AnatomyNeuroanatomyCranial nerves

Optic nerve (CN II)

Learning objectives

After working through this study unit you will be able to:

Identify the properties and functions of the optic nerve.
Discuss the structures and functions of the visual pathway.
Discover the role of the optic nerve in the visual pathway.

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The optic nerve (CN II) is a special somatic afferent (SSA) nerve which carries the sensation of sight (vision) from the retina of the eye to the brain. The optic nerve is a part of the visual pathway, which is a route by which light that falls on the retina is transmitted to the occipital lobe of the brain, where it is interpreted as visual information. More specifically, the visual pathway refers to a series of synapses that start in the retina, where light stimuli are converted into action potentials, and transmitted across several nervous structures to reach the primary visual cortex.

The structures of the visual pathway include the retinal neurons (photoreceptors, bipolar cells, ganglion cells) which pass visual stimuli through the optic nerve, optic chiasm, and optic tract to the lateral geniculate nucleus. Axons from the lateral geniculate nucleus then project via the optic radiation to the primary visual cortex of the occipital lobe.

The optic nerve is formed when the axons of retinal ganglion cells pierce the scleral layer of the eyeball. The nerve runs posteromedially within the orbit and through the optic canal to enter the cranial cavity, where together with its contralateral counterpart, forms the optic chiasm.

This video will help you learn more about the optic nerve and its role in the visual pathway.

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Summary

Key points about the visual pathway
Components	Retina → optic nerve (CN II) → optic chiasm → optic tract → lateral geniculate nucleus (90%) → optic radiation → primary visual cortex Retina → optic nerve (CN II) → optic chiasm → optic tract → superior colliculus (10%) → pulvinar of thalamus → secondary visual cortex
Retina	Sensory neural layer of the eyeball Neurons: Photoreceptors (rod cells and cone cells), bipolar cells, ganglion cells, horizontal cells, and amacrine cells
Optic nerve (CN II)	Formed by axons of ganglion cells coming together at the optic disc
Optic chiasm	Point of decussation of the optic nerves: nasal fibers of retina cross over to the contralateral optic tract, while temporal fibers stay on the same side (ipsilateral)
Optic tracts	Left: Carries left temporal and right nasal fibers of retina Right: Carries right temporal and left nasal fibers of retina
Lateral geniculate nucleus	Receives 90% of the retinal fibers Represents a relay center of the thalamus that projects to the primary visual cortex via the optic radiation
Superior colliculus	Receives 10% of the retinal fibers Integrates visual, auditory, and somatosensory spatial information and controls reflex movements of the eye
Optic radiation	Large bilateral bundle of fibers each containing two divisions that receive visual input from upper and lower quadrants of the contralateral hemifields
Primary visual cortex	Brodmann area 17: Region of the occipital lobe that receives and processes visual information from contralateral visual field

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