Neuronal firing recorded from primary afferents and central spinal neurons in isolated mammalian spinal cord in vitro was analyzed using interspike interval histograms and Poincaré maps (Return maps). Interspike interval histograms described the time between two successive spikes. However, Poincaré maps represented the interspike interval preceding a spike against the interspike interval following the spike. The variability in firing of neurons occurring naturally in different preparations was explored. In a presumptive single central spinal neuron, the firing pattern provided a single cluster. During bursting in a presumptive single primary afferent, the Poincaré maps provided information on interburst intervals. Moreover, lengthening and or shortening of interspike intervals within bursts can be detected in the maps. The firing of a population of neurons was modulated using a pharmacological intervention with 4-aminopyridine and resulted in a complex Poincaré map.