Spike coding during locomotor network activity in ventrally located neurons in the isolated spinal cord from neonatal rat

To characterize spike coding in spinal neurons during rhythmic locomotor ac tivity, we recorded from individual cells in the lumbar spinal cord of neon atal rats by using the on-cell patch-clamp technique. Locomotor activity wa s induced by N-methyl-D aspartate (NMDA) and 5-hydroxytryptamine (5-HT) and monitored by ventral root recording. We made an estimator based on the ass umption that the number of spikes arriving during two halves of the locomot or cycle could be a code used by the neuronal network to distinguish betwee n the halves. This estimator, termed the spike contrast, was calculated as the difference between the number of spikes in the most and least active ha lf of an average cycle. The root activity defined the individual cycles and the positions of the spikes were calculated relative to these cycles. By c omparing the average spike contrast to the spike contrast in noncyclic, ran domized spike trains we found that approximately one half the cells (19 of 42) contained a significant spike contrast, averaging 1.25 +/- 0.23 (SE) sp ikes/cycle. The distribution of spike contrasts in the total population of cells was exponential, showing that weak modulation was more typical than s trong modulation. To investigate if this low spike contrast was misleading because a higher spike contrast averaged out by occurring at different posi tions in the individual cycles we compared the spike contrast obtained from the average cycle to its maximal value in the individual cycles. The value was larger (3.13 +/- 0.25 spikes) than the spike contrast in the average c ycle but not larger than the spike contrast in the individual cycles of a r andom, noncyclic spike trains (3.21 +/- 0.21 spikes). This result suggested that the important distinction between cyclic and noncyclic cells was only the repeated cycle position of the spike contrast and not its magnitude. L ow spike frequencies (5.2 +/- 0.82 spikes/cycle, that were on average 3.5 s long) and a minimal spike interval of 100-200 ms limited the spike contras t. The standard deviation (SD) of the spike contrast in the individual neur ons was similar to the average spike contrasts acid was probably stochastic because the SDs of the simulated, noncyclic spike trains were also similar . In conclusion we find a highly distributed and variable locomotor related cyclic signal that is represented in the individual neurons by very few sp ikes and that becomes significant only because the spike contrast is repeat ed at a preferred phase of the locomotor cycle.