What is the optic nerve?
Like the retina, the optic nerve is part of the brain. It is about four to five centimeters long and begins at the optic disc in the eye (discus nervi optici). This is a whitish, disc-shaped area at the back of the eye where the nerve endings of the retina (retina) bundle together to form the optic nerve. There, at the posterior pole of the eye, is an opening about three and a half millimeters in size for the optic nerve to pass through the sclera (white sclera of the eye).
However, it is not only the retinal nerve endings that collect in the optic disc (eye) – it is also where the retinal vessels enter and exit in a depression located in the center. For this reason, there is no vision at this point (no photoreceptors). Physicians therefore also speak of the “blind spot”.
The nerve fibers that come from the peripheral area of the retina are also located in the optic nerve in the peripheral area. The fibers from the central retinal area and the macula (the area of sharpest vision) run inside the optic nerve. All nerve fibers in the optic nerve are enclosed by protective myelin sheaths.
Optic nerve junction
In the cranial cavity in front of the pituitary gland, the optic nerves of the two eyes join to form an optic nerve junction (optic chiasm). However, the nerve fibers in the two optic nerves are only partially crossed: the fibers coming from the middle (nasal) halves of the retina are crossed; the fibers coming from the outer (temporal) retinal areas are not crossed.
This means that after crossing, the fibers from the left retinal hemispheres of both eyes move to the left cerebral hemisphere, and the fibers from the right retinal hemispheres move to the right cerebral hemisphere.
After the crossing of the two optic nerves, doctors speak of the “tractus opticus”.
The optic nerve function is primarily to transmit the electromagnetic (light) impulses hitting the retina to the visual center in the cerebral cortex. There, the information arriving from the eyes is processed into an image.
Some of the fibers of the optic tract are also important for the pupillary reflex: Normally, both pupils are equally wide. When stronger light hits one eye, the pupil narrows not only in that eye, but also simultaneously in the other, non-illuminated eye.
What problems can the optic nerve cause?
In case of damage in the area of a tractus opticus, there is a visual field loss (scotoma) in the affected half of the retina in both eyes (homonymous hemianopsia). Damage to the optic chiasm results in heteronymous hemianopsia: The visual field loss affects either the lateral half (towards the temple) or the medial half (towards the nose) in both eyes.
Optic neuritis (inflammation of the optic nerve) leads to visual impairment and can also result in blindness.
In optic atrophy, optic nerve fibers are lost – either in only one optic nerve or in both optic nerves. This can happen, for example, as a result of an injury or optic neuritis, or be the result of medication, nicotine, or low-grade alcohol. Increased pressure (e.g. in the case of tumor disease or “hydrocephalus”) can also damage the optic nerve in such a way that nerve fibers die.
