Nerve
Optic nerve (CN II)
Retinal output carried toward the brain
High-yield
CN II travels through the optic canal and provides the incoming signal for the pupil's light response. Unequal optic-nerve dysfunction can produce a relative afferent pupillary defect; equally severe disease in both nerves may not. Raised intracranial pressure can cause papilledema through the communicating CSF sheath.
Overview
Retinal ganglion-cell axons gather at the optic disc and pass through the scleral lamina cribrosa to form CN II. Behind the eye they acquire oligodendrocyte myelin. The nerve is a CNS pathway surrounded by meninges and a CSF-containing sleeve. Its curved orbital course permits eye movement before it enters the optic canal and reaches the chiasm.
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Reference facts
- Origin
- Approximately one million retinal ganglion-cell axons. They are normally unmyelinated in the retina and become myelinated behind the lamina cribrosa; injured human CNS axons have very limited spontaneous regeneration.
- Skull exit
- Optic canal of the sphenoid, accompanied by the ophthalmic artery and sympathetic fibers.
- Course
- Intraocular, intraorbital, intracanalicular and intracranial segments reach the optic chiasm. Optic tracts then project chiefly to the lateral geniculate nucleus, with other retinal targets including pretectal and superior-collicular regions; geniculate neurons reach visual cortex through optic radiations.
- Motor
- No motor output to eye muscles or glands.
- Sensory
- Vision and visual input to light, near-viewing and visually evoked protective responses. General ocular pain and corneal sensation are carried mainly through V1.
Mnemonic
CN II carries the visual input; CN III carries the parasympathetic output for pupil constriction.
Clinical correlation
Optic neuritis commonly reduces vision and color perception, often with pain on eye movement. An asymmetric lesion may produce an afferent pupil defect. It can occur with multiple sclerosis or other inflammatory disorders; an episode does not by itself establish MS. Visual-field patterns help distinguish prechiasmal, chiasmal and postchiasmal disease.
Frequently asked questions
Where does the optic nerve (CN II) arise?
Approximately one million retinal ganglion-cell axons. They are normally unmyelinated in the retina and become myelinated behind the lamina cribrosa; injured human CNS axons have very limited spontaneous regeneration.
What muscles does the optic nerve (CN II) supply?
No motor output to eye muscles or glands.
What is the sensory distribution of the optic nerve (CN II)?
Vision and visual input to light, near-viewing and visually evoked protective responses. General ocular pain and corneal sensation are carried mainly through V1.
Where does the optic nerve (CN II) leave the skull?
Optic canal of the sphenoid, accompanied by the ophthalmic artery and sympathetic fibers.