Anatomy-Lexicon

The optic nerve

definition

As an optic nerve (med. Optic nerve) is the term used to describe the strand of "nerve fibers" that is attached to the retina (lat. retina) transmits signals generated by the eye to the brain. Strictly speaking, it is the optic nerve, which doctors call the nerve (Latin for nerve) Opticus is called, not a real nerve, but a "path" of the brain, since the retina of the eye is in the Embryonic development represents a protuberance of the brain.

Course of the optic nerve

The retina of the eye consists of several layers, the outermost of which is the layer of the visual receptors, the rods and cones. Several cell layers with switching stations for the electrical signals of the sensory cells caused by the light are connected to the inside.

The fibers of the so-called ganglion cells, which are located in the innermost cell layer of the retina, form the actual optic nerve. The place where these fibers collect as the optic nerve leave the eye is called papilla (lat. Optic nerve papilla) and is about 15 ° from the center of each eye to the nose. Since the fibers have to break through the layer of light receptors in order to get to the outside, the area of the papilla is not sensitive to light and is also known as the "blind spot".

After leaving the eyeball, the optic nerve passes through the fatty tissue located in the eye socket between the muscles of the eye and passes through an opening (Optic Canal) into the skull. In the skull, the optic nerves of both eyes form the intersection of the visual pathways (Optic nerve chiasm), an area where the fibers that carry signals from the halves of the eyes facing the nose cross to the other side. Due to the anatomical proximity of this junction to the pituitary gland, the visual path junction is of certain importance in the diagnosis of certain brain tumors. In addition, injuries to the optic nerve before and after the junction produce different deficits in the visual field, which enables the doctor to assess the location of the damage with little effort.

In the further course, the fibers of the left halves of both eyes now pull in the left optic nerve and the fibers of the right halves of both eyes in the right optic nerve. Since the nerves emerging from the intersection now enter the brain (one in each half of the brain), after the intersection of the visual tract one no longer speaks of the optic nerve but of the "visual tract" (lat. Optic Tract).

The refraction of light by the lens of the eye means that the information from the other side of the field of vision arrives in each half of the brain. Everything we see to the right of the middle of our field of vision is processed in the left hemisphere and vice versa. The fibers of the optic nerve find their end in the cerebral cortex at the back of the head, where the information processing of what is perceived takes place.

Further information on the subject can be found at: Visual pathway

Anatomy of the eye

Illustration: Horizontal section through the left eyeball, seen from below

Cornea - Cornea
Dermis - Sclera
Iris - iris
Radiant Bodies - Corpus ciliary
Choroid - Choroid
Retina - retina
Anterior chamber of the eye -
Camera anterior
Chamber angle -
Angulus irodocomealis
Posterior chamber of the eye -
Camera posterior
Eye lens - Lens
Vitreous - Corpus vitreum
Yellow spot - Macula lutea
Blind spot -
Discus nervi optici
Optic nerve (2nd cranial nerve) -
Optic nerve
Main line of sight - Axis opticus
Axis of the eyeball - Axis bulbi
Lateral rectus eye muscle -
Lateral rectus muscle
Inner rectus eye muscle -
Medial rectus muscle

You can find an overview of all Dr-Gumpert images at: medical illustrations

Task of the optic nerve

As with all nerves, the basic job of the optic nerve is to carry electrical signals. The conversion of external light impressions into these electrical signals takes place through a series of biochemical processes within the nerve cells of the retina. From there they are then guided through the optic nerve to those parts of the brain that are responsible for processing the information it contains - the visual center.

Read more on the topic: How does vision work?

Function of the optic nerve

On the way from the sensory organ of the eye to the primary processing areas in the brain, the electrical signals of the perceived pass through four stations, at which they are switched from one nerve cell to the next.

The first two switchings take place in the retina of the eye. The information then leaves the eye with the extensions of the third nerve cell. Around a million of these nerve fibers are bundled here to form the actual optic nerve. At the intersection of the Sehbahn (Optic chiasm) the fibers of the right and left optic nerves meet. This is where the nasal part of the fibers crosses to the opposite side. From this point on, the information from one half of the face is bundled into the downstream processing centers. By definition, the course of the optic nerve ends with the crossing of the fibers. If the visual pathway is damaged in this area, the so-called chiasm syndrome occurs.

The fibers run as what is known as the optic tract towards the cerebral cortex, where they are processed either directly or after being forwarded again. The nerve fibers of the optic tract are involved in the function of the pupillary reflex: If strong light falls into an eye, the pupil of both the illuminated and the non-illuminated eye narrows. This reflex is triggered by a special interconnection of the optic nerve fibers with the muscle responsible for the constriction of the pupil (M. sphincter pupillae) realized.

How is the optic nerve examined?

When examining the optic nerve, visual acuity, field of vision, and fundus are usually checked.

Visual acuity can be checked using standardized writing boards. These must be read from a distance of five meters, whereby the font size becomes smaller with each new line. The visual acuity can then be calculated from the line that the patient can just barely read and the distance.

