So far in our consideration of the mechanisms involved in synaptic transmission, we have been concerned very largely with postsynaptic structures and events. It is time to look at what happens in the presynaptic terminal and the synaptic cleft. How is the neurotransmitter released from the terminal when a nerve impulse arrives there? How is it synthesized and packaged for release? What happens to it after it has dissociated from the postsynaptic receptors? We begin by considering some classic experiments by Bernard Katz and his colleagues on the release of acetylcholine at the frog neuromuscular junction.

Transmitter release from synaptic vesicles

During the controversy in the 1930s as to whether synaptic transmission was essentially an electrical or a chemical process, those who held to the electrical hypothesis could not see how a chemical process could account for the speed of synaptic transmission. The advent of the intracellular microelectrode demonstrated, as we have seen in chapter 7, that the transmission process is dependent in most cases upon the release of a chemical substance, the neurotransmitter, from the presynaptic nerve terminal. It also showed that this release is a very rapid process: fast synaptic transmission really is fast, so that the action of the neurotransmitter begins within a millisecond or so of the nerve impulse reaching the presynaptic terminal. How is this rapid release of the neurotransmitter brought about?