The optic nerve, its structure and functions. Features of the structure and function of the optic nerve

afferent fibres. The optic nerve contains about 1.2 million afferent nerve fibers coming from retinal ganglion cells. Most of the fibers form synapses in the lateral geniculate body, although some of them go to other centers, mainly to the pretectal nuclei of the midbrain. About 1/3 of the fibers correspond to the central 5 fields of view. Fibrous septa extending from the pia mater separate the fibers optic nerve for about 600 bundles (each with 2000 fibers).
Oligodendrocytes provide myelination of axons. Congenital myelination of retinal nerve fibers is explained by the abnormal intraocular distribution of these cells.
Microglia are immunocompetent phagocytic cells, possibly regulating apoptosis ("programmed" death) of retinal ganglion cells.
Astrocytes line the space between axons and other structures. When axons die in optic nerve atrophy, astrocytes fill the resulting spaces.
Surrounding shells
- pia mater - soft (inner) meninges containing blood vessels;
- the subarachnoid space is a continuation of the subarachnoid space of the brain and contains cerebrospinal fluid;
- outer shell is divided into arachnoid and hard shells, the latter continues into the sclera. Surgical fenestration of the optic nerve involves incisions in the outer sheath.

Axoplasmic transport

Axoplasmic transport - movement cytoplasmic organelles in a neuron between the cell body and the synaptic ending. Orthograde transport consists in movement from cell body to the synapse" and retrograde - in the opposite direction. Rapid axoplasmic transport is an active process that requires oxygen and ATP energy. Axoplasmic current can be stopped by various reasons, including hypoxia and toxins that affect the formation of ATP. The cotton-like foci of the retina are the result of the accumulation of organelles when the axoplasmic current stops between the retinal ganglion cells and their synaptic endings. The stagnant disc also develops when the axoplasmic current stops at the level of the cribriform plate.

The optic nerve is covered by three meninges: hard, arachnoid and soft. In the center of the optic nerve, in the nearest segment to the eye, the vascular bundle of the central vessels of the retina passes. Along the axis of the nerve, a connective tissue cord is visible, surrounding central artery and a vein. The optic nerve itself does not have half frequencies of the central vessels of any branch.

The optic nerve is like a cable. It consists of axial processes of all ganglion cells of the retinal rim. Their number reaches approximately one million. All fibers of the optic nerve exit the eye into the orbit through a hole in the cribriform plate of the sclera. At the point of exit, they fill the opening of the sclera, forming the so-called optic nerve papilla, or optic disc, because in normal condition the optic disc lies on the same level with the retina, only the congestive optic papilla protrudes above the level of the retina, which is pathological condition- a sign of increased intracranial pressure. In the center of the optic nerve head, the exit and branching of the central retinal vessels are visible. The color of the disk is paler than the surrounding background (with ophthalmoscopy), since the choroid and pigment epithelium are absent in this place. The disc has a lively pale pink color, more pink on the nasal side, from where the vascular bundle more often emerges. Pathological processes developing in the optic nerve, as in all organs, are closely related to its structure:

many capillaries in the partitions surrounding the bundles of the optic nerve, and its particular sensitivity to toxins create conditions for exposure to the fibers of the optic nerve infection (for example, influenza) and a number of toxic substances ( methyl alcohol, nicotine, sometimes plasmacide, etc.);
with increases in intraocular pressure weak point the optic nerve disc turns out to be (it closes the holes in the dense sclera, like a loose plug), therefore, with glaucoma, the optic nerve disc is “pressed”, a fossa is formed.
excavation of the optic disc with its atrophy from pressure;
elevated intracranial pressure, on the contrary, delaying the outflow of fluid through the intershell space, causes compression of the optic nerve, stagnation of fluid and swelling of the interstitial substance of the optic nerve, which gives a picture of a congestive nipple.

Hemo- and hydrodynamic shifts also have an adverse effect on the optic nerve head. They lead to a decrease in intraocular pressure. Diagnosis of diseases of the optic nerve is based on the data of ophthalmoscopy of the fundus, perimetry, fluorescein angiography, electroencephalographic studies.

A change in the optic nerve is necessarily accompanied by a violation of the function of the central and peripheral vision, limiting the field of view for colors and reducing twilight vision. Diseases of the optic nerve are very numerous and varied. They are inflammatory, degenerative and allergic character. There are also anomalies in the development of the optic nerve and tumors.

