Optic nerve (CN II)

The optic nerve (second cranial nerve, CN II, Latin: nervus opticus) is a sensory nerve, which is formed by the axons of the ganglion cells of the retina. The optic nerve provides the visual sensation and is involved in the afferent part of the pupillary and accommodation reflexes.

The optic nerve develops as a formation of the diencephalon and that is why it is covered by all of the meningeal layers and surrounded by the subarachnoid space up to the eyeball.

The visual pathway begins in the retina as a chain of three functionally different interconnected neurons: 1) photoreceptor cells (cones and rods), 2) bipolar cells, 3) ganglion cells.The axons of the ganglion cells form the optic nerve that leaves the eyeball at the area called the optic disc.

The optic nerve together with the ophthalmic artery enters the middle cranial fossa through the optic canal. The fibers of the right and left optic nerves partially cross on the prechiasmatic sulcus in the middle cranial fossa. This nerve junction is called the optic chiasm. Only the medial fibers transmitting the visual information from the lateral visual field cross here.

The continuation of the chiasm is called the optic tract. The tract consists of the medial fibers from the contralateral eye and lateral fibers from the ipsilateral eye. The optic tract divides in several parts. Most of the fibers wrap around the cerebral peduncles and reach the lateral geniculate body, and these fibers are part of the main visual pathway. Some fibers form synapses with the pretectal nuclei to provide the pupillary light reflex, while other fibers reach the neurons of the superior colliculus and provide reflectory defensive movements when an unexpected visual stimulus is present. There are also fibers which leave the tract at the area of the optic chiasm and reach the suprachiasmatic nucleus in the hypothalamus, which participates in regulation of the day-night cycles.

The axons of the neurons located in the lateral geniculate body form the optic radiation and reach the primary visual centers in the cerebral cortex via two pathways. The superior part of the tract which carries visual information from the upper visual field of the retina reaches the cuneus, but the inferior part carrying the information from the lower visual field forms the so-called Mayer`s loop and reaches the middle occipitotemporal gyrus.