An intracellular study of somatosensory input to the optic tectum of the frog, Rana catesbeiana

A few investigations have been reported about somatosensory projection to the optic tectum of the frog, but characteristics of tectal activity to somatic input are left unknown. In the present study, we investigated the neuronal activity evoked by sciatic nerve stimulation using intracellular recording technique. Somatosensory neurons distributed between layer 1 and 6. Postsynaptic potentials (PSPs) include more IPSPs than EPSPs. The majority of the latencies of PSPs distributed from 5 to 30 ms. Some of somatosensory neurons responded to visual stimuli. Bimodal neurons in middle layers received inhibitory effect from sciatic nerve. According to the spatiotemporal distribution of visual responses, bimodal neurons in middle layers were determined to receive fast conducting component, named class R3 and/or R4 of retino-tectal afferents. From these results we concluded that somatic information from sciatic nerve mostly inhibits the visual response of tectal neurons especially concerning with avoidance behavior. It is also suggested that the tectal neurons in deeper layers constitute the neural circuit of somatosensory process, and a part of them receives inhibitory influence from visual input.