One embodiment is directed to a system for altering the function of a sensory unit that innervates a targeted tissue region in an animal, the sensory unit being configured to express a light-responsive protein, comprising a light delivery element configured to direct radiation to at least a portion of a targeted tissue structure； and a light source configured to provide light to the light delivery element； wherein the targeted tissue structure is illuminated transcutaneously with radiation such that a membrane potential of cells comprising the targeted tissue structure is modulated at least in part due to exposure of the light-responsive protein to the radiation.
