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Communication of information between neurons is accomplished by movement of chemicals across a small gap called the synapse. Chemicals, called neurotransmitters, are released from one neuron at the presynaptic nerve terminal. Neurotransmitters then cross the synapse where they may be accepted by the next neuron at a specialized site called a receptor. The action that follows activation of a receptor site may be either depolarization (an excitatory postsynaptic potential) or hyperpolarization (an inhibitory postsynaptic potential). A depolarization makes it MORE likely that an action potential will fire; a hyperpolarization makes it LESS likely that an action potential will fire. The three major categories of substances that act as neurotransmitters are... (1) amino acids (primarily glutamic acid, GABA, aspartic acid & glycine), (2) peptides (vasopressin, somatostatin, neurotensin, etc.) and (3) monoamines (norepinephrine, dopamine & serotonin) plus acetylcholine. The major "workhorse" neurotransmitters of the brain are glutamic acid (=glutamate) and GABA. The monoamines & acetylcholine perform specialized modulating functions, often confined to specific structures. The peptides perform specialized functions in the hypothalamus or act as co-factors elsewhere in the brain. |
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