Figure 11. Synaptically driven spiking and kinetics of the underlying synaptic events.

A, sample traces of typical spiking patterns elicited by spontaneous synaptic input as well as subthreshold synaptic events in the same cell. The onset of synaptic depolarisation is marked by the arrow. Action potentials are shown truncated. The tonic cell is seen to fire relatively slowly (i.e. moderate delay to onset and long interval between spikes) on the peak of an excitatory postsynaptic potential (EPSP), whereas the phasic cell fired two spikes in rapid succession at the onset of the EPSP, in contrast to the delayed onset neuron in which the elicited spike occurred after a significant delay. Single spike neurons were never seen to spike in response to spontaneous synaptic input. B, kinetics of subthreshold spontaneous EPSPs. Traces show the averaged EPSP (≥ 60 events) for a sample cell of each type. T, tonic; P, phasic; DO, delayed onset and SS, single spike. Though fast events predominated for the delayed onset cells (trace labelled DO_f), a clearly distinguishable subset of events with a much slower decay time course was observed in this cell type (trace labelled DO_s) and occasionally in the phasic and single spike neurons. C, cumulative histograms of decay time constants (τ_epsp) from fittings of individual synaptic events in the same cells shown in B. Though each cell exhibits a range of τ_epsp, differences between cells are clearly discernible. D, summary of decay time constants (τ_epsp) and 10–90 % rise times (RT, 10–90 % of peak) of spontaneous EPSPs from four cells of each type. Bars show means ±s.e.m. The ANOVA was significant for τ_epsp (P < 0.005), with EPSPs in tonic cells being significantly longer than in other cell types (Tukey tests; *P < 0.01). While RT varied significantly across groups (ANOVA; P < 0.05), no single cell type was significantly different from another (Tukey tests). E, membrane time constant (τ_m) was insufficient to explain differences in τ_epsp. Tonic, •; phasic, ▪; delayed onset, ▵; single spike, ⋄. While τ_m can account for some of the variability of τ_epsp within a group, the linear regression for pooled data (line, r²= 0.15) was insignificant (t test) and indicates τ_m cannot explain variability of τ_epsp between groups. F, tonic and phasic cells tended to have longer RTs than delayed onset and phasic cells. Linear regression of the pooled data (line) indicated that RT can explain relatively little of the variability in τ_epsp between groups (r²= 0.36; t test; P < 0.05). More importantly, a long RT alone cannot explain the long τ_epsp in tonic cells given that the RT in phasic cells is equally long though that cell type has a much shorter τ_epsp.