The Optic Nerve
Perception of the eclectic varieties of shapes, colors and sizes that exist across the globe is dependent on the relatively small, spherical eye balls . There are multiple parts of the eye that allows images to be detected. However, recognition and interpretation of these objects is largely dependent on the optic nerve.
The identity of cranial nerve II (CN II), also known as the optic nerve, predates Galenic anatomy. Known to the Greek fathers of anatomy as nervus optikus , the optic nerve has the responsibility of transmitting special afferent impulses of light to the brain. It is also involved in several reflex arcs related to the ocular system. It is a unique structure that functions as the bridge between the retinal layer of the eyes and the visual cortex of the brain.
This article will review the embryology, anatomy, histology, and blood supply of the optic nerve, as well as briefly discuss the optic tracts. The visual reflexes, optic radiation, and some relevant pathologies will also be discussed.
- Optic Nerve Embryology
- Optic Nerve Anatomy
- Optic Nerve Histology
- Optic Nerve Pathway
- Blood Supply of the Optic Nerve
- Clinical Significance
- Related Atlas Images
Optic Nerve 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. The 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.
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.
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 Head
The optic nerve head is the most anterior component of the optic nerve and corresponds to the 1 mm segment that is located within the eyeball (i.e. the intraocular part). Historically, it was thought to be a raised entity protruding from the retinal surface and by extension, was referred to as a papilla (hence the term, papilloedema). However, it has since been discovered that the optic nerve is in fact level with the retina, therefore the term papilla is an old erroneous word that is still used occasionally with reference to the head of the optic nerve.
The optic nerve head (also known as the optic disc) is approximately 1.5 mm wide and is also associated with a physiological cup that corresponds to a central depression in the optic nerve head. The dimensions of the cup and disc are dependent on the orientation, shape and size of the chorioscleral canal that exists at Bruch’s membrane. The conical chorioscleral canal tends to widen in the anteroposterior direction.
The optic nerve head is a unique part of CN II in that it marks an important point of vascular, geometric and tonometric transition. At the optic nerve head, the optic nerves are moving to the relatively low pressure space of the retro-orbital region from the much higher intraocular pressure zone. Furthermore, there is a change in the blood supply from the central retinal artery to the ophthalmic and posterior ciliary arteries. The nerve fibers make a sharp 90 degrees turn to pierce the lamina cribrosa. Finally, they not only become myelinated, but they are also encased in the meningeal layers in the extraocular areas.
Intraorbital Part of CN II
The 25 mm of optic nerve travelling from the posterior part of the eyeball (a few millimetres medial to its posterior pole) to the intraorbital opening of the optic canal is known as the intraorbital part. The optic nerve fibers distal to the lamina cribrosa are myelinated, while those preceding the lamina cribrosa are unmyelinated. Therefore the diameter of the intraorbital optic disc is twice the width of the intraocular part.
The intraorbital optic nerve is also surrounded by all three meningeal layers (i.e. dura, arachnoid and pia mater). The arachnoid and dura mater are usually loosely attached proximally, and is associated with a larger subarachnoid space (giving it a bulbous appearance) at the posterior pole of the eyeball. The subarachnoid space narrows significantly at the orbital orifice of the optic canal.
Just before it enters the optic canal, CN II can be found adjacent to CN III, CN VI and the nasociliary nerve, and superomedial to the ophthalmic artery. Within the canal, there are numerous fibrous dural attachments that extend to the pia mater. Earlier in its course, orbital fat separates the optic nerve from the surrounding extraocular muscles. However, the nerve becomes more closely related to the annulus of Zinn, where the four recti originate.
Intracanalicular Part of CN II
Within the optic canal lies the intracanalicular part of the optic nerve. The optic canal is formed within the lesser wing of the sphenoid bone. There is significant variability in the thickness of the walls of the optic canal such that the thickness increases from medial to lateral, and from superior to inferior.
The optic canal travels posteromedially at a 35 degrees angle relative to the midsagittal plane. Like the preceding intraorbital part, the intracanalicular part is also enclosed by the meninges described earlier. It has a variable length ranging between 4 – 10 mm. The extra length (in addition to the length of the intraorbital component) permits additional movements of the eyeball within the orbit. It is otherwise structurally identical to the preceding intraorbital segment.
The last 10 mm of optic nerve is the intracranial part. It extends from the internal orifice of the optic canal and travels above the diaphragma sellae before passing above the suprasellar part of the cavernous sinus. Here, the optic nerve unites with its contralateral CN II to form the optic chiasm. There are also very important vascular structures that are closely related to the intracranial optic nerve:
- it lies medial to the internal carotid artery
- it is superomedial to the ophthalmic artery
- and inferior to the anterior cerebral artery.
The pia mater is the only meningeal layer covering the intracranial optic nerve.
The optic chiasm marks an important part of CN II. Recall that the retina can be subdivided into nasal and temporal halves, which can be further subdivided into superior and inferior poles. The nasal side of the left eye and the temporal side of the right eye receive input from the temporal and nasal visual fields, respectively (i.e. left visual field). Similarly, light from the right visual field is detected by the temporal side of the left eye and nasal side of the right eye.
