Characterization of 4 light-responsive putative motor neurons in the pedal ganglia of Hermissenda crassicornis.

As part of the analysis of the circuitry underlying phototaxis, 4 light-responsive pedal neurons were identified and characterized. The 4 newly identified neurons have been designated as pedal neurons P7, P8, P9 and P10. Pedal cell P7 has an inhibitory response to light, lasting several minutes. Pedal cells P8, P9 and P10 exhibit excitatory 'on' responses to light that last for a few seconds after light onset. Lucifer yellow fills showed that each identified pedal cell has only one process which exits the nervous system through one of the pedal nerves. Various procedures were used to investigate the responses to illumination expressed by the 4 identified pedal neurons. The results indicate that: (1) the light responses are not intrinsic, but are due to synaptic input from other light-responsive cell(s), and (2) the sources of the synaptic input to the pedal cells are the photoreceptors of the eye, and not extraocular photoreceptors or light sensitive neurons within the circumesophageal nervous system.