= Synaptic vesicle
{wiki}
A <synaptic vesicle> is a small membrane-bounded compartment in a presynaptic terminal that stores <neurotransmitter> for release. Fusion with the <cell membrane> exposes its contents to the <synaptic cleft>; the packet supplied by one <synaptic vesicle> is a quantum of release. <Quantal neurotransmitter release> refers to these discrete packets, rather than the release of one <molecule> at a time. The postsynaptic response to a packet can vary with its contents, <receptors> and recording conditions. See https://www.ncbi.nlm.nih.gov/books/NBK10866/[Purves and colleagues: vesicles as the source of transmitter quanta].
The useful cycle is filling, preparation for fusion, <calcium>-triggered <exocytosis>, and recovery of membrane and <proteins> through <endocytosis> for reuse. Different recovery routes occur; recycling need not preserve a particular <synaptic vesicle> as an identifiable container throughout. Limited availability of prepared <synaptic vesicles> can reduce release during repeated stimulation, while changes in <synaptic release probability> can also change successive responses. See https://www.ncbi.nlm.nih.gov/books/NBK27911/[Basic Neurochemistry: secretion machinery and vesicle reuse] and <Neuronal Dynamics: From Single Neurons to Networks and Models of Cognition by Wulfram Gerstner and others/3.1 Synapses>.
For an illustrative calculation, assume four release sites, each with one prepared <synaptic vesicle>, each releasing with <synaptic release probability> $p=0.25$ during one presynaptic <action potential>. Assume <independent> <Bernoulli trials> and at most one release per site. The released packet count $K$ has a <binomial distribution>:
$$
K\sim\operatorname{Bin}(4,0.25),\qquad \mathbb E[K]=4(0.25)=1,\qquad \operatorname{Var}(K)=4(0.25)(0.75)=0.75.
$$
The <probability> of no release is $\mathbb P(K=0)=0.75^4=0.31640625$; at least one release has <probability> $0.68359375$. An <expected value> of one packet does not mean that every <action potential> releases one packet. If each packet contributes a stipulated $0.5\,\mathrm{nS}$ conductance increment and increments add, the <expected value> of that increment is $0.5\,\mathrm{nS}$, with zero increment on about $31.6\%$ of trials. These assumptions are a teaching model: shared <calcium> fluctuations, unequal sites, depletion and nonlinear <receptor> responses can invalidate them.
Back to article page
