Figure 9.

Synaptic integration time is increased by subthreshold inward currents. A, Model neuron was stimulated with regularly timed EPSCs in a distal dendrite. Bottom graphs show the number of EPSCs necessary to elicit a spike. Top graphs show latency from stimulus onset to spike generation. Two variations of models with both I_Na,P and I_Ca,P were tested: model 1 (gray squares) had the same parameters as the complete model in Figure 7A; for model 2 (inverted black triangles), g_Ca,P was increased to 0.025 mS/cm², and g_K,S was increased to 0.5 mS/cm², like the model in Figure 8 A. Open circles represent basic model without g_K,S. For strong synaptic input (left), both models with I_Na,P and I_Ca,P spiked in response to three to four EPSCs arriving over a broad range of intervals, whereas the basic model was less sensitive and required at least seven EPSCs arriving at short intervals. For a 10-fold decrease in synaptic weight (right), 10× more EPSCs arriving at 10× shorter intervals were required to elicit a spike but, otherwise, the same trends as with stronger synaptic input were observed. Integration time was defined by latency to spike generation during operation at greater than or equal to half-maximal sensitivity (i.e., requiring equal or less than twice the number of EPSCs as the minima in bottom graphs). Integration time was between 12 and 114 ms for the basic model and increased to between 17 and 3665 ms for model 1 and between 17 and 6884 ms for model 2. In other words, the model neuron without intrinsic inward currents responded only to a relatively large number of EPSCs arriving in a short time, whereas the model neurons with intrinsic inward currents responded to a relatively small number of EPSCs arriving over a much broader time window. Gray lines represent linear regression of data used to calculated integration time for model 1. For w = 1 nS, latency = 3.4^*interval^0.956, and for w = 0.1 nS, latency = 38.3^*interval^0.987. B, Example in real lamina I tonic neuron of three spontaneous EPSPs (times indicated by arrows) summating over 650 ms interval. Probability distribution of V_m shown on right highlights stepwise depolarization toward spike generation, reminiscent of data in Figure 8. This demonstrates that the long integration times predicted by simulation data in A accurately describe the behavior of real tonic neurons.