A MATHEMATICAL-MODEL FOR THE COMPONENTS OF THE RECEPTOR CURRENT IN LIMULUS VENTRAL NERVE PHOTORECEPTORS

The light-activated current in the ventral photoreceptor of Limulus co nsists of three components. The components have different physical and biochemical properties indicating the operation of different transduc tion pathways. The second current component is activated via the phosp holipase C pathway, which releases calcium from intracellular stores b y inositol trisphosphate. The channels of the other two components are probably gated by cyclic nucleotides of different types. We demonstra te here a mathematical model which describes the current components. C urrent components were fitted by a function consisting of the Fermi fu nction and a single exponential function, describing the activation an d deactivation, respectively. The model has four parameters for each c urrent component: (1) a scaling factor for the amplitude; (2) the half -time of activation; (3) the time constant of activation; (4) the time constant of deactivation. The light-intensity dependences of these pa rameters are different for the three components, reflecting different underlying enzyme reactions. The model makes the quantitative analysis of the current components under different conditions possible.