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.