A mathematical model predicting many facets of the PDA phenomenon is presented. The phototransduction process is divided into three interacting sublevels: pigment kinetics, transmitter dynamics and coupling of the signal to the membrane potential. The dynamics of each is studied, while showing their characteristics and limitations. Control mechanisms are proposed to overcome these limitations and to achieve good sensitivity in their dynamic range. For example, it is claimed that enzymatic gain modulation underlies the post-stimulus behaviour of the invertebrate photoreceptor cells. This hypothesis is further explored through a quantitative and qualitative analysis of the model. The results agree with the general PDA characteristics inferred from the large amount of data available from the literature in the last decade. This work provides a theoretical framework which unifies the phototransduction data into an overall system organization.