Sensory, interneuronal, and motor interactions within Hermissenda visual pathway.

The visual pathway of Hermissenda was identified by means of intracellular recordings and iontophoretic injection of the fluorescent dye lucifer yellow. This pathway consisted of five neuron types, namely, type B photoreceptors and the medial type A photoreceptor within each of the two eyes, hair cells in the two statocysts, a group of interneurons in the cerebropleural ganglia, and a putative motor neuron (MN1) in each pedal ganglion. The MN1 cells responded during illumination of the eye with increased impulse and excitatory postsynaptic potential (EPSP) activity. This response was often followed by bursting activity for higher light intensities. The medial type A photoreceptor, which was found to be inhibited by medial and intermediate type B photoreceptors, was demonstrated to excite the MN1 cell indirectly via a group of identified interneurons. Hair cells were also found to excite the MN1 cell indirectly via these interneurons. Among the ipsilateral hair cells, cephalic hair cells were least frequently found to excite the MN1 cell. Among the contralateral hair cells, on the other hand, lateral hair cells were most often found to excite the MN1 cell. Interneurons that were shown to excite the MN1 cell received excitatory input from the medial type A photoreceptor and hair cells. Our observations are consistent with the interpretation that these interactions are mediated by monosynaptic chemical synapses. Electrical stimulation of the MN1 cell with positive-current injection produced turning of the posterior half of the animal's foot to the ipsilateral direction consistent with the animal's turning behavior toward light. The visual pathway identified in this experiment was considered to have some significance in explaining, at least in part, a causal role for changes within type B photoreceptors in producing Hermissenda's modified behavior following associative conditioning.