Read more on the topic:

Eye test
Examination of visual acuity

The doctor sits in front of the patient and asks him to fix a point, for example the doctor's nose, to check the field of vision. Now the doctor brings his hands to the edge of the field of vision by stretching out his arms and checks alternately in all directions to see when the patient perceives the movement of his fingers. Special devices, so-called perimeters, can be used to determine minor visual field defects.

Read more on the topic: Examination of the visual field

During the fundus examination, the doctor primarily assesses the point of entry of the optic nerve (papilla) with regard to their shape, border, color and any bleeding, which can provide information about diseases of the optic nerve. The small vessels in the fundus are also checked for changes.

What happens during an examination of the optic nerve?

An optic nerve examination usually means a so-called fundoscopy or ophthalmoscopy. This examination is also known as an ophthalmoscope or an ocular fundus. First of all, special eye drops are administered to ensure that the pupil dilates so that the examining doctor can carry out a full examination. Then the doctor can use a special device close to the eye and a system of magnifying glass and light source to look at the papilla, i.e. the opening of the optic nerve on the eye, and determine any damage. Some patients find this examination a little uncomfortable, but it is usually not painful.

Further diagnostic options in which the optic nerve is (also) examined are, for example, a computed tomography (CT) or a magnetic resonance examination (MRT). Above all, the thickness of the optic nerve and the presence of any injuries can be checked. However, due to the radiation exposure and costs, these procedures are not part of a normal examination of the optic nerve.

Read more on the subject at: Opthalmoscopy - the ophthalmoscopy

What happens when the optic nerve is measured?

The opening of the optic nerve on the eye, i.e. the papilla, can be performed, for example, as part of an optical coherence tomography (OCT for short). This is an imaging examination that shows the retina (retina) together with the papilla. At the end, the doctor receives an image of the retina with its various layers and the part of the retina where the optic nerve enters. Here the diameter can be determined and, depending on the location and extent, possible damage can be diagnosed.
The OCT examination is carried out using a special device and can be compared to a photo without a flash. The whole thing only takes a few minutes and is not painful. The OCT examination is usually not covered by the statutory health insurance.

Diseases of the optic nerve

The most common causes of damage to the optic nerve are accidents or acts of violence (traffic accidents or the like) in which the optic nerve is squeezed or pulled, for example when entering the skull. Even if there is bleeding into the eye socket (e.g. after punching the eye), an increase in pressure can cause the nerve fibers to be crushed.

Bacterial or viral infections of the eye socket (Orbital phlegmon) of various origins can also lead to damage to the optic nerve. In the context of multiple sclerosis, in the course of which different structures of the central nervous system can be affected, it is not uncommon for damage to the optic nerve with visual field defects.

Read more on the subject: Optic nerve inflammation in multiple sclerosis

As part of a green star (glaucoma) there is an increase in pressure in the eye, which squeezes the fine vessels that supply the retina and the optic nerves. The undersupply leads to irreversible damage to the affected cells after a few hours with permanent visual field deficits.

Various brain tumors can cause reversible and irreversible damage by applying pressure to the optic nerves. Pituitary gland tumors (Pituitary gland) are best suited to this due to their close relationship to the optic nerve and call the characteristic picture of "blinker blindness" (bitemporal hemianopia), as the fibers running in the junction of the visual pathway are particularly affected.

Overview of the various diseases of the optic nerve

The optic nerve can be affected by different diseases. An inflammation of the optic nerve can be caused, for example, by an infection. In about 30% of the cases, the inflammation of the optic nerve is also a symptom of multiple sclerosis. Optic nerve congestion or papillae is a swelling of the part of the optic nerve that opens directly at the eye. A so-called optic nerve infarction describes the closure of the artery that supplies the optic nerve head. In the event of an injury to the optic nerve, the field of vision may be impaired or even blindness may occur, depending on the extent. Optic atrophy describes a loss of nerve cell cords, which is usually irreversible and can also lead to complete blindness. Tumor diseases can also play a role. These can either come from outside and compress the optic nerve or arise on the optic nerve itself.

What happens if the optic nerve is injured?

Injury to the optic nerve is usually rare because the optic nerve is behind the eye and is therefore not as prone to injury as other parts of the eye. The injury occurs more often as a result of bruises (for example in the context of a swelling eyeball) or, for example, in the event of a traumatic brain injury. Occasionally, burns also occur, which can occur, for example, from looking directly into the sun for a long time. This can be amplified by using binoculars or the like.

Depending on the location of the injury, this can have different consequences. If, for example, the papilla, i.e. the opening of the optic nerve at the eye, is damaged, this can sometimes lead to complete blindness. If, on the other hand, only parts of the nerve fibers are injured, visual impairments or impairments can occur as a result.

