Signal transmission from photoreceptors to ganglion cells in the visual system of the giant barnacle

The first few integrative stages of the visual systems of almost all animals perform similar functions. These functions include adaptation to a wide range of light intensities, amplification of the small signals generated by dim lights, and conversion of steady responses to transient signals. In an attempt to understand the synaptic and integrative mechanisms underlying early visual processing, we have recorded intracellularly from photoreceptors and postsynaptic cells in the vis-ual system of the giant barnacle. The large size, acces-sibility and small number of cells in this preparation should permit detailed analysis of visual mechanisms. Barnacle photoreceptors, like vertebrate rods and cones, give graded responses to light that spread pas-sively to their presynaptic terminals (Shaw 1972). Al-though barnacle receptors depolarize in response to increases in light intensity, the animal responds be-haviorally only to decreases in light intensity, i.e., shadows. Dimming of light causes repotarization of the receptor but elicits depolarization and impulses in certain postsynaptic eils in the supraesophageal ganglion. These impulses lead to a defensive with-drawal of the animal into its shell. The inversion of the receptor response by ganglion cells underlies the "shadow reflex." Ganglion cells that hyperpolarize in the light and transiently depolarize when the light is dimmed have been described in another barnacle species (Millecchia and Gwilliam 1972); neurons with similar characteris-tics have been found directly postsynaptic to photo-receptors in insects (Zettter and J~irvilehto 1971; Chappell and Dowling 1972). This paper presents an analysis of synaptic transmission from photoreceptors to the ganglion cells which exhibit these inverting properties. Our aim has been to describe (1) the electrical properties of the postsynaptic membrane and (2) the relationship between events in the photorecep-tor and in these particular ganglion cells in the light and dark