Optic nerve

Anatomy

The optic nerves are paired, cylindrical structures that extend from the posterior part of the eyeball (roughly 2 mm medial to the posterior pole) to the suprasellar space in the middle cranial fossa. It is made up of roughly 1 million myelinated axons of the ganglion cells of the retina. CN II is myelinated by oligodendrocytes, and not Schwann cells like the axons of peripheral nerves. While there is significant variability in the dimensions of the optic nerve (even between optic nerves within the same individual), the average length of the structure ranges between 35 mm and 55 mm.

It can be subdivided into four main parts:

• The optic nerve head (i.e. intraocular part) measures about 1 mm in length.
• The intraorbital part is approximately 25 mm in length.
• The intracanalicular part is most variable, ranging between 4 – 10 mm in length.
• The Intracranial part accounts for about 10 mm of the total length of the nerve.

Optic nerve histology

The roughly one million nerve fibers that form CN II can be categorized into five groups:

• The nerves responsible for transmitting visual impulses are the visual afferent fibers. They travel from the retina to the lateral geniculate body of the thalamus .
• Nerve fibers that travel to the tectum to regulate the pupillary light reflex are known as the pupillary afferent fibers.
• Efferent fibers to the retina have been identified, but their function is yet to be elucidated. 
• Some fibers travel to the superior colliculus and are known as photostatic fibers. They participate in the visual body reflexes.
• Finally, there are autonomic fibers that also travel alongside the optic nerve within the optic canal; but they do not penetrate the nerve.

There are subtle differences in the histological architecture of the optic nerve along its course. It is easier to follow the histology by subdividing the optic nerve into its comprising parts (i.e. head, intraorbital, intracanalicular and intracranial parts). 

In addition to having neural tissue, the optic nerve head is also populated by nonneural components such as astrocytes, fibroblasts and capillary-associated cells. The optic nerve head can be further subdivided into the surface nerve fiber layer, prelaminar region and lamina cribrosa. The inner limiting membrane of Elschnig (formed by a collection of astrocytes) is responsible for separating the anterior extremity of the nerve head – the surface nerve fiber layer – from the vitreous humour that fills the eyeball. The inner limiting membrane becomes more thickened centrally at the physiological cup, and is referred to as the central meniscus of Kuhnt. However, as the cup gets larger (i.e. the chorioscleral canal gets wider) the membrane becomes thinner. The surface nerve fiber layer is highly vascular with numerous capillary networks, and large retinal arteries and veins course through it. The layer also contains the tightly packaged axons of the ganglion cells. 

Deep to the surface nerve fiber layer is the prelaminar region of the optic nerve. It has also been referred to as the choroidal or glial layer. The principal constituent of this layer is glial tissue. There are, however, connective tissue found in the perivascular regions of the layer. Histologically, the distinguishing feature between the connective tissue and glial tissue is the diameter of the fibers. The connective tissue fibers are thicker than the glial fibers; and glial fibers travel in a perpendicular direction relative to the nerve bundles. In the periphery of the head of the optic nerve, the glial fibers are attached to the elastic membrane and choroid; while in the centre of the optic disc, it has attachments to the perivascular connective tissue.

The glial cells are densely packed and flattened in the anteroposterior plane, with numerous capillaries lying around the cell layer. The prelaminar glial tissue offers structural support to the axons of the ganglion cells as they make the perpendicular bend from the retina to form the optic nerve. All nerve fibers extending from the surface nerve fiber layer to the prelaminar region are unmyelinated.

Deep to the prelaminar zone is the lamina cribrosa region of the optic nerve head.  This is a highly fenestrated (roughly 200 – 300 holes) area that allows passage of the optic nerve axons into the extraocular space. As the nerve fibers pass (myelinated by oligodendrocytes) through the fenestra of the lamina cribrosa, they form a relatively tight seal in order to protect the retrolaminar tissue from the relatively high intraocular pressures. The lamina cribrosa has concave and convex surfaces anteriorly and posteriorly, respectively.

There is very poor demarcation between the prelaminar and deeper connective tissue zone. It therefore has a relative mixture of both glial and connective tissues, and is referred to as a transitional zone. The connective tissue forms a full thickness layer of tissue that covers the optic nerve head. It is arranged into thick columns along the periphery that anchor the lamina cribrosa to the adjacent sclera.

The intraorbital part of the CN II is surrounded by myelin from oligodendrocytes, as well as the three meningeal layers. The pia mater is intimately associated with the optic nerve. It is separated from the overlying arachnoid mater by cerebrospinal fluid . The dura mater offers the final encasement (similar to the rest of the central nervous system).  There are intervening connective tissue septae that contain vascular entities travelling longitudinally and transversely. They have inter-connective tissue connections, as well as connections to the optic nerve. At the periphery, the transverse septae attach the pia mater to the posterior aspect of the lamina cribrosa. Centrally, it attaches the pia mater to the perivascular fibrous sheaths.

