Pupillary light reflex

The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the eye, thereby assisting in adaptation to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict (miosis/myosis) (allowing less light in), whereas a lower intensity of light causes the pupil to dilate (mydriasis, expansion) (allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye.

The pupillary light reflex pathway has an afferent limb (within CN II) and efferent limb (within CN III). The ganglion cells of the retina project bilaterally to the pretectal nuclei. The pretectal nuclei projects crossed and uncrossed fibers to the Edinger-Westphal nucleus, which gives rise to the preganglionic parasympathetic fibers. These fibers exit the midbrain with CN III and synapse with postganglionic parasympathetic neurons of the ciliary ganglion, which innervates the sphincter muscle of the iris.

Pupillary reflex

Pupillary reflex refers to one of the reflexes associated with pupillary function.

Types include:

See also:

Although pupillary dilation is not usually called a "reflex", it is still usually considered a part of this topic.

Adjustment to close-range vision is known as "the near response", while inhibition of the ciliary muscle is known as the "far response".

In "the near response" there are three processes that occur to focus an image on the retina. Convergence of the eyes, or the orientation of the visual axis of each eye towards an object in order to focus its image on each fovea, is the first of the three responses. This can be observed by the cross-eyed movement of the eyes when a finger is held up in front of a face and moved towards the face. Next, constriction of the pupil occurs. Because the lens cannot refract light rays at the edges well, the image produced by the lens is blurry around the edges so the pupil constricts when one attempts to focus on nearby objects. Lastly, accommodation of the lens occurs. This is an alteration in the curvature of the lens that allows focus on a nearby object.