Release of Acetylcholine

The work of Bernard Katz during the early 1950's formed the basis for what is understood about the processes that cause acetylcholine to be released from the presynaptic membrane, and the subsequent events. The release of acetylcholine causes the opening of ion channels, which changes the electrical potential across the postsynaptic membrane. The smallest change in electrical potential is called an increase in noise. Noise is caused by single channels opening, due to the release of small amounts of acetylcholine from the terminal button of the presynaptic membrane.

A larger change is caused by synchronious opening of channels, also called a miniature endplate potential (MEPP). Each MEPP is caused by the release of a one or more quantum of acetylcholine, consisting of 10,000 molecules each. On a even larger scale, an impulse, or wave or depolarization, triggers 100-200 quanta of acetylcholine to be released, producing what is called an endplate potential (EPP). There are approximately 1,000 active zones in a single axon terminal where EPPs occur. These active zones normally do not release all of their acetylcholine stores when they are depolarized. Each active zone has a certain probability of release. The depolarization of a nerve terminal onto an active zone increases the probability that a MEPP will occur. This, however, does not change the amplitude of the MEPP. The probability of release is not dependent on depolarization of the nerve terminal, but the probability increases during depolarization.

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