The optic nerve

The optic nerve (n. Opticus) is a thick nerve trunk, which consists of axons of ganglionic retinal ganglion cells.

The optic nerve refers to the cranial cerebral peripheral nerves, but in essence it is not a peripheral nerve either in origin, in structure, or in function. The optic nerve is a white substance of the large brain that conducts the pathways that connect and transmit visual sensations from the mesh envelope to the cerebral cortex.

Axons of ganglionic neurocytes gather together in the blind spot of the retina and form a single bundle - the optic nerve. This nerve passes through the vascular membrane and the sclera (the intraocular portion of the nerve). Coming out of the eyeball, the optic nerve goes posteriorly and slightly medially to the visual channel of the sphenoid bone. This part of the optic nerve is called the intraocular part. It is surrounded up to the white shell of the eye by the continuation of the firm, arachnoid and soft membranes of the brain. These membranes form the vagina of the optic nerve (vagina nervi optici). When the optic nerve leaves the orbit in the cavity of the skull, the hard shell of this vagina passes into the periosteum of the orbit. In the course of the intraocular part of the optic nerve, the central retinal artery (branch of the eye artery) is attached to it, which at a distance of about 1 cm from the eyeball penetrates into the depth of the optic nerve. Outside the optic nerve are long and short posterior ciliated arteries. In the corner formed by the optic nerve and the lateral rectus muscle of the eye, lies the ciliary node (ganglion). Upon exit from the orbit near the lateral surface of the optic nerve is the eye artery.

In the visual channel there is an intracannular part of the optic nerve in the length of 0.5-0.7 cm. In the canal, the nerve passes over the eye artery. Leaving the visual canal into the middle cranial fossa, the nerve (its intracranial part) is located in the subarachnoid space above the diaphragm of the Turkish saddle. Here, both the optic nerve - right and left - approach each other and over the furrow of the cross of the sphenoid bone form an incomplete visual intersection (chiasma). Behind the chiasma, both optic nerve pass respectively to the right and left visual tracts.

The pathological processes of the optic nerve are close to those that develop in the neural tissue of the large brain, especially clearly it is expressed in the structures of the neoplasms of the optic nerve.

Histological structure of the optic nerve

1. Afferent fibers. The optic nerve contains about 1.2 million afferent nerve fibers from the ganglion cells of the retina. Most of the fibers form synapses in the lateral geniculate body, although some of them enter other centers, mainly in the preteral nuclei of the midbrain. About 1/3 of the fibers correspond to the central 5 fields of view. Fibrous septa, coming from the pia mater, divide optic nerve fibers into approximately 600 bundles (each with 2,000 fibers).
2. Oligodendrocytes provide myelination of axons. Congenital myelination of retinal nerve fibers is explained by abnormal intraocular distribution of these cells.
3. Microglia are immunocompetent phagocytic cells, possibly regulating apoptosis ("programmed" death) of retinal ganglion cells.
4. Astrocytes lining the space between axons and other structures. When axons die at an atrophy of the optic nerve, astrocytes fill the formed spaces.
5. Surrounding shells
   • pia mater - soft (inner) cerebral membrane containing blood vessels;
   • Subarachnoid space is an extension of the subarachnoid space of the brain and contains cerebrospinal fluid;
   • the outer shell is divided into a cobweb and a hard shell, the latter continues into the sclera. Surgical optic fenestration includes the incisions of the outer shell.

Axoplasmatic transport

Axoplasmic transport is the movement of cytoplasmic organelles in the neuron between the cell body and the synaptic termination. Orthopedic transport consists in the movement from the cell body to the synapse, and the retrograde transport in the opposite direction. Rapid axoplasmatic transport is an active process that requires the expenditure of oxygen and energy of ATP. The axoplasmatic current can stop due to various causes, including hypoxia and toxins that affect the formation of ATP. Vat-like foci of the retina are a consequence of the accumulation of organelles when the axoplasmic current ceases between ganglion cells of the retina and their synaptic endings. A stagnant disc also develops when the axoplasmic current stops at the level of the trellis plate.

The optic nerve covers three meninges: hard, spidery and soft. In the center of the optic nerve, in the nearest segment to the eye, there is a vascular bundle of the central vessels of the mesh shell. On the axis of the nerve is a connective tissue cord surrounding the central artery and vein. The optic nerve itself does not receive any central branch of the central vessels.

