Optic nerve

The optic nerve (II) is a sensory nerve that connects the eyeball to the brain, serving as the neural transmission pathway for the sense of vision.

Number: II

Name: Optic

Sensory, motor, or both: Sensory only

Origin: Retinal ganglion cells

Nucleus: Lateral geniculate nucleus

Function: To transmit visual signals from the retina of the eye to the brain.

Description:

The optic nerve extends from the eyeball to the optic chiasma. It is not a nerve in the usual sense, but rather a thick bundle of white matter belonging to the diencephalon and enclosed in a robust meningeal sheath. It originates directly from the stalk of the embryonic optic vesicle, which is invaded by fibers originating from the retina and projecting toward the metathalamus.

The optic nerve is composed of fibers arising from the multipolar neurocytes of the ganglion cell layer of the retina, which represents its origin. The nerve, still composed of multiple distinct bundles, passes through the sclera and then travels in a mediocaudal direction, forming a double curvature that allows it to accommodate movements of the eyeball. It then reaches and traverses the optic canal. Upon exiting the canal, it enters the cranial cavity and travels a short distance to the optic chiasm.

The optic nerve is classically divided into four segments:

• intrascleral,

• orbital,

• intraosseous,

• intracranial.

The intrascleral segment, the shortest, is where the nerve is formed. Fibers from the ganglion cell layer, situated beneath the internal limiting membrane, converge toward the optic disc. They group into small bundles that curve outward, pass through the choroidea—which surrounds them—and enter the area cribrosa sclerae.

The orbital segment is always the longest. In this part, the nerve is circular in cross-section and has a diameter of 5 to 6 mm in the horse and ox, about 2 mm in the pig and dog, and approximately 1 mm or slightly more in the cat and rabbit. In domestic mammals, this segment lies within adipose tissue and is surrounded by bundles of the retractor bulbi muscle, which separates it from the four rectus muscles of the eye. In humans, who lack a retractor bulbi, the optic nerve is directly surrounded by these rectus muscles. It is also closely accompanied by ciliary vessels and nerves, with the ciliary ganglion located laterally.

The intraosseous segment, ranging from a few millimeters to over a centimeter depending on the species, traverses the optic canal, accompanied only by the internal ophthalmic artery, located ventrally.

The intracranial segment, also short, sees the nerve lose its meningeal sheath. Covered only by the pia mater, it continues through the subarachnoid space to the optic chiasm. Beyond the chiasm, its fibers diverge in a complex and species-specific manner, with most continuing into the optic tracts.

The optic nerve has a unique structure. It is encased in meningeal layers, and its fibers resemble those of white matter more than those of typical peripheral nerves.

It is composed of very fine fibers, most measuring 1 to 2 µm in diameter, grouped into numerous small bundles. These myelinated fibers, unlike those in other nerves but like those in white matter, lack a neurilemma. The bundles are longitudinal and arranged with a retinotopic organization, meaning fibers from each retinal quadrant preserve their relative position within the nerve. This organization is significantly altered at the optic chiasm, where a partial crossing over occurs, the extent and pattern of which vary greatly between species. In general, among mammals, each optic tract contains fibers from the lateral quadrants of the ipsilateral retina and the medial quadrants of the contralateral retina. Most fibers (the lateral root of the optic tract) terminate in the lateral geniculate body, while others (the medial root) project to the rostral colliculus and various midbrain nuclei. This latter group is not involved in vision per se but mediates reflexes essential for visual function. A few centrifugal fibers directed toward the eye have also been identified in the optic nerve. Their origin and role are not fully understood, but in birds, where they are better studied, they originate from a specific group of neurocytes in the caudal midbrain.

In its intracranial portion, the optic nerve is covered only by the pia mater. However, this meningeal layer extends beyond the cranium. As it enters the optic canal, it is joined by the arachnoid and dura mater, forming a sheath that envelops the nerve all the way to the eyeball. Each layer of this sheath continues into one of the layers of the eyeball wall. The outer layer, continuous with the dura mater, is fibrous in nature. It accompanies the ciliary nerves and vessels and attaches to the sclera. The middle layer, continuous with the arachnoid, is thin and delicate, separated from the dura by a potential subdural space and from the pia by a broader subarachnoid space. The innermost layer, the pia mater, carries a dense network of microvessels and numerous collagen fibers, more abundant than in other meningeal regions. This pia mater acts as an epineurium; its inner surface sends numerous septa into the nerve, forming the perineurium and endoneurium around the fiber bundles.

The vascular supply of the optic nerve is dual. The peripheral vascular network of the deep meningeal sheath layer extends into the septa and their subdivisions, forming a rich capillary bed. These capillaries anastomose with those from the central retinal artery, whose origin and point of entry vary by species and which may even be absent in some. This artery runs within the core of the nerve, passing through the scleral cribiform area and supplying adjacent parts of the retina. Its contribution to the nerve's vascularization is minimal. A central retinal artery accompanies this artery. In domestic mammals, however, the arteries remain on the surface of the nerve and penetrate the sclera in multiple branches, which then supply the inner retinal layers.