Library · Physiology
Pupillary reflex
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The pupillary light reflex is what makes the pupil close when light reaches it. The pupil is the black circle in the center of the eye, and what closes it is the iris sphincter muscle, a ring of muscle that surrounds it. It is a camera aperture closing down in too much light.
The route is a long one. Light reaches the photoreceptors of the retina, the cones and the rods; from there the signal continues to the pretectal area of the midbrain and reaches the Edinger-Westphal nuclei. From them the ciliary ganglion sends out the fibers that innervate the iris sphincter, that is, the ones that give it the order to contract.
Because some of those fibers cross to the other side along the way, there is not just one reflex: there is the direct one, in the eye that receives the light, and the consensual one, in the other pupil. And dilation in the dark is not this same circuit in reverse, but a different pathway.
In brief
The pupillary light reflex constricts the pupil when light reaches it, through innervation of the iris sphincter muscle.
It is a camera aperture closing down in too much light.
The pathway runs from the photoreceptors of the retina, cones and rods, to the pretectal area of the midbrain and the Edinger-Westphal nuclei; from there the ciliary ganglion innervates the sphincter.
Because some of the fibers cross, there is both a direct reflex and a consensual one in the other pupil; dilation in the dark uses a different pathway.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPupillary Light Reflex, StatPearlsPublished
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