The optic nerve is the nerve of sight and is the second of twelve paired cranial nerves. It is also known as CN II. This nerve is part of the central nervous system and connects the eye to the brain. Both its composition and pathway to the brain are important in functions of vision such as three-dimensional sight and being able to see depth of field.
The Optic Nerve Begins at the Retina and Extends to the Back of the Brain
This nerve originates from the retinal ganglion cells in the retina and extends from the back of the eye to the brain. A ganglion is a mass of tissue composed of nerve cell bodies and a retinal ganglion cell is a type of neuron or nerve cell near the surface of the retina of the eye. These cells receive incoming visual information from photoreceptors in the eye and transmit the information to the brain via the optic nerve. Retinal ganglion cells vary in size, reaction time and connectivity but each has a long axon that extends to the visual centers of the brain.
The retina is the light sensitive tissue lining the inner surface at the back of the eye. When the eye is open, light hits the retina and initiates nerve impulses that travel along the optic nerves to the visual centers of the brain. The spot on the retina where the optic nerve leaves the eye is called the optic disc. This area does not contain any photoreceptors and is literally a blind spot. However, this does not affect normal vision as each eye covers the missing information from the other eye’s vision.
The Optic Nerve Pathway
Approximately 1.2 million retinal neuron axons form each optic nerve, the optic chiasm and optic tract. The optic nerves run from the back of each of the eyes along the optic canal to the optic chiasm located below and in front of the pituitary gland. Here the retinal neuron axons partially cross over in the shape of an ‘X’ and continue on in the optic tracts to the visual cortex in the occipital lobe at the back of the brain.
This partial crossing of the optic nerves at the chiasm is important for three-dimensional vision and allowing depth of sight field. The retinal neuron cells receive and convey the information for vision as well as other important body functions such as light receptors for circadian rhythm (the sleep-wake cycle) and papillary muscle reflex.
The Composition of the Optic Nerve Fibers
The optic nerve is composed of special sensory fibers which are similar to brain tissue in compostion. During embryonic development it is formed from the diencephalon in the forebrain. As a result the optic nerve fibers are encased with a myelin sheath produced by oligodendrocytes, a type of brain cell, rather than Schwann cells that cover peripheral nerves.
The diameter of the optic nerve increases as it travels from the eye to the brain. It is approximately 1.6 mm within the eye and widens to 4.5 mm in the brain. The entire length of th optic nerve varies ranges from 35 to 55 mm.
The optic nerve also lies completely with the central nervous system and is covered with the meninges layers much like the brain is. It is ensheathed in the three meningeal layers, the dura, arachnoid and pia mater. The optic nerves cannot regenerates and damage due to injury or disease may result in irreversible blindness.
Clark & Kumar: Clinical Medicine Fifth Edition. WB Saunders, London 2002