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Optic nerve (CN II)
The optic nerve (Latin: nervus opticus) or the second cranial nerve (CN II) is a sensory nerve formed by the axons arising from the ganglion cells of the retina. The optic nerve provides the sense of sight and is involved in the afferent part of the pupillary and accommodation reflexes.
During embryonic development, the optic nerve develops as a formation of the diencephalon. That is why it is covered by all of the meningeal layers and surrounded by the subarachnoid space till its intracranial part, where the only meningeal layer covering it is the pia mater.
Optic nerve course and visual pathway
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 second cranial nerve that leaves the eyeball at the area called the optic disc. This is known as the intraocular part of the optic nerve.
Together with the ophthalmic artery, the optic nerve leaves the orbit and enters the middle cranial fossa through the optic canal of the sphenoid bone. The part running from the optic disc to the optic canal is the intraorbital part, while the one going through the optic canal is the intracanalicular part of the optic nerve.
The right and left optic nerve fibers 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 final segment of the optic nerve located in the cranial cavity is known as the intracranial part of the optic nerve.
The continuation of the chiasm is called the optic tract. It is a bilateral structure that passes to each side of the brain. The optic tract consists of the medial fibers from the contralateral eye and lateral fibers from the ipsilateral eye. The tract is divided into several parts.
- Most of the fibers wrap around the cerebral peduncles of the midbrain and reach the lateral geniculate body of the thalamus. These fibers are the visual afferent fibers - part of the main visual pathway.
- Some fibers are known as the pupillary afferent fibers forming synapses with the pretectal nuclei to provide the pupillary light reflex.
- Other fibers reach the neurons of the superior colliculus, and axons of the nuclei located there form the tectospinal tract providing reflectory defensive movements when an unexpected visual stimulus is present.
- Some fibers leave the tract at the area of the optic chiasm and reach the suprachiasmatic nucleus in the hypothalamus, which participates in the regulation of the day-night cycles.
The axons that extend from neurons located in the lateral geniculate body form the optic radiation. These axons 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 (superior retinal fibers) and forms the Baum's loop passing through the parietal lobe and reaching the cuneus.
- The inferior part carrying the information from the lower visual field (inferior retinal fibers) forms the so-called Mayer's loop traveling through the temporal lobe, looping around the inferior horn of the lateral ventricle, and finally reaching the lingual gyrus below the calcarine sulcus on the occipital lobe.