Figure 2. The spontaneous firing pattern of Purkinje neurons is Ca²⁺ dependent.

A, current clamp recordings showing the spontaneous firing pattern of a Purkinje neuron under control conditions (left trace), when Ca²⁺ channels were blocked by cadmium chloride (middle trace) and following removal of the cadmium by perfusion (right trace). B, inter-spike interval (ISI) distributions and log-normal fits are shown for each trace in panel A. Firing frequency components were determined from the peaks of the fits. For the ACSF trace, the peak was at 20 ms, corresponding to a firing frequency of 50 Hz. For the cadmium trace, three peaks could be discerned, corresponding to frequencies of 161, 88 and 56 Hz. After the wash period, the distribution again showed a single peak corresponding to a firing frequency of 45 Hz. C and D, recordings and ISI distributions from a different Purkinje neuron show how the firing pattern and frequency changed as 10 mm BAPTA diffused into the cell from the patch pipette. A few seconds after the whole-cell configuration was established, the cell fired at 84 Hz (left trace). After only 30 s, the firing pattern was irregular, displaying both Na⁺ and Ca²⁺ spikes, with an intra-burst firing frequency of 293 Hz (middle trace). An additional 2.5 min of BAPTA diffusion time had little effect on the intra-burst firing frequency, increasing it to 296 Hz (right trace). E, firing rates (means ±s.e.m.) are shown for each condition. Black bars represent the baseline firing rate for each group while hatched bars represent firing rates under treatment conditions. BAPTA increased the firing rate from 174 ± 40 to 534 ± 70 Hz as it diffused into the cell (n = 5), while cadmium changed the firing rate from 53 ± 11 to 187 ± 26 Hz (n = 4). The firing rate of the control group remained stable over time, changing from 41 ± 16 initially to 47 ± 19 Hz after 10 min in ACSF (n = 6). The BAPTA group differed significantly from the control group both in its baseline rate (P < 0.0001) and in the difference between the treatment condition and the baseline (P < 0.0001).