Modeling and stochastic analysis of the single photon response

Abstract

Rod photoreceptors have the remarkable ability to respond to a single photon. A photon absorption triggers the activation of a receptor which is subsequently amplified by the activation of only 5-10 molecules. Because of such low numbers, the activation process has to be proceed in a coordinated manner in order to generate a reproducible signal. In addition, this signal has to overcome the background noise generated by spontaneous activations and deactivation of millions of enzymatic molecules. We review here recent modeling and stochastic analysis of the molecular events underlying the single photon response and the background noise. The homogenization procedure of the rod geometry is the first step for reducing the three into one dimension, so that numerical simulations become possible and reveal the fundamental relation between proteins concentrations, biochemical rate constant, and rod geometry. The stochastic modeling is used to analyze electrophysiological recordings and to extract in vivo biochemical constants. Modeling phototransduction has evolved at the far front of cell transduction and system biology and thus the approach presented here can be applied to many transduction mechanisms.