Symptoms of damage to the optic nerve

A decrease in visual acuity when fixing near and far objects is often noted (may occur with other diseases).
Afferent pupillary defect.
Dyschromatopsia (impaired color vision, mainly in red and green color). A simple way to detect unilateral color vision impairment is to ask the patient to compare the color of a red object seen by each eye. A more accurate assessment requires the use of Ishihara pseudo-isochromatic tables, the City University test, or the 100-shade Farnsworth-Munscll test.
Decreased light sensitivity, which may persist after the restoration of normal visual acuity (for example, after suffering optic neuritis). This is best defined as follows:
- light from an indirect ophthalmoscope illuminates first the healthy eye, and then the eye with suspected damage to the optic nerve;
- the patient is asked if the light is symmetrically bright for both eyes;
- the patient reports that the light seems less bright to him in the diseased eye;
- the patient is asked to determine the relative brightness of the light seen by the diseased eye compared to the healthy one.
Decreased contrast sensitivity is defined as follows: the patient is asked to recognize the gratings of gradually increasing contrast of different spatial frequencies (Arden tables). This is a very sensitive, but not specific for the pathology of the optic nerve indicator of vision loss. Contrast sensitivity can also be tested using Pelli-Robson charts, which read letters of gradually increasing contrast (grouped in threes).
Visual field defects that vary with disease include diffuse depression in the center of the visual field, central and centrocecal scotomas, nerve bundle defect, and altitudinal defect.

Optic disc changes

There is no direct correlation between the type of optic nerve head and visual functions. In acquired diseases of the optic nerve, 4 main conditions are observed.

The normal appearance of the disc is often characteristic of retrobulbar neuritis, initial stage optic neuropathy Leber and compression.
Disc edema is a sign of congestive disc, anterior ischemic optic neuropathy, papillitis and acute stage optic neuropathy Leber. Disc edema can also appear in compression lesions before optic nerve atrophy develops.
Opticociliary shunts are retino-choroidal venous collaterals on the optic nerve head that develop as a compensatory mechanism for chronic venous compression. This is often caused by meningioma and sometimes by optic nerve glioma.
Optic nerve atrophy is the outcome of almost any of the aforementioned clinical conditions.

Special Studies

Manual kinetic perimetry according to Goldmann is useful for diagnosing neuro-ophthalmic diseases, since allows you to determine the state of the peripheral field of vision.
Automatic perimetry determines the threshold sensitivity of the retina to a static object. Most useful are programs that test the central 30" with objects spanning the vertical meridian (eg Humphrey 30-2).
MPT is the method of choice for visualization of the optic nerves. The orbital part of the optic nerve is better seen when the bright signal from adipose tissue is eliminated on T1-weighted tomograms. The intracanalicular and intracranial parts are better visualized on MRI than on CT, as there are no bone artifacts.
Visual evoked potentials - registration electrical activity visual cortex caused by retinal stimulation. The stimuli are either a flash of light (flash VEP) or a black and white checkerboard pattern reversing on the screen (VEP pattern). Several electrical responses are obtained, which are averaged by the computer, and both the latency (increase) and the amplitude of the VEP are evaluated. In optic neuropathy, both parameters are changed (latency increases, VEP amplitude decreases).
Fluorescein angiography may be useful in differentiating a congestive disc, in which there is dye leakage on the disc from disc drusen, when autofluorescence is observed.

Responsible for the optic nerve important function. It is responsible for the transmission of visual information that is projected onto the retina. Then it enters the visual center of the brain and is perceived by us as an image. Vision is extremely important for a person, as it provides up to 90% of information about the outside world. How is the optic nerve arranged, and what do its pathologies lead to?

The organs of vision begin to form as early as the 5th week of pregnancy, which corresponds to the third week of embryo development. At this time, the laying of the optic nerve begins, which is the second of 12 pairs of nerves lying in the cranial region. It develops in the area between the eyeball and the diencephalon. Visually, it represents the stem of a glass, the bowl of which is an eyeball.

The function of the optic nerve is the direct transmission of impulses from photosensitive receptors to the thalamus, the visual center in the brain. This is a special optical neuron that works separately from other neurons. Its difference is that it has no pain receptors. Therefore, the diagnosis of diseases of the optic nerve is difficult.

As the fetus grows, the nerve stretches along with the meninges, which will eventually provide a secure sheath for the optic bundle. The case differs from the shell in that it completely isolates the through hole from the brain. The shell, however, only closely adjoins the beam and consists of connective tissue.

Structure

What is the structure of the optic nerve (ON)? It begins with the optic disc - an area on the retina pierced by nerve fibers. Then they are assembled into nerve bundles, the structure of which consists of 4 sections:

intrabulbar(inside the eye). It is located between the disc and the sclera at the place from which the nerve exits. The length of the section is about 1.5 mm. It is formed by extended nerve endings of the retina, formed by ganglion cells. In this segment, the nerve fibers are devoid of sheaths.
retrobulbar(or orbital segment). Has a length of approximately 33 mm. It originates from the cribriform scleral plate and thickens to 4 mm due to the sheath around it formed by the three meninges. The fibers also contain myelin.
intracanal plot. Located between the orbital and intracranial segments of the optic nerve. It is approximately 4 mm long. At this intervals of the optic nerve sheath fuse with the periosteum. In this case, the distance between the protective sheaths is reduced, which leads to a decrease in the thickness of the nerve fibers.
intracranial(or intracranial area). It originates at the end of the optic canal and extends all the way to the chiasm, the place where the optic fibers intertwine. The length of the segment is from 4 to 16 mm. In this area, the nerves are flattened, their outlines become ovoid.

