The optic nerve is the second nerve that is part of the group of twelve cranial nerves , but is considered to be part of the central nervous system: in fact the fibers arecovered with myelin produced by oligodendrocytes, and the optic nerve is wrapped in the meninges (dura mater, arachnoid, pious mother). It is an extension of the nerve endings of the photoreceptors of the retina: these cells transform images into electrical impulses, which are transmitted to the brain via the optical nerves (similar to cables that carry current). After about five centimeters, the nerves from the two eyes cross and divide: a section called chiasma begins.
Functions
The optic nerve has the function of transmitting visual information transduced into electrical impulses from the eyes.
Anatomy
The optic nerve is about 5 cm long and is surrounded by the meninges which also protect the entire brain (they are composed of three layers: dura mater, arachnoid and pia mater). It is similar to an electrical cable made up of many internal wires, each of which is protected by a sheath called “myelin”. Each individual fiber (similar to a copper wire) corresponds to a small area of the retina, while each bundle corresponds to a retinal area. The fibers located in the center of the optic nerve carry the bioelectric signals from the macula, the central and most sensitive area of the retina. This organization is maintained up to the occipital cerebral cortex or the area of the brain responsible for the interpretation of visual signals located above the nape of the neck. The optic nerve leaves the orbit through the optic channel, reaching the optic chiasm, in which there is a partial decussation (crossing) of the nerve fibers: in fact those coming from the nasal emiretins cross and continue in the contralateral optic tract. Most axons of the optic nerve end in the lateral geniculate body, from where visual information is transmitted to the visual cortex. The remainder ends in the quadrigeminal lamina of the midbrain. Based on the relationships that the optic nerve contracts, its course is divided into 4 parts: where the visual information is transmitted to the visual cortex. The remainder ends in the quadrigeminal lamina of the midbrain. Based on the relationships that the optic nerve contracts, its course is divided into 4 parts: where the visual information is transmitted to the visual cortex. The remainder ends in the quadrigeminal lamina of the midbrain. Based on the relationships that the optic nerve contracts, its course is divided into 4 parts:
- intrabulbar part: through the chorioid membrane and sclera of the eye;
- intraorbital part: from the posterior pole of the eye to the optic hole of the sphenoid;
- intracanalicular part: through the optic hole of the sphenoid;
- intracranial part: from the optic hole of the sphenoid it reaches the optic chiasm.
Diagnostic tests
Instrumental tests allow to study the optic nerve both by observing its morphology (thanks to imaging techniques such as magnetic resonance imaging), and to evaluate its functionality (electrofunctional examinations and visual field). We can study the visible part of the optic nerve (optic papilla) directly – by examining the ocular fundus – or by HRT (laser tomography for examining the fibers of the optic nerve). In addition, the nerve fibers that make up the optic papilla can be examined by OCT . The MRI(RM), however, allows us to follow and visualize the nerve in its path inside the head (intracranial tract). Electrofunctional examinations (in particular visual evocative potentials or PEV) allow to study the bioelectric activity of the optic nerve and retina. Another examination, the visual field, then allows us to know what type of damage has occurred. The visual capacity depends on the functionality of the eye itself, on the signal conduction system between the latter and the brain, as well as on the processing of the cerebral cortex. The analysis of the visual field allows us to see at what point is the possible deficit in the pathways that carry the signal of the eye to the areas assigned to the processing of visual information.
Optic nerve alteration
In case of damage to the optic nerve, a reduction in visual acuity can occur both from a distance and from near (a frequent symptom in many other diseases). There is a reduced ability of the pupil to react to light by contracting (alteration of the pupillary reflex). The vision of colors is altered, especially for hues such as red, green and a combination of the two: a very easy test to highlight defects from one eye is to observe a red object with one eye at a time, comparing the perception of the color. The reduction in light sensitivity – like contrast sensitivity – also decreases in the event of optic nerve suffering. Visual field defects are varied and can range from a generalized depression of the central visual field to areas of non-vision. However,
diseases
They are multiple and, if not treated promptly, they can lead to blindness.
They are distinguished in acquired and congenital . The causes are various: metabolic, infectious, toxic pathologies, due to demyelinating diseases (which damage the external lining of the nerves), autoimmune (the immune system attacks one’s body), vascular (heart attacks, aneurysmal compressions) and due to drugs.
The inflammation of the optic nerve are divided into anterior or retrobulbar optic neuritis. The latter have the appearance of the initially normal optic papilla (examination of the fundus of the negative eye) and in adults they are often associated with multiple sclerosis; this may be due to metabolic causes such as alcohol and tobacco intoxication (which causes a reduced intake of B complex vitamins).
The front optic neuropathies of non-arteritic origin, they can be considered a total or partial infarction of the optic papilla. It is fundamental for the preservation of sight to recognize the arteritic form of the anterior optic neuropathy (due to giant cell arteritis). This disease affects patients over 65 years of age, is associated with chewing pain, increased Ves and PCR; it leads to blindness if left untreated, as well as between 30 and 50% involvement of the other eye.
The papilledema is a condition in which both the optic nerves appear swollen due to increased pressure within the brain (due, for example, to tumors and haemorrhages). It is a pathology that affects the optic nerve due to the increase in pressure not in the brain, but in the eye: it is glaucoma, which involves characteristic visual field deficits and a typical aspect of the optic papilla, which increases its normal characteristic cup-shaped. Excavation increases due to the progression of the pathology.
The pale optic nerve is so called when the patient suffers from glaucoma: due to the high ocular pressure the optic nerve appears flattened and thinner than normal, hence the name “pale”.
L ‘ optic atrophy is an important sign of optic nerve disease in advanced stage or pathologies of the visual pathways. For example, terminal phase glaucoma is characterized precisely by optic atrophy
