A theoretical explanation for some effects of calcium on the facilitation of neurotransmitter release

The theory provides phenomenological hypotheses concerning the entry of Ca ions into the terminal, removal of these ions, and release of transmitter as a function of the internal Ca concentration C at the time of the triggering impulse. As consideration is limited to experiments with a single control impulse, entry is regarded as dependent only on the external Ca concentration C_e. Evidence is given that release is a saturating function of C, based on the experimental results of Dodge & Rahamimoff (1967) that release is a saturating function of C_e. The principal novelty of the theory rests on the finding of Rahamimoff (1968) that facilitation F increases (decreases) as C_e increases when the interval between impulses is long (short). It is shown that a linear removal law is not consonant with these experimental results; saturation in removal is required. With such saturation, but not without it, one can explain the fact that the duration of facilitation is observed to be an increasing function of C_e. Several other points of comparison between theory and experiment are discussed.