Final answer:
Shining light in the eye causes the pupils to constrict, a response known as the pupillary light reflex (PLR), mediated by the optic and oculomotor nerves. The reflex is bilateral, and abnormalities in it can indicate damage to the optic or oculomotor nerves.
Step-by-step explanation:
When you shine light in the eye, the response is pupillary constriction. This reaction, known as the pupillary light reflex (PLR), involves sensory input through the optic nerve and a motor response through the oculomotor nerve to the circular fibers of the iris. The afferent branch consists of the optic nerve, which detects light and transmits the signal to the brain. The efferent branch is the oculomotor nerve, which carries signals from the brain to the ciliary ganglion and then to the circular fibers of the iris, causing them to contract and constrict the pupil. The pupillary light reflex serves to protect the retina by adjusting the amount of light entering the eye. It is a bilateral response, meaning that shining light into one eye results in both pupils constricting.
In the competing reflex, which is the dilation of the pupils, the sympathetic nervous system plays a role. When light levels are low, the sympathetic system will dilate the pupil. Damage to the optic nerve or oculomotor system can be assessed through this reflex by observing changes in pupillary size when light is shone in one or both eyes.