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Optic nerve
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The optic nerve is the second cranial nerve, one of the nerves that come directly off the brain, and its job is to carry visual information. It carries only afferent, or sensory, fibers: it moves information inward and does not send orders outward.
It is not a single cable but a bundle. It is made of the long extensions, the axons, of about 1.2 million ganglion cells of the retina, the layer at the back of the eye that picks up light. All those axons converge at a point called the optic disc and leave the eyeball through the lamina cribrosa, a sieve-like perforated area of the sclera, the white layer of the eye.
Two curious consequences follow. The first: the optic disc has no photoreceptors, the cells that detect light, and that is why each eye has a blind spot there. Where the cables exit there is no room for the sensors. The second: those axons are not covered with myelin, the insulating sheath, until after they cross the lamina cribrosa.
In brief
The optic nerve is the second cranial nerve and carries visual information; it contains only afferent, that is sensory, fibers.
It is made of the axons of 1.2 million retinal ganglion cells, which converge at the optic disc and leave the eye through the lamina cribrosa.
The optic disc has no photoreceptors and forms the blind spot of each eye; the axons only become myelinated after crossing the lamina cribrosa.
Where the cables exit there is no room for the sensors.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingNeuroanatomy, Cranial Nerve 2 (Optic), StatPearlsPublished
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