrine gland system. Thus, communication can occur via release of neurotransmitters (the nervous component), and also via secretion of particular hormones into the bloodstream (the endocrine component). We will now describe some specific examples.