The amount of quantal release is determined by both the number of available synaptic vesicles as well as by the probability that an available vesicle is released.
EPSCs can be described as the product of the number of available vesicles (n), release probability of each quanta (Pr) and size of the quantal unit q (mEPSC size):
EPSC = n·Pr·q
If N is the number of release sites (= maximum number of available vesicles), and Pa is the number of available vesicles.
n = N·Pa
EPSC = N·Pa·Po·q
This is the release-site model (Vere-Jones, 1966), assuming that the number of release site is limited.
The value q can be determined by recording mEPSC, sustaining presynaptic neuron depolarized, or stimulating the presynaptic neuron in low-Ca saline.
N can be estimated by giving a depleting stimulus. N is dependent on active zone constituents and mechanisms of vesicle recycling.
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