| In the nervous system, a message-carrying impulse travels from one end of a nerve cell to the other by means of an electrical impulse. When it reaches the terminal end of a nerve cell, the impulse triggers tiny sacs called presynaptic vessicles to release their contents, chemical messengers called neurotransmitters. The neurotransmitters float across the synapse, or gap between adjacent nerve cells. When they reach the neighboring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to “fire,” or generate an electric message-carrying impulse. As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic vessicles in a process called neurostransmitter reuptake. |
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