The optic nerve is like a cable. It consists of axial processes of all ganglion cells of the reticular rim. The number of them reaches about one million. All fibers of the optic nerve through the hole in the trellis plate of the sclera exit from the eye into the orbit. At the exit site, they fill the scleral aperture, forming the so-called nipple of the optic nerve, or the optic nerve disc, because in the normal state the optic nerve disk lies at the same level with the retina. Only the stagnant nipple of the optic nerve appears above the retina, which is a pathological condition - a sign of increased intracranial pressure. The exit and branching of the central retinal vessels are visible in the center of the optic nerve disc. The color of the disc is paler than the surrounding background (with ophthalmoscopy), since there is no choroid and pigment epithelium in this place. The disc has a live pale pink color, more pink on the nose, from where the vascular bundle comes out more often. Pathological processes developing in the optic nerve, as in all organs, are closely related to its structure:

1. a lot of capillaries in the septums surrounding the fascicles of the optic nerve, and its special sensitivity to toxins create conditions for influencing the fibers of the optic nerve infection (eg, influenza) and a number of toxic substances (methyl alcohol, nicotine, sometimes plasmacid, etc.);
2. when the intraocular pressure rises the weakest point is the optic nerve disc (it, like a loose cork, closes the holes in the dense sclera), so when glaucoma the optic nerve disk is "pressed in", a hole is formed.
3. Excavation of the optic disc with atrophy of the optic nerve;
4. increased intracranial pressure, on the contrary, delaying the outflow of fluid through the intercostal space, causes compression of the optic nerve, fluid stagnation and swelling of the interstitial substance of the optic nerve, which gives a picture of the stagnant nipple.

Hemodynamic and hydrodynamic changes also have an adverse effect on the optic disc. They lead to a decrease in intraocular pressure. Diagnosis of optic nerve diseases is based on data from ophthalmoscopy of the fundus, perimetry, fluorescent angiography, electroencephalographic studies.

The change in the optic nerve is necessarily accompanied by a disruption in the function of the central and peripheral vision, limiting the field of vision to colors and reducing the twilight vision. Diseases of the optic nerve are very numerous and diverse. They are inflammatory, degenerative and allergic. There are also anomalies in the development of the optic nerve and tumor.

Symptoms of damage to the optic nerve

1. Reduction of visual acuity when fixing near and distant objects is noted often (can occur in other diseases).
2. Afferent pupillary defect.
3. Dyshromatopsia (violation of color vision, mainly in red and green). A simple way to identify a one-sided violation of color vision: the patient is asked to compare the color of the red object seen by each eye. A more accurate estimate requires the use of Ishihara pseudo-isochromatic tables, the City University test, or the 100-ton Farnsworth-Munscll test.
4. Decrease in light sensitivity, which can persist after restoration of normal visual acuity (for example, after a neuritis of the optic nerve). This is best defined as follows:
   • light from an indirect ophthalmoscope is first illuminated by a healthy eye, and then - an eye with suspicion of optic nerve damage;
   • the patient is asked if the light is symmetrically bright for both eyes;
   • the patient reports that the light seems to him less bright in the diseased eye;
   • the patient is asked to determine the relative brightness of the light visible to the diseased eye, compared to the healthy
5. Reduction of contrast sensitivity is defined as follows: the patient is asked to identify the gratings of the gradually increasing contrast of different spatial frequencies (Arden tables). This is a very sensitive, but not specific for the pathology of the optic nerve, the index of vision loss. Contrast sensitivity can also be investigated using the Pelli-Robson tables, in which the letters of gradually increasing contrast (grouped by three) are read.
6. Defects of the field of vision that vary depending on the disease include diffuse depression in the center of the field of vision, central and centrocecal scotoma, a defect of the bundle of nerve fibers and an altitudinal defect.

Changes in the disc of the optic nerve

There is no direct correlation between the type of optic disc and visual functions. With the acquired diseases of the optic nerve, four basic conditions are observed.

1. The normal form of the disc is often characteristic of retrobulbar neuritis, the initial stage of Leber's optical neuropathy and compression.
2. Disk edema is a sign of a stagnant disk of anterior ischemic optic neuropathy, papillitis, and the acute stage of Leber's optical neuropathy. Disk edema can also appear with compression lesions before the development of optic nerve atrophy.
3. Opticociliary shunts are retino-choroidal venous collaterals in the lisque of the optic nerve, which develop as a compensatory mechanism in chronic venous compression. The cause of this is often meningioma and sometimes glioma of the optic nerve.
4. Atrophy of the optic nerve is the outcome of almost any of the aforementioned clinical conditions.

Special researches

1. The manual kinetic perimetry according to Goldmann is useful for the diagnosis of neuro-ophthalmic diseases. Allows you to determine the state of the peripheral field of vision.
2. Automatic perimetry determines the threshold sensitivity of the retina to a static object. The most useful programs testing the central 30 ', with objects covering the vertical meridian (for example, Humphrey 30-2).
3. MPT is the method of choice for visualization of the optic nerves. The orbital part of the optic nerve is better seen when T1-weighted tomograms eliminate a bright signal from the adipose tissue. Intracanalicular and intracranial parts on MRI are better visualized than on CT, as there are no bone artifacts.
4. Visible evoked potentials are the recording of the electrical activity of the visual cortex caused by stimulation of the retina. Stimuli are either a flash of light (flash VZP), or a black and white chess pattern reversing on the screen (VIZ pattern). A number of electrical responses are obtained which average the computer, evaluate both the latency (increase) and the amplitude of the VIZ. With optical neuropathy, both parameters are changed (the latency increases, the amplitude of the VLP decreases).
5. Fluorescent angiography can be useful for differentiating a stagnant disc, in which there is percolation of the dye on the disc from the drusen disc when autofluorescence is observed.