Fig. 4.

Systematic changes in the IPSC time course and synaptic delay as the duration of action potential increases.A₁, A₂, andA₃ illustrate a series of IPSCs, Ca²⁺ transients, and action potentials recorded simultaneously as the block of K⁺ channels becomes more pronounced. The dotted traces represent responses activated by the broadest action potential. The Ca²⁺transient and IPSC elicited by the broadest action potential exhibit the slowest initial rate of rise. The double arrows inA₁ define the time window within which the integral of IPSC was calculated (∫IPSC). The arrow inA₃ identifies the point at which presynaptic membrane potential was measured, 1.5 msec after the peak (V_pre). Each tracerepresents the average of 100 trials. B, A depolarization–release coupling plot obtained by plotting ∫IPSC against V_pre. Results measured from the preparation shown in A are in open circles. Three additional preparations are also shown using different symbols. ∫IPSCs calculated in each preparation were normalized by scaling their averaged values to 1. Facilitated ∫IPSCs, shown as filled symbols, were then scaled according to their corresponding control ≥IPSCs. Two of the preparations (filled diamonds, plus signs) only include data obtained with the broadest action potentials. In these cases, their control ∫IPSCs were scaled to fall on the smooth curve, which is drawn by hand.C, A systematic increase in synaptic delay as the duration of presynaptic action potential increases. Action potential duration was measured at 0 mV (see Results for the measurement of synaptic delay). Results obtained from different preparations are shown using different symbols. Action potential duration ranging from 2 to 3.5 msec is not available because action potentials broaden in a large step as Ca²⁺ spikes appear.