After the chiasm, the place where the nerves of the right and left eyes cross each other, the visual path begins. It's meant to be delivered nerve impulses to the visual center, represented by a process of the brain called the thalamus.

Structure of the optic nerve

The optic nerve is made up of numerous nerve fibers. They originate from the third neuron of the retina. The third neurons have long processes that are collected in the fundus in a bundle. They conduct electrical impulses from the photosensitive receptors of the retina to the following fibers, which form the optic nerve.

The optic disc, or ONH, is located at the bottom of the eye and forms a papilla that stands out visually. The retina in the region of the disc does not have light-receptive cells, since the axons of the first neuron are located above it. They cover the photosensitive cell layer. This place is called the blind spot. The position of the blind spots on the right and left eyes does not match. Therefore, the brain, receiving an image from two eyes at once, corrects the picture, and a person does not notice blind spots at all. But they can be detected with the help of special tests.

To detect a blind spot:

Close your right eye.
Look at the picture below.
Fix the gaze of the left eye on the circled cross.
Move away or approach the monitor until the cross on the left disappears from the field of view. This is the blind spot.

The optic disc lies below the zone responsible for maximum visual acuity. It is on it that the concentration of photosensitive receptors in the retina is maximum.

The structure and purpose of the sheaths of the optic nerve

The GN is externally covered with three meninges. They begin to cover the nerve fibers at the exit from the sclera. In this place, the myelin sheath is immediately included in the composition of the nervous tissue. The optic tract is protected by it along its entire length up to the center of vision in the brain. And thanks to the meninges, the optic nerve thickens and reaches 3.7-4.7 mm in diameter.

Brain layers:

Soft;
gossamer;
Solid.

All three layers at one end are in close contact with the sclera, and at the other end with the visual structures in the brain, being their continuation.

The outer covering of the optic nerve forms a hard shell. It is the thickest of the three layers and consists mainly of coarse, to a lesser extent, elastic collagen. The outer side consists of the endothelial layer of cells. Where the hard shell connects to the sclera, there are blood vessels and trunks of ciliary nerve fibers penetrating this sclera.

The first shell covering the GL is soft. It and the nerve are separated only by a small glial gap. In places where the fibers are closely intertwined with the soft shell, partitions are formed - septa. It is they who divide the nerve into separate bundles, thanks to which it acquires greater strength.

The arachnoid medulla lies between the pia mater and the dura mater. It is a thin collagen layer consisting of flat cells. It is connected to the soft shell by trabeculae. The result is a web-like network. Trabeculae are formed by mesothelial and collagen cells. The arachnoid usually has two mesothelial layers, but sometimes there may be more or less.

Chiasma

After the optic nerve has passed through the canal located inside sphenoid bone, it is converted to chiasma. This is the name of the place where the threads of nerve fibers partially intersect and mix with each other. The width and length of the cross is about 1 centimeter. The thickness of the chiasma is not more than 0.5 cm. The structure of the neural chiasm is very complex. But it is thanks to the chiasm that the visual functions with some types of eye damage.

In the chiasm, fibers extending from the nasal part of the retina of the eye are directed in the opposite direction. And those fibers that pass from the temporal part continue their way along the same side. The result is a partial cross, which is endowed with an interesting property. If it is cut from front to back, then the image will not be obtained either on the left or on the right side.

The nerve bundle after passing through the chiasma is called the "optic tract". These are the same neurons, but they have only one task left - to deliver an impulse from the chiasma to the thalamus.

The thalamus and the path to the visual center

The optic tract is formed from the same neurons as the nerve of the eyeball. It originates from the chiasm and continues down to the subcortical zones of the visual center in diencephalon. The length of the optic tract is approximately 5 centimeters.

From the intersection of the nerve bundles, the visual path passes under the base of the temporal lobes of the brain and reaches the geniculate body and thalamus. It transmits information from the retina of its side. If the optic tract is damaged immediately after exiting the chiasm zone, then vision problems begin only on the side where the nerve bundle was damaged.

From the first neuron in the primary zone of the crankshaft, an electrical impulse is transmitted to the next neuron. An additional branch also departs from the visual pathway, which reaches the auxiliary subcortical zones of the thalamus. But in front of the geniculate body, it goes to the pupillary-motor and pupillary-sensitive nerve, and only then they go to the thalamus. This branch is designed to close the reflex networks of the friendly reaction of the pupils to light, beveling the eyeballs. It is also responsible for changing the focus on objects that are located at different distances from a person (accommodation).

Near the subcortical zone of the thalamus are the centers of balance, hearing, smell, and some other nerve nuclei. spinal cord and skulls. Basic behavior, such as responding to rapid movement, is ensured by the coordinated work of all these centers together. The thalamus has close connection with all brain structures. He takes part in the performance of visceral and somatic reflexes.

