Augmentation is a potentiation of the exocytotic process

Short-term synaptic enhancement is caused by an increase in the probability with which synaptic terminals release transmitter in response to presynapt ic action potentials. Since exocytosed vesicles are drawn from a readily re leasable pool of packaged transmitter, enhancement must result either from an increase in the size of the pool or an elevation in the fraction of rele asable vesicles that undergoes exocytosis with each action potential. We sh ow here that at least one major component of enhancement, augmentation, is not caused by an increase in the size of the readily releasable pool but is instead associated with an increase in the efficiency with which action po tentials induce the exocytosis of readily releasable vesicles.