Definition of the optic nerve
Also known as cranial nerve II or simply as CN II, the optic nerve is a paired cranial nerve that carries visual impulses from the innermost layer of the eye, retina to the brain. It establishes the connection between the eye and the brain. These visual impulses, dispatched through the optic nerve to the brain, form the building blocks of the image of the object. CN II is the only visible part of the brain (or it’s extension), and the optic nerve head (optic disc) can be easily viewed by using an ophthalmoscope.
The second cranial nerve is a part of the central nervous system as it is derived from the out-pouching of the diencephalon during embryogenesis. Being the cranial nerve, the optic nerve is covered with myelin produced by oligodendrocytes, rather than Schwann cells of the peripheral nervous system.
Like other cranial nerves, the optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in peripheral nerves.
CN II is formed by glial cells and more than 1 million nerve fibers which are axons of the retinal ganglion cells of the retina.
Optic Nerve Function
All sorts of visual information, such as the perception of brightness, contrast, color perception, are transmitted via the CN II. It also plays a role to conduct two important neurological reflexes, light reflex, and accommodation reflex. The light reflex is necessary for constriction and dilation of both pupils according to the amount of light shone into the eyes. Likewise, accommodation reflex facilitates the eye to adjust the lens thickness for clear near vision.
Examining the anatomical integrity and functions of optic nerve, eye care professionals can determine the health status of the visual pathway and the areas nearby visual pathway and visual cortex. For instance, the pituitary adenoma can be suspected from the abnormal functioning of the CN II. Likewise, increased intracranial pressure leads to papilledema, which can be easily examined.
CN II develops in the framework of the optic stalk. To form the optic nerve fibers, the fibers from the retinal nerve fiber layer grow into optic stalk by passing through the choroidal fissure. The glial system of the nerve is developed from the ectodermal cells of the walls of the optic stalk.
The fibrous septum develops from the vascular layer of mesenchyme at the third month of gestation. Similarly, the nerve sheaths are developed from the mesenchyme layer similar to meninges of other parts of the central nervous system. Myelination of nerve fibers starts from the brain and extends up to the lamina cribrosa just before birth. If myelination extends up to around the optic disc, it presents as congenital myelinated nerve fibers.
Anatomy of the optic nerve and visual pathway
The visual pathway starts from the innermost layer of eyeball, retina and extends up to the cortical region of the brain consisting of the optic nerve, optic chiasma, optic tracts, lateral geniculate bodies, optic radiations, and the visual cortex.
Parts of Optic nerve
About 47-50 mm long CN II is divided into 4 parts: intraocular (1 mm), intraorbital (30 mm), intracanalicular (6-9 mm), and intracranial (10 mm).
Optic chiasma is an 8-12 mm flattened structure that lies over the tuberculum and diaphragm sellae. The optic nerve fibers from the nasal halves of the retina get decussated at the optic chiasma.
The cylindrical nerve fiber bundles run posteriorly from the optic chiasma. It consists of nerve fibers from the nasal half of the retina of the opposite eye and the temporal half of the same eye. The optic tract end in the lateral geniculate body.
Lateral geniculate bodies
It is located at the posterior termination of the optic tract. It is formed by six layers of grey matter (neuron) alternating with white matter. The second-order neurons coming through optic tract relay information to the lateral geniculate bodies.
It is formed by the axons of third-order neurons of the visual pathway and extends from the lateral geniculate body to the visual cortex.
The visual cortex is located on the occipital lobe, below and above the calcarine fissure. The subdivisions of the visual cortex are visuosensory area (striate area 17) that receives optic radiation fibers, and the surrounding area (peristriate area 18 and parastriate area 19).
Blood supply of the visual pathway
Ophthalmic branch of the internal carotid artery, posterior ciliary arteries, and central retinal artery supply blood to different parts of the nerve. Most of the parts of the visual pathway are supplied by the pial network of blood vessels except the orbital part of the optic nerve. The pial network (pial plexus) is formed from different arteries.
The capillaries derived from the retinal arterioles supply the surface layer of the optic disc. The prelaminar region of optic nerve head gets blood supply from the branches of the peripapillary choroid and some contributions from the vessels of lamina cribrosa. Likewise, the posterior ciliary arteries and arterial circle of Zinn supply blood to the lamina cribrosa.
The retrolaminar part of the nerve gets blood supply from the centrifugal branches of central retinal artery and branches from pial plexus formed by branches from the choroidal arteries, the circle of Zinn, central retinal artery and ophthalmic artery.
Visual afferent fibers: transmit visual impulses from the retina to the lateral geniculate body of the thalamus
Pupillary afferent fibers: regulate the pupillary light reflex
Efferent fibers: travel to the retina but have an unknown function
Photostatic fibers: responsible for visual body reflexes
Optic Nerve: Definition, Function & Anatomy (PPT)
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- Sapkota B (2015), Anatomy of Optic Nerve https://www.slideshare.net/BikashSapkota/anatomy-of-optic-nerve-49607410