It is assumed that nerve impulses from the retina to the thalamus through the optic canal affect the sequence of periods of sleep and wakefulness, menstrual cycle, psycho-emotional state, autonomic regulation of organs, carbohydrate, lipid and water-salt exchanges synthesis of sex hormones and growth hormones.

The central channel transmits information about visual stimuli from the primary visual center to the cerebral hemispheres. The higher center responsible for vision is located in the cortex inside the occipital lobes, lingual gyrus and spur groove. Moreover, he receives an inverted picture of a mirror type. But he transforms it so that we see the world as it is.

Blood supply to the optic nerve

The frontal part of the optic nerve is powered by a system of shortened ciliary posterior arteries. The optic disc is divided into 4 sections, each of which is fed by different vessels:

The retinal zone of the disc is nourished by the retinal ciliary artery approaching it;
Tepolyarna zone is fed by branches coming from the choroidal vessels;
The prelaminar region of the optic disc receives nutrients with blood going through the choroidal vessels themselves;
The laminar zone of the optic disc receives nutrition and oxygen from the arterioles belonging to the peripapillary choroid.

From the frontal section of the optic nerve, blood flows out due to the central vein passing in the retina of the eye. The ONH in the prepalaminar zone transmits venous blood to the peripapillary veins with high concentration decomposition products and carbon dioxide. Blood from them enters the vorticose veins of the eye.

The optic canal of the nerve carries blood to the posterior central vein. After leaving the trunk of the optic nerve, it enters the cavernous sinus. This vein usually bleeds into nervous tissue eyes with damage to the bone canal.

The segment of the optic nerve inside the skull is enriched with nutrients through a branched network of vessels formed by the anterior cerebral and internal carotid arteries. The anterior communicating and ophthalmic arteries also participate in nutrition.

Diseases of the optic nerve and consequences

Pathologies of the optic nerve may be associated with its incorrect formation, involvement in inflammatory process, and mechanical injury or organic damage to the fibers. Any violations cause serious consequences, in the worst case, irreversible blindness develops.

Possible pathologies of the optic nerve:

Anomalies in the formation of the ONH;
Inflammatory diseases of the peripheral bundles (intrabulbar and retrobulbar neuritis);
Congestive optic disc (edema with increased intracranial pressure);
Toxic damage to the optic canal;
Optochiasmatic arachnoiditis (inflammatory process affecting the meninges covering the nerve);
Ischemic neuropathy of the optic nerve (impaired blood supply).

Methods for diagnosing pathologies of the ONH and optic nerve:

Ophthalmoscopy of the visual disc to assess its boundaries, color, shape and condition of the vessels in it;
Optical coherence tomography, or OCT;
Campimetry to identify central scotomas in the field of view and calculate the size of the blind spot.

With the help of such studies, congenital anomalies can be detected:

Drusen of the optic disc;
Atrophy of the optic disc;
False neuritis;
Coloboma of the optic disc;
An increase in the size of the optic nerve disc;
Hypoplasia or aplasia of the disc.

Optic disc drusen are formed as a result of the formation of mucopolysaccharides and mucoproteins, leading to calcification of this nerve. They are found in every hundredth person. The disease progresses over time, leading to ischemic neuropathy, an enlarged blind spot, and impaired peripheral vision.

With the help of OCT and other diagnostic methods, acquired pathologies can also be detected: a congestive optic disc, its atrophy, or circulatory disorders.

ND is normal

The optic disc is normal:

Has a round or oval shape with a long vertical meridian;
Painted reddish or pink. In older people, the optic disc becomes yellow;
The nipple of the disc thickens towards the nasal edge, so from this side it seems that it is bright red. In the temporal part, the color is normally always paler. Excessive pallor may be due to myopic refraction;
clearly visible on the disk. pigment rings: choroidal and scleral;
The boundaries of the visual disk must be clear. The clearest border runs at the temporal edge;
The disc is normally located at the level of the retina;
The central vessels in the optic disc are clearly visible. Sometimes optociliary or cilioretinal branches can be seen.

The study of the structure of the optic nerve is very important for humanity. Thanks to the accumulated knowledge, the causes of many problems associated with vision have become known. And finding out the cause of the pathology is half the way to cure it. And for some patients, it became possible to see again thanks to an operation on the optic nerve, which would not have been possible without studying its structure and functions.

The optic nerve delivers nerve messages to the area of the brain responsible for processing and perceiving light information.

Initially, these signals are received on the retina of the eye by converting light pulses. The visual center itself is located in cortical structure brain, in its occipital lobe. The totality of all nerve fibers that make up the nerve exceeds one million. This is the very first area visual analyzer, from chiasm to sclera, its length is about thirty millimeters. The optic nerve is divided into several sections.

In prenatal development, the optic nerve, just like the retina, is formed from the same structures as the brain. Thus, we can say that the optic nerve is a continuation of the brain, advanced to the periphery, beyond the edge of the cranium. This nerve is different from the normal cranial nerves.