At the optic chiasm, the optic nerve fibers originating from the temporal side of the retina of the right eye continue in the right optic tract (post chiasmatic part of the optic nerve). At the point of the decussation, the fibers that originated from the nasal field of the left eye, cross over and enter the right optic tract. Therefore, visual input from the left visual field travels in the right optic tract. A similar decussation occurs with fibers arising in the nasal side of the contralateral eye.
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.
Pupillary Light Reflex
The optic nerve is responsible for relaying information to the pretectal nucleus at the level of the superior colliculus, which is responsible for bilateral regulation of the activity of the Edinger-Westphal nucleus (parasympathetic nucleus of CN III). The fibers of the optic tract that synapse at the pretectal nucleus, do not communicate with the lateral geniculate body.
The presynaptic parasympathetic fibers of each Edinger-Westphal nucleus then travels with the fibers of the oculomotor nerve, after which it synapses at the ciliary ganglion. The postsynaptic parasympathetic fibers then travel with the short ciliary nerve, to innervate the constrictor pupillae of the iris. Therefore, the pupillary light reflex pathway has one afferent limb arising from the ipsilateral optic tract, and two efferent limbs that provide bilateral innervation to the Edinger-Westphal nuclei. As a result, it is able to produce both direct (ipsilateral pupillary constriction in response to light stimulus) and consensual (contralateral pupillary constriction in response to light stimulus) pupillary response.
Optic Radiation (Geniculocalcarine Tract)
This dorsal lateral geniculate nucleus is divided into layers 1 - 6, and has a somatotopic arrangement such that fibers arising from the contralateral eye will synapse on layers 1, 4, and 6. Conversely, those fibers originating from the ipsilateral eye will synapse on layers 1, 3, and 5. Additionally, axons arising from the superior quadrants (excluding the macula) project to the anteromedial part of the nucleus. Similarly, those arising from the inferior quadrants (excluding the macula) project to the anterolateral part of the nucleus. The upper and lower quadrants of the macula will project to the posteromedial and posterolateral parts of the lateral geniculate nucleus, respectively.
The fibers leaving the lateral geniculate body are collectively known as the geniculocalcarine tract. They transmit integrated visual impulses from the thalami to the visual cortex. They can be divided into upper and lower loops that are also known as Baum’s and Meyer’s loops, respectively. The fibers of Meyer’s loop take a far anterior course around the temporal horn of the lateral ventricle (passing through the temporal lobe) before curving posteriorly toward the calcarine sulcus. Fibers of Baum’s loop pass directly through the parietal lobe to enter the retrolentiform part of the internal capsule. Eventually, fibers arising from the peripheral upper and lower quadrants will project to the anterior two-thirds of the primary visual cortex, while those arising from the upper and lower central quadrants (i.e. the macula) will project to the posterior third of the primary visual cortex.
Frontal Eye Fields
In addition to the numerous thalamocortical projections associated with vision, postsynaptic fibers from the primary visual field also travel to the cerebrum to synapse and integrate with the frontal eye fields (Brodmann 6, 8, and 9). This area of the brain has numerous connections with the thalamus, as well as parietal and temporal lobes; where it processes afferent signals related to sight and sound. Through its efferent tracts to the ipsilateral Edinger-Westphal and primary oculomotor nuclei, the frontal eye fields are able to regulate rapid eye movements between fixation points (saccadic movements).
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.
There are various pathological processes that can affect the optic nerve. The major ones discussed here are related to disorders of circulation, intraocular pressure, or inflammation of the nerve. However, the nerve is also susceptible to a similar array of pathologies that affect the brain, including tumors such as gliomas and meningiomas.
Anterior Ischaemic Optic Neuropathy (AION)
Anterior ischaemic optic neuropathy (AION) shares many similarities with the cerebrovascular phenomenon referred to as a stroke. The pathology occurs due to disruption of the blood supply to the optic nerve. This may result in a varying spectrum of disorders ranging from ischemia to infarction with necrosis.
The severity of the injury depends on the degree and duration of the vascular obstruction. Milder versions of AION may occur with temporary disruption of blood flow to the optic nerve; known as transient vision loss. Like the brain, the optic nerve does not regenerate once severely damaged (infarctions). Therefore, visual impulses mitigated by that region of the optic nerve will be permanently lost. While underlying disorders such as vasculitis, embolic phenomenon, or a thrombotic event may cause AION, the likelihood of advanced atherosclerosis as the cause of vascular occlusion should not be ignored.
Clinicians should have a high degree of suspicion with arterial injuries of the eye, as they usually indicate that there is an increased risk that a similar ischaemic event may occur elsewhere in the body (i.e. in the heart leading to a myocardial infarction, in the brain leading to a stroke, or in the kidneys leading to marked renal impairment). On funduscopic examination, the optic disc in AION will appear pale, and the margins will seem blurry (swollen).
Papilledema refers to swelling (oedema) at the optic nerve head of both optic nerves. This may present unilaterally or bilaterally. In the case of unilateral papilledema, the likely etiology is a mass effect from a primary optic nerve tumor. Bilateral papilledema is more likely related to raised intracranial pressure.
Increase in the perineural pressure may compromise the venous drainage of the nerve. Subsequently, the transportation of intracellular contents through the neuron will be disrupted; resulting in swelling of the disc. Clinically, this phenomenon is observed as blurring of the optic disc margins and the disc appears hyperaemic on funduscopic examination.