Nerve fibers which respond to illumination of the sixth abdominal ganglion were isolated by fine dissection from connectives at different levels in the abdominal nerve cord of the crayfish. Only a single photosensitive neuron is found in each connective; its morphological position and pattern of peripheral connections are quite constant from preparation to preparation. These cells are "primary" photoreceptor elements by the following criteria: (1) production of a graded depolarization upon illumination and (2) resetting of the sensory rhythm by interpolated antidromic impulses. They are also secondary interneurons integrating mechanical stimuli which originate from appendages of the tail. Volleys in ipsilateral afferent nerves produce short-latency graded excitatory postsynaptic potentials which initiate discharge of one or two impulses; there is also a higher threshold inhibitory pathway of longer latency and duration. Contralateral afferents mediate only inhibition. Both inhibitory pathways are effective against both spontaneous and evoked discharges. In the dark, spontaneous impulses arise at frequencies between 5 and 15 per second with fairly constant intervals if afferent roots are cut. Since this discharge rhythm is reset by antidromic or orthodromic impulses, it is concluded that an endogenous pacemaker potential is involved. It is postulated that the increase in discharge frequency caused by illumination increases the probability that an inhibitory signal of peripheral origin will be detected.

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