The ganglionic neurocytes of the retina are considered to be the beginning of the optic nerve, they are the main component of the optic disc, and it ends with the chiasm (the segment where the intersection of the nerves following from the two eyes is carried out). The S-shaped bend of the optic nerve does not allow it to stretch and injure in the event of a sharp change in the location of the eyeball.

The structure of the optic nerve

Intrabulbar (intraocular) segment.
The site is located from the disk of the optic nerve to the gap where the nerve comes out of the sclera. The length is about 1.5 mm. This segment is represented by long nerve endings of ganglion cells directly in the retina. They accumulate at the back of the eyeball and form the optic disc. Those axons that are located at the edges form it outer part, and the rest, as they join, take on a more central location. In the intraocular segment of the ON, the optic fibers do not have myelin sheaths. In the central part of the optic disc, there is a recess (excavation), inside which the central vein and artery of the retina are located.
Retrobulbar (orbital) segment.
Its length has a length of about thirty-three millimeters. The department begins immediately behind the scleral cribriform plate. It is noteworthy that the optic nerve in this area becomes much thicker due to the attachment of the three meninges and the presence of myelin on the nerve fibers. Initially, the central retinal artery is not located inside the optic nerve. But approximately at an interval of 6-14 millimeters from the eyeball, it makes a smooth bend and comes to the optic nerve, then follows inside it, along the axis of the optic nerve trunk. This vessel is covered by a connective sheath along its entire length, which protects the nerve fibers from the pressure of the impulse of pulse impulses.
Intracanal department.
It occupies the gap between the orbital and intracranial entrances of the optic nerve canal. The length is approximately four millimeters. At this end outer covering The outer layer of the optic nerve smoothly comes and connects with the periosteum, and the gaps that exist between all the integumentary layers of the ON are reduced.
Intracranial (intracranial) segment.
The department is localized between the place of departure from the visual canal and the chiasma. Length from 4.22 to 16.22 mm. In this area, the nature of the optic nerve changes, it flattens and acquires an ovoid shape. Both nerves converge and form a crosshair - chiasma, it is covered with soft and arachnoid membranes. After the chiasm, the optic nerves go to the visual area of the brain, at this interval they are called visual pathways.

What functions are provided by the optic nerve

The optic nerve is the most important part of the entire process of converting light information. Its first and most significant function is to deliver visual messages from the retina to the areas of the brain responsible for vision. Even the smallest injury to this area can have severe complications and consequences.

Ruptures of nerve fibers threaten with loss of vision. Many pathologies are caused by structural changes in this area. This can lead to impaired visual acuity, hallucinations, and the disappearance of color fields.

Scheme of movement of the visual impulse

The photosensitive cells of the retina of the eye are cones and rods. The largest number of such cells is concentrated in the location yellow spot. The signals received by the light-sensitive cells follow first to the bipolar and then to the ganglion cells of the retina. The nerve endings of these cells form the optic nerve. A separate optic nerve at this gap includes fibers exclusively from its "own" eye.

At this interval, the optic nerve is formed by fibers from the outer, inner part of the retina, as well as fibers coming from the macula. These fibers represent the macular ligament of the optic nerve.

Having formed on the retina, through the axons of ganglionic neurocytes, the impulse moves to the optic nerve head, which is also on the retina. In the future, through the visual channel, each from its own edge, they enter cranium. After that, the optic nerves follow through the frontal parts of the brain, and then partially approach each other, forming a decussation. This part of the partial intersection of the optic nerves is called chiasm. Only the fibers that come from the inner halves of the retina cross. Nerve fibers coming from the outer part of the retina do not cross. Some fibers of the macular bundle also form a decussation.

After crossing, the left and right visual pathways appear, which absorb nerve fibers from two eyes: part of the non-crossed fibers coming from the “own” eye, and the remaining amount is made up of fibers from the second eye that got to this side as a result of the crossing. Therefore, each optic nerve after the chiasma has fibers from equivalent areas of the retina - right or left.

Further, the visual pathways move backwards and outwards, bypassing the brain stem, reach the visual areas in the subcortex of the brain. Neurons and axons are concentrated in these centers, which then go to the deep sections of the parietal and temporal lobes, but they follow different paths. As a result, the optic fibers are sent to the occipital lobe, where they reach the visual analyzer. In this area, the analysis and synthesis of the optical image and the identification of what is seen take place.

Sheaths of the optic nerve

Outside, the optic nerve is lined with three meninges. The initial segment of the optic nerve is formed immediately after its exit from the sclera. Here, the nerve immediately acquires a myelin sheath, which is preserved along its entire length. The diameter of the optic nerve grows from 3.7 mm to 4.7 mm due to the presence of these three medulla:

soft;
cobweb;
solid.

All these layers from one side are in close interaction with the sclera, and from the opposite side - with the structures of the brain and are their direct extrapolation.

