Studies in our lab suggest that microelectrode penetration may significantly affect cell activity. The rhythmically active pairs of interneurons that pace the leech heartbeat were thought to require reciprocal inhibition to produce bursting activity. Intracellular recordings demonstrate that blocking synaptic transmission with bicuculline causes tonic spiking, not rhythmic bursting. However, when this experiment is performed with the non-invasive technique of extracellular recording, the cells often maintain rhythmic bursting (Cymbalyuk et al., SFN Abstr 164.2, 2000). This result suggests that microelectrode-induced soma shunt affects cell behavior during synaptic isolation. In a single compartmental model, activity changes from endogenous bursting to tonic spiking when somatic shunt is simulated (Cymbalyuk et al., SFN Abstr 164.2, 2000). Since synaptic transmission, reception and spike-initiating and -propagating currents appear situated in the primary and secondary neuritic processes, and not in the soma...