Miniature potentials, light adaptation and afterpotentials in locust retinula cells

Abstract

SUMMARY In dark-adapted retinula cells of the locust, the miniature potentials or bumps have amplitudes of 3-7 mV when recorded in the dark-adapted state with bevelled electrodes at very low light levels. For a sample of N = 378 they had a mean amplitude of 4*55 mV and s.D. 1-59 mV. These are called dark-adapted bumps, and their distribution of amplitudes is presumably a component of the noise of the visual signal in addition to the irregularity of occurrence of bumps. The afterpotential which follows an intense flash is caused by the summation of bumps which become individually visible as they decline in frequency over a period of many minutes after the stimulus. When recorded over a period from 17 to 23 min after the stimulus, the amplitude of these light-induced dark bumps in a sample of N = 231 had a mean of 2-89 mV and s.D. of 0-92 mV. They also have a shorter duration than dark-adapted bumps. Light-adapted bumps, which are denned as bumps which arise when light of low intensity falls on light-adapted cells, are statistically indistinguishable in amplitude or duration from light-induced dark bumps. The reduction in size, as compared to dark-adapted bumps, cannot be attributed to a change in membrane potential or to an increase in membrane conductance, which actually decreases during the period of decline of the after-depolarization. Therefore in the light-adapted state, each bump is caused by the opening of fewer sodium channels, or of less effective sodium channels than in the dark-adapted state.