The dura mater is the outermost covering of the optic nerve. It unites with the sclera, is characterized by a considerable thickness and is formed from coarse collagen formations, with a small admixture of elastic ones. The outer part of the membrane is lined with endothelial cells. In the epicenter of the connection of the hard shell and the sclera, there are many vessels and trunks of the ciliary nerves penetrating the sclera.

The trunk of the optic nerve itself is lined with a soft sheath, which is delimited from the nerve by only the thinnest glial gap. This layer is extremely tightly connected to the optic nerve. Between them, a lot of connective tissue partitions (septa) are identified, which divide the optic nerve into bundles. Septa go inside the nerve bundles, giving the optic nerve itself greater strength. Through these partitions, blood vessels enter the trunk of the optic nerve, they do not go inside the nerve bundles, therefore, the nutrition of individual nerve fibers is carried out through these glial partitions.

The vessels of the pia mater are not perforated, as are those located in the optic nerve. Intercellular connections are distinguishable between adjacent endothelial cells. The soft shell does not represent a barrier to metabolites, despite the presence of intercellular contacts.

The arachnoid is located between the hard and soft shells. It is represented by the thinnest layer collagen tissue which is lined with flat cells. Numerous trabeculae connect it to the pia mater, forming a network. Trabeculae are composed of collagen and mesothelial cells. The number of mesothelial layers varies, but usually there are two. If the trabecula carries blood vessel, then there are more such layers. The border of the arachnoid is located at the cribriform scleral plate, and it smoothly connects to the sclera. This membrane divides the intervaginal space into subarachnoid and subdural. The subarachnoid cavity ends at the sclera and is filled with subarachnoid fluid.

Optic disc (DND)

DNZ is represented by neural processes of ganglionic neurocytes of the retina. This is the junction of all optical fibers of the retina. The thickness of the nerve fibers and the retina itself increases as it approaches this area, so the disk protrudes slightly deeper into the eye and resembles a papilla.

DNZ is localized in the nasal region of the fundus and is a non-pulmonic nerve substance (according to the characteristics of the tissue structure). He has no brain surface layers, and in the optic fibers that form it, there is no myelin sheath. In the center of the disk there is a funnel-shaped depression into which the central artery enters and the central retinal vein exits. This place is called excavation or vascular funnel.

Normal ophthalmic picture of the optic disc

The optic disc has an elliptical or rounded shape, with a large vertical meridian.
The size of the optic disc can vary and is dependent on the method of ophthalmological testing.
A pinkish or slightly reddish color is considered normal. In old age, the presence of a yellowish color is possible.
The optic disc nipple is thicker towards the nasal margin, which is why it bow has a redder color than the temporal. Fine temporal part always a little paler. At nearsighted people The optic disc is generally paler and this is the norm.
ONH has clear edges, the temporal margin stands out more sharply.
Allocate the presence of scleral and choroidal rings.
The optic disc is localized, as a rule, at the level of the retina.
physiological excavation must be present.
On the optic disc itself, retinal vessels are distinguishable - central, cilioretinal and optociliary vessels.

Diseases caused by disorders in the optic nerve

Incorrect formation of anatomy, inflammation, organic lesions and injury can become a serious pathology and lead to grave consequences including blindness.

anomalies in the development of the ONH.
neuritis - retrobulbar and intrabulbar.
inflammation of the optic nerve.
ischemic neuropathy.
optochiasmal arachnoiditis.
congested optic disc.
toxic damage to the optic nerve.

How is the blood supply to the optic nerve

Through the system of short posterior ciliary arteries, the anterior part of the optic nerve is supplied. A. retinae centralis is responsible for the blood supply to the retinal part of the optic disc. Branches from the choroidal vessels feed the temporal region of this layer. Peripapillary choroidal vessels supply blood and nutrients to the prelaminar region of the optic nerve head. And its laminar part receives nutrients and oxygen thanks to the terminal arterioles of the peripapillary choroid.

From the frontal segment of the optic nerve, blood flow is organized through the participation of the central retinal vein. In the prelaminar section of the optic disc deoxygenated blood, saturated carbon dioxide and decay products, flows into the peripapillary veins, which deliver it to the ophthalmic vorticose veins. Blood enters the posterior central vein from the intracanal part of the optic nerve. Upon completion of the exit from the optic nerve trunk, it passes into the cavernous sinus. With injuries of the bone canal, it is this vein that usually causes hemorrhages into the nerve tissue. The intracranial segment is fed by a branched vascular network, formed by the internal carotid artery and the anterior cerebral artery, the ophthalmic artery and the anterior communicating artery are also involved.

The optic nerve (n. opticus) provides the transmission of nerve impulses caused by light irritation from the retina to the visual center in the cortex of the occipital lobe of the brain.

The structure and function of the optic nerve

Nerve fibers from sensitive cells the retinas eventually assemble at the posterior pole of the eye into the optic nerve. Total There are more than 1 million nerve fibers that form the optic nerve, but their number decreases with age. The location and course of nerve fibers from different areas of the retina has a strictly defined structure. As it approaches the optic nerve head (OND), the layer of nerve fibers thickens, and this place rises somewhat above the retina. Then, the fibers collected in the optic nerve head (OND) are bent at an angle of 90˚, forming the intraocular part of the optic nerve.

