The nervous system produces at least thirty different kinds of neurotransmitters. Some neurons release only one type; others produce two or three kinds. Neurotransmitters include acetylcholine, which stimulates skeletal muscle contractions (see chapter 9, p. 303); a group of compounds called monoamines (such as epi-nephrine, norepinephrine, dopamine, and serotonin), which are formed by modifying amino acid molecules; a group of unmodified amino acids (such as glycine, glutamic acid, aspartic acid, and gamma-aminobutyric acid—GABA); and a large group of peptides (such as enkephalins and substance P), which are short chains of amino acids.

Most types of neurotransmitters are synthesized in the cytoplasm of the synaptic knobs and stored in synap-tic vesicles. When an action potential passes along the membrane of a synaptic knob, it increases the membrane's permeability to calcium ions by opening its calcium ion channels. Calcium ions diffuse inward, and in response, some of the synaptic vesicles fuse with the presynaptic membrane and release their contents by exo-cytosis into the synaptic cleft. The more calcium that enters the synaptic knob, the more vesicles release neurotransmitter. Table 10.4 lists the major neurotransmitters and their actions. Tables 10.5 and 10.6 list disorders and drugs that alter neurotransmitter levels.

O^1 Reconnect to chapter 3, Exocytosis, page 92

After a vesicle releases its neurotransmitter, it becomes part of the cell membrane. Endocytosis eventually returns it to the cytoplasm, where it can provide material to form new secretory vesicles. Table 10.7 summarizes this vesicle trafficking.

In order to keep signal duration short, enzymes in synaptic clefts and on postsynaptic membranes rapidly decompose some neurotransmitters. The enzyme acetylcholinesterase, for example, decomposes acetylcholine on postsynaptic membranes. Other neurotransmitters are transported back into the synaptic knob of the presynap-tic neuron or into nearby neurons or neuroglial cells, a process called reuptake. The enzyme monoamine oxidase inactivates the monoamine neurotransmitters epinephrine and norepinephrine after reuptake. This enzyme is found in mitochondria in the synaptic knob. Destruction or removal of neurotransmitter prevents continuous stimulation of the postsynaptic neuron.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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