Pupillary light reflex

Pupillary light reflex (PLR) is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retina of the eye, to assist in adaptation to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict (reducing the amount of light that enters through the pupil), whereas a lower intensity of light causes the pupil to dilate (increasing the amount of light that enters through the pupil). Thus, the pupillary light reflex regulates the intensity of light entering the eye.

Anatomy

The pupillary light reflex involves a reflex arc that includes the retina, optic nerve, pretectal area of the midbrain, Edinger-Westphal nucleus, oculomotor nerve, and the sphincter muscle of the pupil. Light entering the eye strikes the retina, and this signal is passed through the optic nerve to the pretectal area of the midbrain. From here, neurons send signals bilaterally to the Edinger-Westphal nucleus that stimulate the parasympathetic part of the oculomotor nerve, which leads to constriction of the pupil.

Clinical significance

The pupillary light reflex is an important diagnostic tool for assessing the integrity of the optic nerve and brainstem. An absent or abnormal pupillary light reflex may indicate damage to the optic nerve or brainstem, which can be caused by conditions such as optic neuritis, multiple sclerosis, or brainstem death. The pupillary light reflex is also used to assess the function of the cranial nerves, particularly the optic nerve (cranial nerve II) and the oculomotor nerve (cranial nerve III).

See also

• Pupil
• Reflex
• Optic nerve
• Oculomotor nerve

References

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