The optic disc has a diameter of about 1.75-2.0 mm and occupies an area of 2-3 mm. The projection zone of the optic disc in the field of view corresponds to the area of the blind spot. The blind spot was first discovered by the physicist E. Marriot in 1668.

The optic nerve starts at the optic nerve head and ends at the chiasm. The length of the optic nerve in an adult is from 35 to 55 mm. The optic nerve has an S-shaped bend, which prevents its tension when the eyeball moves. Almost the entire length of the optic nerve, like the brain, has three shells: hard, arachnoid and soft. The spaces between them are filled with a liquid of complex composition.

Topographically, the optic nerve is divided into 4 sections: intraocular, intraorbital, intratubular and intracranial.

The optic nerves of both eyes enter the cranial cavity, and joining in the region of the Turkish saddle, form a chiasma. In the region of the chiasm, a partial decussation of the optic nerve fibers is carried out. The fibers coming from the inner (nasal) halves of the retina cross, and the fibers coming from the outer (temporal) halves do not cross.

After decussation, the optic fibers form the optic tracts (tractus opticus). Each tract contains fibers from the outer half of the retina on the same side and the inner half of the opposite side.

Methods for examining the optic nerve disc and the optic nerve

The optic nerve head is available for detailed examination and research:

Ophthalmoscopy of the optic disc with an assessment of the shape, color, borders, vessels.
Campimetry - determines the central scotomas in the field of view and the size of the blind spot
Optical coherence tomography OCT

In these studies, congenital anomalies can be detected:

Increasing the size of the optic disc
Aplasia and hypoplasia of the optic disc
Disc drusen
disc coloboma
False neuritis
Atrophy of the optic disc

Acquired disorders are also very diverse:

Atrophy of the optic disc of various origins
True neuritis and congestive optic disc
Vascular disorders - narrowing of the arteries, dilated veins

Clinically, these changes in the optic nerve can be manifested by the following symptoms:

Decreased visual acuity
Violation of color perception
Changes in the visual field of the diseased eye, with localization of lesions above the chiasm - in both eyes
Increased electrical sensitivity threshold of the optic nerve

The composition of the optic nerve includes four sections, which are conventionally distinguished, based mainly on its topography.

Intrabulbar department

In the structure of the optic nerve, the main part is played by the axons of the ganglion cells of the retina itself. These axons, passing through inner layer retinas, flock to the posterior pole of the eye and form the optic disc at the exit site. In this case, the axons, the course of which comes from the periphery, lie outside, and the axons that joined them later lie inside.

The optic fibers have an arcuate bend. This affects the fact that the optic papilla in its center has a small depression, the anatomy of which resembles a funnel in shape (the so-called physiological excavation). Through this funnel, the retinal vein and the central artery pass into the eye. The latter in the embryonic period of development also penetrates into the vitreous body.

The area of physiological excavation is covered from above by the glial cover, in which there is an admixture of connective tissue, referred to by the term "Kunt's connective tissue meniscus". The optic disc is devoid of photoreceptors. In relation to the macula of the eye, the optic nerve papilla is located 3 mm nasally and 0.5 mm downwards. Such a structure and location of the disc contributes to the formation of a negative, absolute, physiological scotoma in the upper temporal part of our field of vision, referred to in ophthalmology as a blind spot. The optic nerve fibers located where the optic disc and retina are located are devoid of myelin. The total path of the intrabulbar section in millimeters is slightly more than 0.5.

Intraorbital department

Immediately in the area behind the cribriform plate of the sclera, the nerve fibers acquire a myelin sheath, which then continues throughout the rest of the optic nerve. The diameter of the nerve behind the sclera increases from 3.5 mm to 4–4.5 mm. This happens due to the fact that the structure of the nerve is undergoing changes - three shells are attached to it from the outside, surrounding the nerve trunk from all sides. The arachnoid, hard and soft membranes are connected, on the one hand, with the membranes located in the brain in the corresponding sections, and on the other, with the sclera.

The hard (outer) sheath of the optic nerve merges with the sclera at the eyeball. Its anatomy is represented by coarse collagen fibers with an admixture of elastic fibers. The thickness of the hard shell is the greatest, from the inside it is lined with endothelium, separated by a fascial sheet from the fatty tissue of the orbit. Where the hard shell completely merges with the sclera, the optic nerve is equipped around the circumference with trunks and vessels of the ciliary nerves, the course of which goes through the sclera and ends inside the eye.

The soft sheath surrounds the nerve trunk and is separated from it by the glial mantle, which is a thin layer of glia. The soft shell is in close connection with the nerve trunk itself and sends it inward to in large numbers connective tissue partitions of the first and second order, called septa. The function of these septa is to divide the optic nerve into separate bundles. Septa also increase the strength of the optic nerve, possibly due to the fact that their anatomy is represented by elastic tissue, collagen and glia, which, in turn, penetrates into the nerve bundles.

