Figure 8.

Subthreshold inward currents encourage temporal summation but at the expense of spike time precision. A, Traces show responses to a train of brief current pulses in a typical tonic neuron (top) and in the model neuron (bottom). Before TTX (black), repolarization was slow and allowed for temporal summation and spike generation. After TTX (gray), repolarization was much faster, thereby preventing temporal summation. Spikes in this and subsequent panels are truncated. B, In this sample recording, inward and outward currents were balanced so that the neuron acted as a nearly perfect integrator (i.e., very long integration time). Note the near-horizontal trajectory of V_m after each stimulus pulse, resulting in a stepwise depolarization toward each action potential. C, Sample responses to pairs of stimuli under control conditions (top) show that inward currents can produce slow depolarizing ramps that lead to spike generation at long and variable latencies after stimulation. The inset shows that latency to spike generation in response to spontaneous synaptic excitation was also highly variable, despite approximately the same initial depolarization and same prestimulus V_m. Blocking I_Ca,P with Ni²⁺ (bottom) prevented slow depolarizing ramps, and spike timing was more precisely associated with stimulation. D, Response of a typical single-spike neuron to trains of current pulses of three intensities. When spikes did occur, they were at consistently short latency after the stimulus onset. Decreasing stimulus intensity revealed the cell to be incapable of temporal summation.