Fig. 6.

The S-cell, an interneuron of the fast conducting pathway, responded vigorously and with short latency to light stimulation at both the head and the tail. A semi-intact preparation was used to evaluate neuronal responses. It conserved continuity of the CNS and all major nerves from head to tail. See Materials and methods for a detailed description. Representative S-cell intracellular (IC_S10) and extracellular (EC_7-8) voltage recordings are shown. Arrowheads indicate stimulus onset. The S-cell was impaled in midbody ganglion 10. Ganglia 9 and 11 were routinely exposed for impalement, if necessary. The resting potential was approximately −50 mV and the cell was sporadically active, but generally silent until stimulated with light. The S-cell produced short latency action potentials in response to stimulation using UV, blue and green LEDs. In those instances where the red LED evoked responses, these were longer latency from stimulus onset. These S-cell spikes were always distinguishable in the extracellular (EC) recordings as the largest multiphasic spike that could be related 1:1 with the intracellular (IC) action potential.