Discrete Waves and Phototransduction in Voltage-damped Ventral Photoreceptors

Discrete waves in the voltage-clamped photoreceptor of Limulus are remarkably similar in all essential properties to those found in an unclamped cell. The latency distribution of discrete waves is not affected by considerable changes in the holding potential in a voltage-clamped cell. Both large and small waves occur in voltage-clamped and unclamped cells and in approximately the same proportion. Large and small waves also share the same latency distributions and spectral sensitivity. We suggest that small waves may result from the activation of damaged membrane areas. Large waves have an average amplitude of approximately 5 nA in voltage-clamped photoreceptors. It probably requires several square microns of cell membrane to support this much photo-current. Thus the amplification inherent in the discrete wave process may involve spatial spread of activation from unimolecular dimensions to several square microns of cell membrane surface. Neither local current flow, nor pre-packaging of any transmitter substance appears to be involved in the amplification process. The possible mechanisms of the amplification are evaluated with relationship to the properties of discrete waves.