Photoreceptor sensitivity and the shot noise of chemical processes.

The general modeling of dose-response curves to very low stimuli in a photosensory-effector system is critically reshaped starting from basic assumptions on the fluctuations of chemical signals inside the receptor cell, which add to those of the stimulus itself, both arising from their granular (or quantal) structure. We have shown, both through the analytical treatment of a simple kinetic scheme and by means of Monte Carlo simulations of the same, that shot noise arising from chemical transduction ("chemical shot noise") contributes considerably to the output noise of the receptor-effector system, thus affecting both the shape and the abscissa shift of dose-response curves under these conditions; the latter phenomenon has indeed been reported in Halobacterium halobium. After evaluating the general properties of a single-step amplifying mechanism, the effects of introducing several low-amplifying steps in cascade were investigated briefly. The results obtained were qualitatively and quantitatively at variance from those of earlier models on the same phenomenon, and the discrepancies are discussed in order to highlight the fundamental contribution of chemical shot noise to the response of any kind of sensory system to very low stimuli.