OPTIC NERVE

It is the 2nd Cranial Nerve.

ORIGIN
The optic nerve arises from cells of the ganglionic layer of retina.
The optic nerve leaves the eye ball in the orbital cavity passes through the optic canal to enter the middle cranial fossa.

OPTIC CHIASMA
It unites with the optic nerve of opposite side to form the optic chiasma. In the optic chiasma the fibers from the medial half of retina cross the midline and enter optic tract of opposite side
Fibers of lateral half pass posteriorly to the same side tract.

Most fibers of optic tract terminate by synapsing with the nerve cells in the lateral Geniculate body (a small projection from posterior thalamus)

A few fibers pass to pretectal nucleus and superior colliculus of midbrain however are concerned with light reflexes.

The axons of nerve cells of lateral geniculate body pass posteriorly as optic radiation and terminate in visual cortex of cerebral hemisphere.

EFFECT OF LESION OF DIFFERENT PARTS OF VISUAL PATHWAY

LESION AT RETINA
Scotoma (small area of blindness)

LESION OF THE OPTIC NERVE
Total loss of vision in one eye; consensual reflex is retained


LESION OF OPTIC CHIASMA
a) Central Lesion
Bitemporal Hemianopia

b) Bilateral peripheral Lesion
Binasal Hemianopia

LESION OF OPTIC TRACT, LATERAL GENICULATE BODY AND OPTIC RADIATION
Homonymous hemianopia on opposite side without macular sparing. Lesions of optic radiation usually produce quadrantic visual defects


LESION OF THE VISUAL CORTEX
Homonomous hemianopia with macular sparing