This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

Transmitter Substances

Most of the findings of the quiet revolution date from the 1970s, when the electron microscope became available to researchers. They are still not widely discussed outside the field of neuroscience, probably because of the complexity. In the 1980s, another revolution occurred in neuroscience. This one was loud in comparison to the first. (In other words, most people have heard about it!) The second revolution had a huge effect on scientists' understanding of how drugs work and what causes certain types of mental illness. The focus was upon neurotransmitters , also known as transmitter substances or just transmitters

What are three different names for "transmitters"?


What areas are considered part of a synapse?

First, let us revisit the synapse, which we described earlier as an area where two neurons come very close together. The word synapse actually refers to the entire area and its associated structures. It includes the presynaptic (before-the-synapse) area, the postsynaptic (after-the-synapse) area, plus associated structures (such as thickenings which occur on both sides of the synapse) and also the separation or cleft which separates the presynaptic and postsynaptic areas. Gap junctions are synapses without such a cleft, but the vast majority of synapses are the chemical type that have a small gap and no direct junction. The cleft is very small—about 200 angstroms—so chemicals diffuse across it almost immediately. An artist's depiction of this process is shown in following figure.

What do vesicles contain? Why is a synapse a one-way street?


Transmitters are released into the synaptic cleft

Chemical particles are stored on the presynaptic side of the synaptic cleft, in containers called vesicles (VESS-ik-ulls). Vesicles are little spheres made of membrane. One neuroscientist (Palay) said they are like chocolates, coming in a variety of shapes and sizes with different fillings. The "fillings" inside vesicles are the transmitters. Because the chemicals are stored in vesicles only on one side of the synapse (the presynaptic side), each chemical synapse is a one way street. The nerve impulse can be propagated only from the side with the vesicles to the side without.

What process occurs when the nerve impulse arrives at a synapse?

When a nerve impulse arrives at a chemical synapse, transmitters are released from the vesicles. They rapidly cross the synaptic cleft and bind with receptor sites on the post-synaptic (after-the-synapse) neuron. A common metaphor used in describing receptor sites is locks and keys. Transmitter substances are little keys that fit into particular locks, receptor sites with specific shapes. Specific transmitters are matched with specific receptor sites. The vesicle and its contents are recycled. It is passed back to the presynaptic side, filled again with transmitter substances that have been "taken up" from the postsynaptic side, and soon—within a minute—the vesicle is on the presynaptic side ready to release chemicals again (Betz & Bewick, 1992).

When the transmitter reaches the postsynaptic neuron and lodges in the receptor sites, it typically alters the postsynaptic neuron's permeability to ions. Depending on which ion species are involved, this may make the postsynaptic neuron more likely or less likely to fire a nerve impulse itself. So, in most cases, firing a neuron either stimulates other neurons (makes them more likely to fire) or inhibits them (makes them less likely to fire).

In the 1960s there were three widely recognized transmitter substances: acetylcholine (assiteel-KOH-leen or a-SEE-tyl-koh-leen), norepinephrine (NOR-ep-en-EF-rin) and serotonin (sara-TOE-nin). Next came Gamma-aminobutyric acid (GABA, pronounced as it appears). Then the dam broke: large numbers of small peptides were identified as possible transmitters.


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