An analysis of rod outer segment adaptation based on a simple equivalent circuit.

This model of rod outer segment adaptation is based on the hypothesis that transmitter substance released by bleached rhodopsin closes sodium channels in the outer segment plasma membrane, leading to hyperpolarization of the receptor. The outer segment adaptation processes of the model are associated with the transmitter release, the transmitter background concentration and the plasma membrane leakage. Changes in the three model parameters correspond to the three types of outer segment adaptation process. According to the model the stimulus-response function is in every adaptational state U/Umax = I/(I + IH). The model predicts how each adaptation process affects IH and Umax. Specifically, when the number of liberated transmitter molecules per isomerizing quantum decreases, the amplitude Umax remains constant but IH increases. A short description of this model has been published in a paper reporting experimental results on background adaptation (Hemilä, 1977).