Optic nerve inflammation

Inflammations of the optic nerve are divided into two basic types according to their location. If the inflammation occurs at the point of entry (papilla) of the optic nerve in the eyeball, it is called papillitis. On the other hand, if it is outside the eyeball (Globe) localized, one speaks of a retrobulbar inflammation or retrobulbar neuritis. The causes for both types of inflammation can be varied. Often there is an allergic reaction or impaired function of the body's immune cells. However, inflammatory processes from adjacent structures such as the paranasal sinuses or the base of the skull can also spread to the optic nerve. Other causes can be infectious diseases such as viral infections or borreliosis as well as harmful substances such as methanol, lead or quinine (in medicines or as bitter substances in food).

Read more on the topic: Causes of Optic Nerve Inflammation

In rare cases, retrobulbar inflammation can be an early symptom of multiple sclerosis.

Read more on the topic: Optic nerve inflammation in multiple sclerosis

The inflammation usually manifests itself in a very strong and sudden decrease in visual acuity and a dull pain behind the eye, which is intensified by pressure on the eyeball. From the outside, however, no irritation to the eye can be seen.

Read more on the topic: Symptoms of optic nerve inflammation

To detect papillitis, the doctor will do a fundus of the eye, where he will examine the papilla for signs of inflammation or bleeding. In the case of retrobulbar inflammation, a special EEG is usually carried out to check the electrical conduction within the nerve and thus its function - this is technically referred to as visual evoked potentials (VEPs). Optic nerve inflammation is treated with cortisone, which is administered directly into the bloodstream over several days. The success of the therapy depends on the underlying disease. Complete healing can be achieved, but damage to the nerve cells usually remains and thus a permanent reduction in visual acuity.

Read more on the topic: Optic nerve inflammation

Optic nerve atrophy

Optic nerve atrophy is unfortunately a mostly irreversible loss of nerve cells in the optic nerve. This can arise from a wide variety of causes. Examples of this are toxic damage such as alcohol or drugs, reduced arterial blood flow due to an arterial blockage, an inflammatory change due to, for example, a syphilis infection or the hereditary disease of liver optic atrophy. Optic nerve atrophy can lead to deterioration in vision, color perception disorders and even blindness. Since the damage is irreparable, therapy consists only in preventing the atrophy from progressing and treating the underlying disease if it is present.

Optic nerve tumors

The optic nerve can have different types of Tumors develop. The distinction is made on the basis of the type of tissue from which the respective tumor cells are derived.

Both Neurinoma these are the mantle cells of the nerve, the so-called Schwann cells. This type of tumor is benign, but can become a problem if it exerts pressure on the nerves due to its space-consuming growth and damages them.

They also develop from the nerve sheaths Neurofibromas. However, these are usually a side effect of the hereditary disease Neurofibromatosis Type 1, which is associated with further symptoms and organ involvement. They are primarily harmless, but carry a certain risk of degeneration.

Since the optic nerve, as an outgrowth of the brain, is also affected by the Meninges is surrounded, tumors can also originate from these, so-called Meningiomas. These grow very slowly and usually occur in middle age. Furthermore you can Gliomas develop from the supporting tissue of the nerves. These also show a rather slow growth, but mostly occur in children.

The therapy For all types of tumors, it depends primarily on the location and whether they cause discomfort or restrictions. Most of the time they are easily accessible surgically removed. If this is not possible, you can Radiation and chemotherapy can be used.

Swelling of the optic nerve

A swelling in the optic nerve can have different causes. If the optic nerve itself is swollen, this is usually a sign of inflammation. The inflammation of the optic nerve and the associated swelling can be caused by infections such as syphilis (syphilis), Sarcoid, or a fungal infection. However, it can also be caused by systemic multiple sclerosis. Tumorous swellings can also occur on the optic nerve. The symptoms are very diverse, depending on the localization and severity of the swelling and range from slight visual impairments to color perception disorders to complete blindness with severe compression of the optic nerve due to the swelling.

What happens to the optic nerve in glaucoma?

In glaucoma, also known as glaucoma, increased internal pressure in the eye leads to long-term damage to the papilla, i.e. the opening of the optic nerve on the eye. The causes of the increase in intraocular pressure are not always clear. Risk factors can include diabetes mellitus, inflammation or certain medications. The main symptoms of glaucoma are a reduction in vision, as the direct compression of the papilla leads to a lack of transmission of visual perceptions to the brain. Glaucoma is often associated with pain and reddening of the eye and should be treated by a doctor as soon as possible.

Read more on the subject at: Green Star

How does a damaged optic nerve regenerate?

Injury to the optic nerve is a very sensitive issue in medicine, as the prognosis usually looks rather poor. So far, the opinion is that nerves are generally hardly able to regenerate. There are various studies that show, especially in animal models, that partial regeneration of the optic nerve can take place after an injury. However, these studies have so far hardly been transferable to humans. Therefore, in the case of a damaged optic nerve, the primary goal is to prevent further or progressive injury and to try to protect the optic nerve as much as possible. Often the damage caused by an injury is irreversible. This is due to the inability of the nerve cells in the optic nerve to divide and thereby replace other dead or injured cells.