Various systems and methods are implemented for in vivo use in a living animal. One such method involves stimulating target cells having light-responsive proteins and includes providing an elongated light-delivery structure in a narrow passageway in the animal, the elongated light-delivery structure having separately-activatable light sources located along the length of the elongated light-delivery structure. The method also includes activating less than all the light sources to deliver light to light-responsive proteins adjacent to the activated light sources along the length of the elongated light-delivery structure, thereby stimulating target cells in vivo.