Optic nerve: parts, course and related diseases

Sight is one of our most essential senses, being probably the most developed exteroceptive sense in the human being. Not surprisingly, we dedicate a large part of our brain to the processing of visual information, being able to perceive a wide variety of parameters such as color, shape, depth or luminosity with sharpness and precision notorious.

But to be able to process all that information, and indeed to be able to see in general, it is first necessary that the information that the eyes capture reaches the relevant brain nuclei. AND this would not be possible without the existence of the optic nerve, about which we are going to talk next.

• Related article: "The 11 parts of the eye and their functions"

Optic nerve: basic description and location

We give the name of the optic nerve to a tract or set of nerve fibers that go from the eye to the central nervous system and whose presence allows vision. This tract is part of the cranial nerves, specifically pair II, and consists of more than one million neurons (approximately It is estimated that around one and a half million) of a sensory type, not transmitting information to the eye but only receiving it from he.

This nerve can be located in a space between the back of the eyeball, having one of its ends in the ganglion cells of the retina, on the one hand, and the optic chiasm, on the other. This small section, between 4 and 5 cm in length, is of vital importance and without it we would not be able to see.

From the chiasm most of the fibers of the optic nerves of both eyes will decuse (that is, that of the left eye will pass to the right hemisphere and vice versa), forming a tract that will go to the lateral geniculate nucleus and from there to different nuclei of the cortex cerebral.

The optic nerve has the peculiarity that initially the fibers that are going to make it up (the neurons that connect with the cells ganglion cells) are not myelinated until they meet in the so-called optic disc or blind spot, an area where there are neither rods nor cones and from which the neurons will form the optic nerve itself, already myelinated in order to allow a fast and efficient transmission of the visual information.

Thus the optic nerve, which consists mainly of myelinated axons, is mainly white matter. Although it originates outside the skull (in the retina), once it has entered it and especially in the bony part, the optic nerve is covered and protected by the meninges.

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What is it for?

The main function of the optic nerve, as you may already guess, is to transmit the visual information that we capture through the photoreceptors of the retina to the rest of the brain in order to be able to process it and interpret it.

First the photoreceptor captures the external information, generating a series of electrochemical reactions that in turn will transform the data into bioelectric impulses that will activate ganglion cells of the retina, which in turn will travel to the blind spot where the nerve fibers unite to form the optic nerve, which will proceed to send the message.

Curiously, despite being perhaps the most important nerve when it comes to seeing its location in the retina, it is the one that causes the existence of our blind spot.

Parts of the optic nerve

Although the optic nerve is relatively small in size on its journey to the optic chiasm, the truth is that different segments can be seen on their journey between the eye and the chiasm. Among them, the following stand out.

1. Intraocular segment

This first segment of the optic nerve is the one that still passes inside the eye, in the section that goes from the ganglion cells to the blind spot and then passes through the lamina or cribriform zone, which crosses the sclera and choroid.

2. Intraorbital segment

It is the part of the optic nerve that runs from the exit of the eye to its exit from the eye sockets. In this part the nerve passes around the muscles that control the eye and the fat after it.

3. Intracanacular segment

It is in this third segment that the optic nerve finally reaches the skull, next to the ophthalmic artery. For this the nerve enter through a hole called the optic foramen. This area is one of the most sensitive and easy to injure.

4. Intracranial segment

The last of the segments is the intracranial, in which the optic nerve is already totally inside the skull and travels to the optic chiasm. This is where you receive the protection of the meninges.

Pathologies and problems associated with your injury

The optic nerve is one of the most important in our vision and without it, vision as such would not be possible. There are multiple possible conditions that can occur in this nerve and cause us either blindness or alterations and difficulties in vision.

Among them we can find the atrophy of the optic nerve derived for example from a neuropathy (for example derived from metabolic problems such as diabetes), an intoxication, meningitis (Remember that the meninges cover this nerve in some portions, so in case of inflammation they could compress and damage it), strokes or tumors that create pressure or destroy the nerve.

Another possibility is that the nerve itself becomes inflamed, a condition called optic neuritis that is often linked to infections and autoimmune problems. Accumulations of substances that form the so-called abrupt can also appear, especially in the head of the optic nerve (the area where it begins in the blind spot).

Finally, and probably the best known and most frequent problem that can cause blindness related to the optic nerve, is glaucoma. This disease is derived from a progressive increase in intraocular pressure, which progressively damages the nerve.

Bibliographic references:

• Miller, N.R. & Newman. N.J. (eds) (2005).. Walsh and Hoyt´s clinical neuro-opthalmology. 6th edition. Baltimore: Williams & Wilkins, 385-430.
• Sánchez, F. (2001). The optic nerve and vision disorders. Integral Medicine, 38 (9): 377-412. Elsevier.