A, experimental probability density distributions of calcium spike latency in the absence (a and c) and presence of BayK 8644 (b and d) at different tail potentials. a and b, 1.5 ms prepulse; c and d, 5 ms prepulse. The responses to tail potentials of −120, −80 and −40 mV are shown as white, grey and black columns, respectively. B, comparison of experimental probability density distribution of calcium spike latency at −120 mV tail potential in the absence (a and c) and presence of BayK 8644 (b and d) with the theoretical distribution. a and b, 1.5 ms prepulse; c and d, 5 ms prepulse. White and black columns denote, respectively, the experimentally measured probability density and the best fit by eqn (8) with parameters shown in Table 1. C, calculated spike probability at −120 mV (filled symbols) compared with the experimentally determined PSpike (hatched columns).