Fig. 3.

Most dCINs are rhythmic during NMDA- and 5-HT-induced locomotor-like activity. A, Application of NMDA and 5-HT to the isolated spinal cord resulted in rhythmic locomotor-like activity in the cL2 (top trace) and a depolarization and rhythmic firing observed in the dCIN (bottom trace). Calibration: 20 mV, 24 sec. B–D, Analysis of the activity of the three types of dCIN during locomotor-like activity. Highly rhythmic hS-dCINs (B) were characterized first, on the basis that they exhibited pronounced oscillations in membrane potential (bottom trace, Bi; calibration, 40 mV) during locomotor activity (top trace,Bi). Second, their spikes were also locked to a specific phase as revealed by the spike frequency histogram of 50 locomotor cycles (Bii, top trace). Third, the circular plot vector for 25 spikes as shown by the bold arrow in the bottom trace of Biiwas highly significant (p < 0.001). The duration of cL2 bursting is indicated in Bii–Dii by thegray bars (±SD; n = 50) under thehistogram and the gray segment in thecircular plot. C, Rhythmic S-dCIN that preferentially fires out-of-phase with cL2 activity.p values for S-dCINs were in the range of 0.001 <p < 0.05 (Cii, bottom trace). D, Nonrhythmic NS-dCIN with spikes not locked to any particular phase of the locomotor cycle and withp > 0.05 (Dii, bottom trace). In all subsequent plots, hS-dCIN data are shown in dark gray; S-dCIN data are light gray; and NS-dCIN data are white.E, Distribution of dCIN r values (n = 84). F, Analysis of the instantaneous firing frequency exhibited by the three classes of dCIN.