Corpus: Optic nerve

The optic nerve is the 2nd cranial nerve and forms the middle section of the visual pathway. It enables vision by transmitting signals generated in the retina to the brain.

The optic nerve arises from the stalk of the embryonic eye cup, which is derived from the neuroectoderm. From the 6th week of development, the nerve cell processes of the inner neuroblastic retinal layer grow into the eye cup stalk and make contact with the midbrain within the following two weeks. At the same time, the optic fissure closes, and its edges fuse together, enclosing the hyaloid artery and vein in the center. The central retinal artery and vein develop from their proximal parts. The eye cup stalk is filled out by the sprouting of further neurites, completing its formation around the end of the 9th week.

The optic nerve of an adult has a diameter of about 4 mm and contains around one million nerve fibers, which are the neurites of the retinal ganglion cells, the third afferent neurons of the visual pathway. From the point of entry into the optic nerve, these nerve fibers are encased in myelin sheaths of oligodendrocytes, which increase the speed of conduction. They transmit signals to the fourth neurons in the diencephalon, predominantly located in the lateral geniculate nuclei (LGN). Unlike peripheral nerves, which are sheathed by Schwann cells, the optic nerve is sheathed by oligodendrocytes.

In contrast to other cranial nerves, the optic nerve is surrounded along its entire length by the dura mater, pia mater, and cerebrospinal fluid. It lacks an epineurium, perineurium, or endoneurium, which limits its regenerative capacity, making it more like a brain tract that runs through the orbit.

The optic nerve is approximately 50 millimeters long from the optic disc (papilla nervi optici) to the optic chiasm. It can be divided into intraocular, intraorbital, intracanalicular, and intracranial sections.

4.1. Retina

After the axons have merged, the optic nerve leaves the retina at the optic disc, also known as the blind spot due to the absence of sensory cells. From this exit point, the nerve is surrounded by myelin sheaths.

4.2. Orbit

In the orbit, the optic nerve is embedded in fat and accommodates the central retinal artery and vein, which supply the retina. It is encircled by the tendon ring (annulus tendineus communis) of the eye muscles as it exits the eye socket.

4.3. Sphenoid bone

The optic nerve then enters the optic canal of the sphenoid bone, accompanied by the ophthalmic artery.

4.4. Cranial cavity

In the cranial cavity, the optic nerves run in the subarachnoid space. The nasal fibers of the nerve cross at the optic chiasm, located in front of the pituitary stalk. The resulting optic tract continues to the diencephalon.

The optic nerve's mainly receives arterial blood from the central retinal artery, with venous outflow through the central retinal vein.

Experimental animal research indicates that the optic nerve has a transmission bandwidth equivalent to that of a DSL line, transmitting about 1 Mbit/s from the eye to the brain. The retinal ganglion cells that form the optic nerve primarily use glutamate as a neurotransmitter.

Complete destruction of the optic nerve results in blindness in the affected eye. Damage to the middle part of the optic chiasm causes heteronymous hemianopsia, while destruction of an optic tract leads to contralateral hemianopsia.