Optic Nerve: Definition, Anatomical, Extracranial and Intracranial Course

The optic nerve is in the back of the eye.

It is also called the second cranial nerve or cranial nerve II. It is the second of several pairs of cranial nerves.

The work of the optic nerve is to transfer visual information from the retina to the vision centers of the brain through electrical impulses, or we can also say that, whose job is to transport sensory nerve impulses from more than one million ganglion cells of the retina towards the visual centers in the brain.

The vast majority of optic nerve fibers transmit information about central vision. The optic nerve is formed by ganglion cells or nerve cells. It consists of more than one million nerve fibers.

Our blind spot is caused by the absence of specialized photosensitive (light-sensitive) cells, or photoreceptors, in the part of the retina where the optic nerve leaves the eye.

Glaucoma

Glaucoma is one of the most common diseases affecting the optic nerve.

Glaucoma is caused by high intraocular pressure, or high pressure in the fluid that is inside the eye (vitreous fluid).

This high pressure compresses the optic nerve and causes the cells to die. It is called optic nerve atrophy.

Although the optic nerve is part of the eye, it is considered part of the central nervous system. The optic nerve transmits special sensory information for sight. It is one of the two nerves that do not join with the brainstem (the other is the olfactory nerve, CN I).

Embryologically, the optic nerve develops from the optic vesicle, a part of the forebrain. Therefore, the entire nerve can be considered part of the central nervous system and as a result, the examination of the optic nerve (usually performed through ophthalmoscopy) allows an assessment of intracranial health.

Due to its unique anatomical relationship with the brain, the optic nerve is surrounded by cranial meninges (not by epi-, peri- and endoneurium like most other nerves).

Anatomical Course

The anatomical course of the optic nerve describes the transmission of special sensory information from the retina of the eye to the primary visual cortex of the brain. It can be divided into extracranial (outside the cranial cavity), and the intracranial parts.

Extracraneal

The optic nerve is formed by the convergence of the axons of the ganglion cells of the retina. These cells in turn receive impulses from the photoreceptors of the eye (rods and cones).

After its formation, the nerve leaves the bony orbit through the optic canal, a passage through the sphenoid. It is introduced into the cranial cavity, which runs along the surface of the middle cranial fossa (very close to the pituitary gland).

Intracranial (The visual path)

Within the middle cranial fossa, the optic nerves of each eye come together to form the optic chiasm. In the chiasm, the fibers of the nasal (medial) half of each retina are crossed, forming the optic tracts:

  • Left optic tract – contains fibers from the left temporal (lateral) retina and the right nasal retina (median).
  • Right optic treatment – contains fibers from the right temporal retina, and the left nasal retina.
  • Each optic tract travels to its corresponding cerebral hemisphere to reach the Lateral Geniculate Nucleus (NGL), a relay system located in the thalamus; The synapse fibers here.

The axons of the NGL then carry visual information through a path known as optical radiation. The road itself can be divided into:

  1. Superior optical radiation: transports the fibers of the upper quadrants of the retina (corresponding to the quadrants of the lower visual field). Travel through the parietal lobe to reach the visual cortex.
  2. Lower optical radiation: transports the fibers of the lower retinal quadrants (corresponding to the upper quadrants of the visual field).

This, in turn, runs through the temporal lobe, through a path known as the Meyers loop, to reach the visual cortex. Once in the visual cortex, the brain processes the sensory data and responds appropriately.

Where does the optic nerve start?

The optic nerve begins in the optic disc, a structure that is 1.5 mm (0.06 in) in diameter and located in the back of the eye.

The optic disc is formed from the convergence of the fibers of the exit ganglion cells (called axons) as they pass outside the eye.

When the nerve emerges from the back of the eye, it passes through the rest of the posterior orbit (eye socket) and through the optic bone channel to emerge intracranially in the lower part of the front of the brain.

At this point the optic nerve of each eye meets and forms an X-shaped structure called the optic chiasm.

Here, approximately half of the nerve fibers in each eye continue on the same side of the brain and the remaining nerve fibers cross in the chiasm to join the fibers of the opposite eye on the other side of the brain.

This arrangement is essential to produce binocular vision.

Posterior to the optic chiasm, the nerve fibers travel in optical frames to various portions of the brain-predominantly the lateral geniculate nuclei.

The fibers of the lateral geniculate nuclei form the optical radiations that are directed towards the visual cortex located in the occipital lobes in the posterior part of the brain.

Some nerve fibers leave the optic tract without entering the lateral geniculate nuclei and instead enter the brain stem to provide information that ultimately determines the size of the pupil.

The retina, the optic disc, the optic nerve, the optic chiasm, the optic tracts, the optical radiations and the visual centers of the brain are topographically organized to correspond to particular areas of the visual field.

Therefore, damage, alteration or pressure on particular portions of these structures can produce characteristic deficits in a person’s visual field. The affected person may or may not notice these visual field defects.