Optic nerve (II)

At a glance

English

Optic nerve (II)

Latin (TA2)

Nervus opticus (II)

System

Nerve

Region

Head

Side

Paired, left and right

About the optic nerve (ii)

Cranial nerve II, the optic nerve, carries everything we see from the retina back to the brain: light and dark, colour, contrast and fine detail. Although counted among the cranial nerves, it is really a tract of the central nervous system. It grows from the optic stalk, an outpouching of the embryonic forebrain, so oligodendrocytes, not Schwann cells, make the myelin of its fibres, and it is wrapped in dura, arachnoid and pia like the brain.

The nerve is formed by ganglion cell axons from the retina, which converge on the optic disc, slightly to the nasal side of the back of the eye. The disc has no photoreceptors and makes the blind spot. Four parts are described. The intraocular part, the nerve head, is about 1 mm long. The orbital part, about 24 mm, runs back in a gentle S-curve that allows the eye to move, surrounded by the cone of eye muscles, with the central retinal artery entering it from below. The canal part, about 9 mm, passes through the optic canal with the ophthalmic artery. The intracranial part, about 16 mm, runs back and in to the optic chiasm above the pituitary.

At the chiasm the fibres from the nasal half of each retina, which see the outer half of the visual field, cross to the opposite side; those from the temporal halves stay on their own side. Behind the chiasm the optic tracts carry most fibres to the lateral geniculate nucleus, which relays to the visual cortex of the occipital lobe.

Damage to one optic nerve causes loss of vision in that eye and a weak direct response of its pupil to light. A pituitary tumour pressing on the chiasm takes out the outer fields of both eyes, while a lesion of one optic tract takes out the opposite half of the field of both eyes. Common diseases are glaucoma, in which raised eye pressure slowly damages the nerve head and peripheral vision goes first; optic neuritis, often linked to multiple sclerosis, with painful and usually temporary loss of vision; and ischaemic optic neuropathy, a sudden loss of blood flow to the nerve head in older people.

Course and supply

Arises from

Ganglion cell axons of the retina, gathering at the optic disc and leaving the eyeball through the lamina cribrosa of the sclera.

Ends in

Ends at the optic chiasm, where the nasal retinal fibres cross; the fibres continue in the optic tracts mainly to the lateral geniculate nucleus.

Supplies

Special sensory: carries vision from the retina and forms the afferent limb of the pupillary light reflex and the near response.

Blood supply

The orbital part is supplied by the central retinal artery and small branches of the ophthalmic artery; the nerve head by the short posterior ciliary arteries; the intracranial part by branches of the internal carotid and anterior cerebral arteries.