Final answer:
The pupillary light reflex involves sensory input through the optic nerve and motor response through the oculomotor nerve to the ciliary ganglion, causing the pupil to constrict. The sympathetic system is responsible for dilation of the pupil when the retina is not receiving enough light.
Step-by-step explanation:
The pupillary light reflex involves sensory input through the optic nerve (afferent branch) and motor response through the oculomotor nerve to the ciliary ganglion. The ciliary ganglion then projects to the circular fibers of the iris, causing the pupil to constrict. The efferent branch of the competing reflex (dilation) involves the sympathetic system, which dilates the pupil when the retina is not receiving enough light.