Optic nerve: parts, course and related diseases.

Let's see how one of the most important cranial nerves, responsible for vision, works.

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

But in order to be able to process all this information, and indeed to be able to see in general, it is first necessary for the information captured by the eyes to reach the relevant brain nuclei. Y This would not be possible without the existence of the optic nerve, which we are going to talk about in the next section.which we are going to talk about next.

Optic nerve: basic description and location.

We give the name 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 one and a half million) of sensory type, not transmitting information to the eye but only receiving it from it.

This nerve can be located in a space between the posterior part of the eyeball, having one of its ends in the ganglion cells. one of its ends in the ganglion cells of the retina, on one side, and the optic chiasm on the other.. This small section, between 4 or 5 cm long, 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 decussate (i.e., the one from 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 cerebral cortex.

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

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

What is it used for?

The main function of the optic nerve, as you can 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 and interpret it.

First of all, the photoreceptor captures the external information, generating a series of electrochemical reactions.First, the photoreceptor captures the external information, generating a series of electrochemical reactions that in turn will transform the data into bioelectrical impulses that will activate the retinal ganglion cells, 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, in spite of being the nerve that is perhaps the most important when it comes to being able to see, its location in the retina 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 as it travels to the optic chiasm, it is true that different segments can be observed on its journey to the retina. different segments can be observed on its journey between the eye and the optic chiasm.. Among them are the following.

Intraocular segment

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

2. Intraorbital segment

This is the part of the optic nerve that goes from the exit of the eye to its exit from the ocular orbits. In this part the optic nerve passes around the musculature that controls the eye and the fat behind it. and the fat posterior to it.

3. Intracanacular segment

It is in this third segment that the optic nerve finally reaches the skull, together with the ophthalmic artery. For this purpose, the nerve will enter through an orifice called the optic foramen.. This area is one of the most sensitive and easily injured.

4. Intracranial segment

The last of the segments is the intracranial segment, in which the optic nerve is already is already completely inside the skull and travels to the optic chiasm. It is here that it receives the protection of the meninges.

Pathologies and problems associated with its injury

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

These include atrophy of the optic nerve resulting for example from neuropathy (e.g. due to metabolic problems such as diabetes), intoxication, meningitis (remember that some portions of the meninges cover this nerve, so that in case of inflammation they could compress and damage it), cerebrovascular accidents or tumors that generate pressure or destroy this nerve.

Another possibility is that the nerve itself becomes inflamed, a condition called optic neuritis, which is usually linked to infections and autoimmune problems. There may also be accumulations of substances that form so-called brushea, especially in the optic nerve head (the area where it starts in the blind spot).

Finally, and probably the best known and most frequent problem that can lead to blindness linked to the optic nerve, is glaucoma. This disease is caused by 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.
• Sanchez, F. (2001). The optic nerve and vision disorders. Integrative Medicine, 38 (9): 377-412. Elsevier.