The Retina, Optic Nerve, and Autonomic Nerve Supply Controlling the Pupil Can BeTested with a Flashlight
When a light is shone into the eye, the pupil of that eye constricts. This action is called the direct pupillary light reflex. The light triggers the photoreception mechanism, leading to ganglion cell action potentials transmitted along the optic nerve.
Some of the ganglion cell axons of the retino-tectal pathway synapse in the pretectal region of the brain. The pretectal neurons then synapse on the parasympathetic preganglionic neurons of the Edinger-Westphal nucleus, whose axons travel in the oculomotor nerve to synapse on parasympathetic postganglionic neurons in the ciliary ganglion. Stimulation of these postganglionic neurons causes constriction of the pupil by stimulating the constrictor smooth muscle fibers of the iris. A normal direct pupillary light reflex tests the integrity of the retina, the ipsilateral second and third cranial nerves, a limited region of the brainstem, and the iris. The axons of pretectal neurons also cross the midline, so that when a light is shone into one eye, not only does the pupil on the same side constrict (direct pupillary light reflex), but the contralateral pupil also constricts. This action is called the indirect or consensual pupillary light reflex. It also requires the integrity of the contralateral oculomotor (third) cranial nerve.
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