Dynamical differences in rod and cone photoresponses

Understanding how photoreceptor cells respond to light is crucial for comprehending the intricacies of vision. These cells, known as rods and cones, play a pivotal role as they convert light into electrical signals that the brain can interpret. If these cells share this exquisite machinery, how can photoresponses be so different? In this work, we study the factors influencing the dynamics of photoreceptor responses. Based on a detailed model of the underlying biochemical steps, we analyzed the impact of various processes on the response, with particular emphasis on the cyclase feedback. Our study focused on the transition between monophasic and biphasic regimes in photoreceptor responses. Critically, the influence of intracellular messengers' turnover rates, such as for Ca²⁺ and cGMP, initial concentrations, maximum currents, and the modulation by other parameters was studied in depth. By analyzing both dark-adapted and light-adapted responses for rods and cones, we highlighted the importance of Ca²⁺ concentration and the cGMP turnover in darkness to determine bi- or mono-phasic responses. Through this systematic exploration, we aimed to provide valuable insights about the underlying mechanisms driving the dynamic behavior of photoresponses and to answer why similar experiments give rise to different dynamical behaviors.