FIGURE 3.
Two classes of readily releasable synaptic vesicles. A, at the calyx of Held, the presynaptic terminal was stimulated to generate a series of presynaptic Ca2+ currents (Ipre; upper trace), which elicited a series of excitatory postsynaptic currents (EPSC; middle trace). Calculation of the total neurotransmitter release showed a rise and fall in the release rate during the train (lower trace). Following the train, a single light flash was given (arrow), and the resulting excitatory postsynaptic current and release were recorded. Integration of the excitatory postsynaptic currents showed that 66% of the total pool of synaptic vesicles was released during the train of action potentials, and 34% was released by uncaging of Ca2+. The rate and Ca2+ sensitivity of the synaptic vesicles released by the light flash were the same as those released by action potentials. This figure was reproduced from Ref. 84 with permission. B, drawing illustrating four docked synaptic vesicles, two tethered near CaV2.1 channels and two docked farther away. Left, Ca2+ entry through CaV2.1 channels induces exocytosis from the two synaptic vesicles docked near the channels but not from the two docked farther away. Right, a general increase in Ca2+ from photo-uncaging causes neurotransmitter release from all four docked synaptic vesicles.