Quantal events shape cerebellar interneuron firing

Many small synaptic inputs or one large input are needed to influence principal cell firing, whereas individual quanta exert little influence. However, the role of a quantum may be greater for small interneurons with high input resistances. Using dynamic clamp recordings, we found that individual quanta strongly influence rat cerebellar stellate cell firing. When the frequency of synaptic inputs was low, the timing of recent spikes regulated the influence of excitatory quanta. In contrast, when input frequency was high, spike timing was less important than interactions with other inputs. Inhibitory quanta rapidly terminated firing, whereas small numbers of coincident excitatory quanta reliably and rapidly triggered firing. Our results suggest that stellate cells achieve temporal precision through coincidence detection and disynaptic inhibition, despite their high resistances and long membrane time constants. Thus, we propose that small interneurons can process synaptic inputs in a fundamentally different way from principal cells.