The course of the vessels involved in supplying the optic nerve trunk with food is limited by its partitions. Vessels do not go inside the bundles of nerves; therefore, the nutrition of individual nerve fibers is carried out by glia. The endothelium covers the soft shell from the outside. In front, the soft shell gradually passes into the cribriform plate, sending a certain amount of its fibers to the choroid. Pathological accumulation of fluid in this place leads to squeezing soft tissue optic nerve, resulting in swelling of the optic nerve papilla.

The arachnoid is located in the space between the hard and soft sheath of the nerve. In structure, it is tender and loose, and in function it divides the intervaginal space into subarachnoid and subdural. In the subarachnoid space, there are beams consisting of elastic and collagen fibrils, which are lined with endothelium.

The course of the central retinal artery begins outside the optic nerve at the level of its bottom side. The artery at a distance of 7–12 mm from the eyeball has an arcuate bend, after which it enters the optic nerve trunk at a right angle and then is located along its axis. Throughout the nerve, the artery is shrouded in a connective tissue sheath, called the "central connective tissue cord." The function of this sheath is protective - it protects the nerve fibers from the effects of a pulse wave.

The optic nerve in eye orbit makes an S-shape. Due to this, the entire length of the optic nerve increases. This length makes the eyeball mobile, and in addition, protects the optic fibers from injury and tension when the eyeball makes large and sharp movements in amplitude. different sides. The intraorbital part of the nerve in length can be from 25 to 35 mm.

Intracanalicular department

The dura mater at the nerve in the bone canal connects to the periosteum. The optic nerve canal in this place has the narrowest intershell space. The length of the intracanalicular area can be from 5 to 8 mm.

intracranial department

The shape of the intracranial region is ovoid and somewhat flattened, the length is short. The left and right optic nerves approach each other. As a result, chiasm is formed. Covered by chiasm arachnoid and soft shells, it is located in the Turkish saddle (on its diaphragm). The optic tracts located posterior to the chiasm are referred to as the optic tract.

Visual pathways and their role in the visual analyzer

Where the visual pathway connects the retina and the cortical center of the visual analyzer, there are two neurons, they are designated as central and peripheral. The path of the peripheral neuron starts from the axons of the ganglion cells located in the retina. The peripheral neuron ends in the structure of the lateral geniculate body. The peripheral neuron is subdivided into three divisions of the optic pathway, these include the chiasm, optic tract, and optic nerve.

The central neuron starts from the lateral geniculate body, more precisely from its nerve cells. In the place of its beginning, the central neuron forms the so-called Graziole bundle, it passes through the internal capsule and ends in the brain - the cortex of its occipital lobe in the region of the spur groove.

The optic nerve forms the initial part of the visual pathways. The axons of ganglion cells located in the retina run in the form of bundles of nerves and have a specific location in the optic nerve trunk. The order of location corresponds to the parts of the retina from which they originate.

Fibers starting at upper divisions retina, pass in the dorsal, upper side of the optic nerve. The fibers of the lower sector occupy its ventral, that is lower part. The same correspondence exists in the inner and outer sectors of the optic nerve and retina.

The papillomacular bundle starts from the macular area, which is considered one of the most functionally important. This bundle is located in the nerve disk in its temporal sector. It occupies a beam of 2/5 of the cross section. Own peripheral location the beam saves only in anterior section nerve, as it moves away from the eye, it somewhat changes its shape. In the orbital region, its posterior part, the papillomacular bundle shifts to the central part of the optic nerve and then goes along its axis. central position the bundle ends at the place where the chiasm is located.

Chiasma is the intersection of the optic nerves. Nerve fibers exiting the nasal areas of the retina undergo a complete decussation. The fibers pass to the opposite section in the medial part of the retina. Laterally located fibers do not cross on the temporal side and remain on it. Similarly, an incomplete decussation is also determined in the papillomacular bundle. Chiasma, subjected to pathological processes, leads to the development of bitemporal hemianopia.

The optic pathways located behind the chiasm are referred to as the optic tract. Due to the semi-crossing of nerve fibers, the right optic tract includes fibers from the right retina. When it is destroyed, the left halves of the visual field fall out and left-sided homonymous hemianopsia develops. The left optic tract is connected to the left sections of both retinas. If the conduction of the left tract is disturbed, the right visual fields fall out and right-sided hemianopsia occurs.

Blood supply to the optic nerve

The ophthalmic artery is predominantly involved in the blood supply to the optic nerve. The ophthalmic artery departs from the fifth bend of the internal carotid artery. move ophthalmic artery has several branches, which, next to the optic nerve, go in front to the eyeball, and behind - to the bone canal. The blood supply to the optic nerve is also provided by more large arteries, which include the lacrimal artery, the posterior ciliary artery and the central retinal artery.