Complex sentence: Phototransduction is the complex process whereby the energy of a photon is used to change the inherent membrane potential of the photoreceptor--and thereby signal to the nervous system that light is in the visual field.
Split into simpler sentences: Phototransduction is the complex process whereby the energy of a photon is used to change the inherent membrane potential of the photoreceptor. This change thereby signals to the nervous system that light is in the visual field.