READILY RELEASABLE OR NOT?
Short-Term Plasticity in synaptic transmission is related to Katz’s variables -the quantum, the release site, and the release probability- which have morphological counterparts. The quantum is, as originally proposed, the exocytosis of a single synaptic vesicle; the release site corresponds to the single active zone; and the morphological correlate of release probability is the number of docked vesicles. Several authors have identified the docked vesicle pool with the physiologically defined readily releasable vesicle pool.
The number of readily releasable synaptic vesicles shapes the postsynaptic response to presynaptic action potential. In a recently published paper Moulder and Mennerick (Reluctant vesicles contribute to the total readily releasable pool in glutamatergic hippocampal neurons. J. Neurosci. 25, 3842-3850, 2005) describe a population of “reluctant” vesicle that might increase the range of potential outputs at some synapses.