Facilitation and depression

When the motor axon is stimulated repetitively, the successful end-plate potentials produced are often of different sizes. At the beginning of a series, and especially in low calcium ion concentrations, successive potentials increase in size. This phenomenon is known as facilitation. The larger responses are made up of more quantal units. Thus facilitation seems to be caused by an enhancement of the release process, perhaps because of an accumulation of calcium ions at some site within the presynaptic terminal.

The opposite of facilitation is depression, in which succeeding responses are smaller and composed of fewer quantal units. Depression occurs when a large number of quanta have recently been released, as in the later stages of a train of stimuli in the presence of an adequate concentration of calcium ions. It appears to be caused by a temporary reduction in the number of vesicles which are available for release.

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