Ionic current model for the inner-segment of a retinal photoreceptor

From the morphological point of view, the retinal photoreceptor is composed of the outer segment, the inner segment, and the synapse. Each of these has its own functional role. The outer segment receives light stimulation and changes it into the photosensitive current. The inner segment forms the photoresponse through the interaction between that current change and the various ionic currents existing in the inner segment. The synapse adjusts the release of the information transmission substance according to the photoresponse, and transmits the information to the bipolar cell and the horizontal cell. Although there have been many physiological studies concerning these photoreceptor functions, a model must be constructed and a simulation analysis must be applied to that model in order to clarify such details of the information processing function as the behavior of the various ionic currents of the photoreceptor in the photoresponse, as well as their roles. From such a viewpoint, this paper constructs a model for the ionic currents based on physiological knowledge of the membrane potential-dependent current existing in the inner segment of the photoreceptor. Then, a Ca²⁺-dependent current is considered and the behavior of the ionic current corresponding to the change of Ca²⁺ density in the cell is modeled, considering the intracell mechanism for Ca²⁺. It is verified that the model exhibits the same behavior in terms of the membrane potential and the membrane current response of the inner segment of the photoreceptor. The time-course of each ionic current in the Ca²⁺ spike period is determined by simulation, which has been difficult to measure by physiological experiments. As a result, the modification effect of each ionic current on the Ca²⁺ spike is indicated. The difference of the spike waveform between the membrane potential-dependent K⁺-current blocking period and the control state is also accounted for, based on the dynamic behavior of the ionic currents. The effect of the intracellular Ca²⁺ density change on the membrane potential response is analyzed, and it is shown that the termination of the Ca²⁺ spike is greatly affected by the activation of the Ca²⁺ dependent Cl⁻ current accompanying the intracellular Ca²⁺ density increase. The authors believe that the model for the inner segment of the photoreceptor presented in this paper will contribute greatly to future studies of the retinal function through analysis from a physiological engineering point of view. © 1998 Scripta Technica. Syst Comp Jpn, 28(13): 55–66, 1997