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Pupillary Light Reflex (ARCHIVE)

Notes

Pupillary Light Reflex (ARCHIVE)

Pupillary Light Reflex

ANATOMY

Lies at the level of the midbrain-diencephalic junction.

Involves the...

  • Pretectal olivary nucleus of the pretectal area, which lies anterolateral to the superior pole of the superior colliculus.
  • Posterior commissure, which forms the roof of the cerebral aqueduct (of Sylvius).
  • Paired Edinger-Westphal nuclei of the oculomotor complex.
  • Bilateral ciliary ganglia in the posterior orbit.
  • Sphincter pupillae of the iris, which are small circumferential rings of smooth muscle in the pupillary margin of each eye
    • Parasympathetic-innervated sphincter pupillae muscle fibers are circumferentially arranged to constrict pupil size. - Sympathetic-innervated pupillary dilator muscle fibers are radially arranged to widen pupil size.

PUPILLARY LIGHT REFLEX PATHWAY

  • Light activates the retinae.
  • The optic nerve fibers combine to form the optic tracts, which pass toward the lateral geniculate nucleus.
  • Whereas the majority of optic fibers synapse within the lateral geniculate nucleus, fibers of the pupillary light reflex, instead, synapse in the pretectal olivary nucleus of the pretectal area.
  • Prectectal olivary nucleus projects directly to the ipsilateral Edinger-Westphal nucleus and to the contralateral Edinger-Westphal nucleus via the posterior commissure.
  • Each Edinger-Westphal nucleus projects to the ipsilateral ciliary ganglion.
  • The ciliary ganglion sends short ciliary nerves to innervate the sphincter pupillae muscles to produce pupillary constriction.
  • The short ciliary nerves pass initially within the sclera and then within the suprachoroidal space as they wrap around the globe to innervate the sphincter pupillae.
  • The majority of short ciliary nerves actually innervate the ciliary muscle of the ciliary body, which assists in the accommodation reflex.

CLINICAL COROLLARIES

  1. Adie's tonic pupil results from pathology within the ciliary ganglion or the short ciliary nerve, which prevents the pupil from constricting in reaction to light.
  2. Impaired pupillary constriction can be an early warning sign of uncal herniation (brain herniation) because of its midbrain-diencephalic location.