While at first it was believed that there was only one kind of neurotransmitter, acetylcholine, it is now clear that there are many. Most neurons manufacture a particular type of neurotransmitter that is stored within each synaptic knob in microscopic containers called vesicles. On of my articles shows how an anion potential causes some vesicles to release neurotransmitters into the synaptic cleft. These can be taken up at specific receptor sites on the other neuron and affect that neuron’s activity. Each neurotransmitter fits like a ”key” into a specific receptor site “lock.” This whole process is called synaptic transmission.
Synaptic transmission occurs in less than half a millisecond at some ”fast synapses,” with any excess neurotransmitter quickly broken down or reabsorbed into the synaptic knob (a process known as reuptake). The entire sequence can occur again within a few milliseconds. Thus many action potentials can be evoked by repeated synaptic transmission in a very short time. In contrast synaptic transmission between some neurons may take a second or more to occur, with a slow breakdown and reabsorption of excess neurotransmitter. These ”slow synapses” provide another means of communication between neurons that is not completely understood.
Our bodies also produce chemicals than can influence synaptic tracks mission without actually serving as neurotransmitters. These chemicals are called neuromodulators (some of which may also serve as neurotransmitters in certain locations). Neuromodulators are sometimes produced outside the nervous system and carried to it by the blood-for example the hormones estrogen and testosterone produced by the endocrine system. Others are released by neurons but act as neuromodulators rather than neurotransmitters. For example, enkephalins released by internecrons in our spine seem to block the transmission of pain signals at certain synaptic junctions (see the Highlight on enkephalins and endorphins).
Our knowledge of neurotransmitters and neuromodulators has grown rapidly in the last ten years, yet we are still a long way from fully understanding their complex role. What is clear is the degree to which these complex neurochemical processes can be altered by subtle changes in our body chemistry.
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