Response transfer functions of Limulus ventral photoreceptors: interpretation in terms of transduction mechanisms

In recent years, our knowledge of the biochemical mechanisms underlying the transduction process in photoreceptors has expanded rapidly. However, a full picture of the temporal dynamics of these mechanisms remains elusive. To study the dynamics in the Limulus ventral photoreceptor, we measure its light-evoked transfer function under voltage clamp. Comparison of this transfer function to biochemically realistic theoretical models of transduction provides insights into the photoreceptor dynamics. This comparison supports the suggestion that the low-frequency behaviour of the Limulus photoreceptor, corresponding to light and dark adaptation, is that of a nonlinear negative feedback loop. The main reactions of this loop have time constants between about 1 and 40 s. Such a feedback loop does not account, however, for the high-frequency behaviour of the responses, which implies the existence of a further, fast-acting, mechanism.