General information
The optic nerve (Nervus opticus, ancient Greek “belonging to the sight”) is the second cranial nerve and the first part of the visual pathway. It serves to transmit optical stimuli from the retina to the brain. For this reason it belongs to the nerves of sensory quality. It runs from the lamina cribrosa to the optic nerve junction, the optic chiasma, and is about 4.5 cm long.
History of development
The second cranial nerve (optic nerve) as well as the first cranial nerve (bulbus and tractus olfactorius) originate from the diencephalon and are thus a protuberance of the brain. Since all other cranial nerves originate from the spinal ganglia of the neural crest, the first two cranial nerves are often called “fake cranial nerves”.
Origin
The axons of the various ganglion cells of the retina unite to form a large nerve, the optic nerve. For this reason, the optic nerve does not have an actual core area but three neurons in the retina. The individual nerve fibers are interconnected. The cells of the rod and cone layer (1st neuron) are connected to the bipolar cells (2nd neuron) and these to the ganglion cell layer (3rd neuron). The axons of the ganglia then unite to form the large optic nerve, which leaves the retina and travels to the brain.
Course of the optic nerve
The course of the optic nerve can be roughly divided into three parts. It begins with an intrabulbar part located in the eyeball, then runs inside the orbit (intraorbital part) and finally ends in the skull (intracranial part). After the axon union in the retina, the optic nerve leaves the retina at the optic nerve papilla (discus nervi optici).
Since there are no sensory cells at this point, this point is called a blind spot. As soon as the nerve leaves the retina, it is surrounded by the three meninges and the myelin sheaths of the oligodendrocytes. This myelin sheath enables information to be passed on particularly quickly.
However, if the optic nerve is damaged, the astrocytes (connective tissue cells) prevent regeneration of the nerve. The optic nerve then continues to run through the bony eye socket. It is embedded in fat for protection and allows the central retinal artery and the central retinal vein to access the retina.
The two vessels run in the middle of the optic nerve and can thus enter the retina through the optic nerve papilla. When leaving the orbit, the optic nerve is surrounded by the tendon ring (anulus tendineus communis) of the eye muscles. After the orbit, the optic nerve enters the optic canalis of the sphenoid bone and is accompanied on its way by the ophthalmic artery.
In the cranial cavity itself, the nerve fibers of the optic nerve run in the subarachnoid space. In front of the pituitary stalk, in the optic chiasm, there is a crossing of the nasal nerve fibers of both optic nerves. This is how the signals from the left visual field reach the right hemisphere of the brain and vice versa.
The partially crossed and partially uncrossed fibers now form the optic tractus. In the Corpus geniculatum laterale, the nerve fibers of the optic tractus are switched to the fourth neuron. This neuron then projects the information into the area striata with its fibers via the visual radiation (Radiatio retinae). This is the site of primary vision (primary visual cortex, area 17). It is located in the area of the back of the head (occipital lobe) and transmits the information to Area 18, the secondary visual cortex, and to higher visual cortex areas for processing.