Date of this Version
Parmelee et al. BMC Neuroscience 2015, 16(Suppl 1):P41
The synaptic ribbon is a specialized structure in photoreceptor neurons that tethers vesicles prior to release (Figure 1A). When a cell is stimulated, vesicles are released from the ribbon and later replenished from the population of mobile vesicles in the synaptic terminal. A train of depolarizing pulses causes the ribbon to alternate between periods of release (lasting Δt = 25 ms) and replenishment (lasting T = 50ms), which occur on estimated timescales of τr = 5 ms (for release) and τa = 815ms (for replenishment). After the first few pulses, the system approaches a limit cycle, and the amount of vesicles released on each pulse converges to a limiting value, R (Figure 1B). This can be used to determine the maximum available pool size on the ribbon, A. The standard method for estimating A is to measure the rate of replenishment in the limit, and then back-extrapolate from the cumulative release plot to obtain the available pool size at the start of the pulse train . When comparing pulse trains of different strengths, this method yields substantially different values for A, a somewhat paradoxical result. Back-extrapolation assumes, however, that the replenishment rate is constant, even though it is thought to be proportional to the available space on the ribbon .