QS Study

Light reflex: If light enters the eye, the pupil of this eye constricts (direct light reflex) and there is an equal constriction of the pupil of the other eye (consensual light reflex). It is a reflex that controls the diameter of the pupil, in response to the intensity of light that falls on the retinal ganglion cells of the retina in the back of the eye. It has two main parts: an afferent limb and an efferent limb.

Afferent Pathway of Light Reflex:

The optic nerve enters the optic chiasm where the nasal retinal fibers cross to contralateral optic tract, while the temporal retinal fibers stay in the ipsilateral optic tract. The pretectal nuclei project fibers to the ipsilateral Edinger-Westphal nuclei and also to the contralateral Edinger-Westphal nucleus via the posterior commissure.

Efferent Pathway of Pupillary Light Reflex:

The Edinger-Westphal nucleus projects pre-ganglionic parasympathetic fibers, which exit the midbrain and travel along the oculomotor nerve (CN III) and then synapse on post-ganglionic parasympathetic fibers in the ciliary ganglion.

Pathways of Light Reflex 1

Fig- Pathway of the light reflex

Pathway:

  • Light enters the eye,
  • Passes through the refractive media of the eye,
  • Falls on rods & cones of the retina,
  • Optic nerve,
  • Partial decussation in the midbrain,
  • Edinger-Westphal nucleus on each side,
  • Third nerve (Oculomotor nerve),
  • Branch to the inferior oblique,
  • The short root of the ciliary ganglion,
  • Short ciliary nerves,
  • Sphincter papillae,
  • Constriction of papillae.
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