Final answer:
Pupillary constriction in response to light is known as a pupillary reflex, involving the optic nerve as the sensory (afferent) input and the oculomotor nerve for the motor (efferent) response to stimulate the iris.
Step-by-step explanation:
A pupillary constriction in response to light is known as a pupillary reflex. The pupillary light reflex is an automatic reaction that involves the constriction of the pupils when they are exposed to bright light. The sensory input for this reflex comes through the optic nerve, while the motor response is mediated by the oculomotor nerve to the ciliary ganglion, which then activates the circular fibers of the iris, causing pupil constriction. The afferent branch of the competing reflex (dilation) for dim light conditions involves the same sensory pathway via the optic nerve. The efferent branches include sympathetic response pathways that stimulate the radial fibers of the iris to dilate the pupil under low-light conditions. This reflex is critical for regulating the amount of light that reaches the retina, protecting it from damage due to excessive light and enhancing vision under varying light intensities.