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Light reflex - the test for second and third cranial nerve

Afferent pathway - Optic nerve.
Efferent pathway - Oculomotor nerve.
Centre - Midbrain.

Pathway of light reflex
1.Exposure of eye to brightlight send impulses along the optic nerve, optic chiasma and optic tract.
2.The fibers concerned with light reflex do not reach the lateral geniculate body but they pass from optic tract to the pretectal nucleus in the midbrain where they relay.
3.The new fibers pass to Edinger Wesphal nucleus of both sides, the deccusating fibers pass aroud the aqueduct of sylvius and account for consensual light reaction.
4.From the Edinger Westphal nucleus of both side preganglionic fibers pass through the oculomotor nerve to relay in ciliary ganglion.
5.The post ganglionic fibers pass by the short ciliary nerve to the constrictor pupillae muscle.

Method of testing
1. The patient is asked to look at a distant object to eliminate the contraction of pupil on accommodation.
2. The eye not being tested must be covered to eliminate the consensual reaction.
3. A direct source of bright light is focussed directly into the eye.
4. Normally there is constriction of both pupils.
The response of pupil of the eye upon which bright light falls is the directlight reflex and that of the opposite eye is called consensual reflex.
Consensual reflex occur due to decussation of fibers both in optic chiasma and Edinger Westphal nucleus.

Significance of testing light reflex
1. In lesions of the second cranial nerve direct light reflex on the same side and consensual light reflex on the opposite side is absent due to lesions in the afferent pathway.
2. In lesions of third cranial nerve direct light reflex is absent on the affected side  but the consensual light reflex is present.