Figure 6. Stepwise pattern is a sign of suboptimal DCC rate. A, F-I curves from a triceps surae motoneuron (Rin = 2.0 MΩ; τm = 3.8 ms), injected with a triangular ramp of current (1 nA/s) and recorded in DCC mode at three different DCC switching rates. At low DCC rates, a clear stepwise pattern is apparent, which corresponds to multiples of the switching rate (1050 Hz in this instance): 95.5 Hz (or one spike every 11 DCC periods), 87.5 Hz (1:12), 80.8 Hz (1:13), 75.0 Hz (1:14), 70.0 Hz (1:15), 65.6 Hz (1:16), etc. B, The same phenomenon can be observed in a simple integrate-and-fire motoneuron model. The model was that of a typical FF motoneuron (Rin = 1.5 MΩ; τm = 2.0 ms), injected with a 10-nA slow ramp of current (1 nA/s), and recorded in DCC mode at 8, 3, and 1 kHz. A stepwise pattern is apparent at the top of the F-I curve at 3 kHz and is evident at 1 kHz (see distinct peaks in the distributions of the firing frequencies in B2). The horizontal dotted lines represent the multiples of the period of the 1-kHz switching rates. The vertical dash-dotted line represents the region zoomed-in in C. C, Comparison of the behavior of the model recorded in Bridge (gray line) and DCC mode at 3 kHz. The thick black line represents the Vamp output of the amplifier, while the thin green line represents the true membrane potential Vm(DCC) which is hidden from the experimenter by the sample-and-hold circuit. The membrane potential ripples created by the DCC shorten the ISIs (gray arrows) and entrain the firing with ISIs that are multiples of the DCC period. D, Distribution of the ISIs obtained in DCC mode at 1, 3, and 8 kHz. The intervals have been normalized by the DCC period (1, 0.33, and 0.125 ms, respectively) and plotted on a logarithmic scale. At 1 kHz, the ISIs are concentrated at multiples of the DCC period.