Prolonged depolarization in turtle cones evoked by current injection and stimulation of the receptive field surround.

1. Responses evoked by stimulation of the receptive field surround were recorded intracellularly from cone photoreceptors in the retina of the turtle (Pseudemys scripta elegans). 2. A distinctive depolarizing response was evoked by flashing an annulus of light while steadily illuminating the centre of the receptive field. The response, here called 'the prolonged depolarization', was found in 67% of a sample of 125 cones and could reach some 20 mV in amplitude. 3. The prolonged depolarization is characterized by a set of properties which include: the capacity to persist up to 17 s after the flash, a stereotypical waveform, a long period of temporal facilitation, a very narrow dynamic range, and a long refractory period (30-45 s). 4. Depolarizing current pulses (0.01-0.1 nA) evoke a prolonged depolarization which is similar to and functionally interchangeable with that evoked by light. The prolonged depolarization is thus apparently generated by a voltage-sensitive mechanism intrinsic to the cone. 5. Brief depolarizing spikes were recorded in a small fraction of cones. The spikes appear to be dissociable from the prolonged depolarization although both might arise for similar regenerative mechanisms. 6. The prolonged depolarization is typically preceded by a graded, stimulus-locked depolarization which can also be recorded in isolation by flashing annuli of low intensity. The graded depolarization is probably a manifestation of the depolarizing influence arising from synaptic feed-back from horizontal cells first described by Baylor, Fuortes & O'Bryan (1971). 7. It is suggested that the graded depolarization triggers the prolonged depolarization and that complex responses arise from the interaction of these disparate components.