Optic nerve pathway

The optic tract is the intracranial continuation of the optic nerve. Like CN II, the optic tract is paired. Each is made up from temporal fibers arising from the retina of the ipsilateral eye, as well as nasal fibers originating from the retina of the contralateral eye. Majority of these fibers will continue to the lateral geniculate body of the thalamus, where they synapse on the dorsal lateral geniculate nucleus.

However, a minority of these fibers will bypass the lateral geniculate body to terminate in the pretectal nucleus (to participate in the pupillary light reflex) and the superior colliculus (regulation of saccadic eye movements). However, there are fibers that leave the lateral geniculate body to form the optic radiation (geniculocalcarine tract). This tract is made up of six layers, which are formed after their fibers leave the lateral geniculate body via the retrolenticular part of the internal capsule. The fibers subsequently terminate in Brodmann area 17 (i.e. the visual cortex) within the calcarine sulcus. 

Blood supply of the optic nerve

There is some degree of variability in the blood supply of the optic nerve depending on the segment of the nerve being discussed. In essence, the optic nerve is indirectly supplied by the ophthalmic branch of the internal carotid artery. As the internal carotid artery emerges superiorly from the cavernous sinus, the ophthalmic artery diverges and travels along the ventral surface of the optic nerve within the optic canal. As the artery emerges at the apex of the orbit, it produces a triplicate of posterior ciliary arteries, along with the central retinal artery. The vessels travel anteriorly in the substance of the optic nerve after they pierce the structure roughly 10 – 12 mm from the globe. 

The surface nerve fiber layer of the optic nerve head is perfused by branches from the central retinal artery. Blood supply to the prelaminar zone arises from the peripapillary and choroid vessels arising from the posterior ciliary arteries. The lamina cribrosa derives its blood supply from the short posterior ciliary arteries, and in some cases the circle of Zinn-Haller. The latter vascular structure is inconsistent, however, it can be found in the sub-scleral space surrounding the optic nerve at the neuro-ocular junction. Tributaries to the Zinn-Haller network arise from the choroidal vessels, pial arterial network, perineural arteries and about 4 – 8 posterior ciliary arteries. The Zinn-Haller arterial network also gives branches to the laminar part of the optic disc.

The posterior ciliary arteries supply the orbital part of CN II via the pial vessels, which access the nerve via fibrous septae. The internalized part of the central retinal artery may also provide arterial supply to the nerve here. Collateral blood supply arising from the middle meningeal branch of the external carotid artery may also supply the intraorbital CN II near the orbital apex.

The intracanalicular part of CN II lies in a watershed area supplied by the collaterals ophthalmic artery (anteriorly) and pial derivatives of the superior hypophyseal and internal carotid arteries (posteriorly). Within the cranial vault, CN II is supplied by the A1 portion of the anterior cerebral, the internal carotid, and the superior hypophyseal arteries. Although the optic nerve is considered as a diencephalic extension, the supplying vessels do not have a blood-brain barrier. 

Venous tributaries from all parts of the optic nerve eventually drain to the central retinal vein. It then drains either to the superior ophthalmic vein or directly into the cavernous sinus.

Embryology

The optic placode, located at the cranial end of the developing embryo, develops into the eye and surrounding structures. Evagination of a segment of the forebrain wall gives rise to the retina of the eye. The outer part of the optic cup is rich in small pigment granules, and becomes the pigment layer of the retina.

The pars optica retinae make up the posterior 80% of the neural layer of the optic cup and develop into the rods and cones, which are responsible for light perception. The bipolar and horizontal cells (of the outer nuclear layer), as well as the ganglion and amacrine cells (of the inner cell layer), arise from the mantle layer of the retina; which is also adjacent to the pars optica retinae. Bipolar cells permit communication between the photoreceptors and the ganglion cells. The amacrine and horizontal cells are supportive cells found in the inner and outer plexiform layers of the retina, respectively.

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The distal axons of the ganglion cells coalesce in the fibrous nerve fiber layer of the retina and converge in the direction of the optic stalk, which becomes prominent around the fourth week of gestation. It has a groove known as the choroid fissure on the ventral aspect of the stalk that houses the hyaloid vessels.

By the 7th gestational week, closure of the choroid fissure produces a lumen that communicates with the forebrain proximally and the optic vesicle distally. The walls of the optic stalk are formed by a monolayer of primitive epithelial cells. Invagination of the stalk results in a shallow ventral depression. As the stalk lumen continues to shrink, the stalk itself simultaneously lengthens. Glial cells are also added to the proximal part of the stalk, where the optic disc forms. As more cells are added to the luminal surface of the stalk, the hyaloid arteries regress.  During gestational week 8, there is subsequent differentiation of the glial cell populations (of both the optic stalk and ventral forebrain) into type 1 and type 2 astrocytes, and oligodendrocytes.

Growth and development of the optic nerve continues into adolescent years. There is progressive increase in the diameter of the nerve, with the intraocular component being smaller than the extraocular part. This disparity in diameter can be accounted for by the myelination that occurs in the extraocular part of the nerve.