Kinetics of synaptic transfer from receptors to ganglion cells in turtle retina

Abstract

Synaptic transfer between the retinal input and output was studied in turtle [Pseudemys scripta elegans and Chelydra serpentina] eyecups by injecting rectangular current pulses into a single cone or rod while recording externally from a ganglion cell. When a receptor was activated with weak steps of polarizing current, the probability of obtaining a ganglion cell impulse rose after an S-shaped delay to a peak at about 0.1 s and then declined. The transmission chain apparently behaves like an electrical band-pass filter containing delay and differentiating elements. To further characterize the kinetics of excitation in the subthreshold region, the duration and polarity of the polarizing current pulses were varied while determining the magnitude of the threshold current and the delay to the ganglion cell impulses. The results of these experiments were described with linear models which assumed that synaptic transfer occurred over a cascade of 1st-order delay stages and a single differentiating stage. The pathways which relay off responses to light from rods and red-sensitive cones were formally similar, but the time scale in the rod path was several times slower. The path carrying off responses from the red-sensitive cones was faster than the on path. These kinetic differences indicated that independent pathways mediated each of the 3 categories of response and suggested that the kinetics of each path were matched to the input signals generated by light. The strength-latency relations for the responses of on-center ganglion cells to flashes and steps of light were approximately predicted from the description of synaptic transfer developed here and the description of visual transduction in red-sensitive cones from a previous study. The retinal paths apparently have kinetics which might be useful in transmitting light-evoked signals while attenuating noise present near the input.