The second cranial nerve, or optic nerve (CN II), is in charge of carrying unique relevant stimuli for vision. It originates from the optic vesicle, a forebrain out pocketing. Since the optic nerve is a vital part of the central nervous system, its examination permits the evaluation of the intracranial environment.
The cranial meninges are located around the optic nerve because of their specific physical relationship to the brain. In this article, we’ll examine the optic nerve’s structure, including its path, sensory capabilities, and therapeutic use (1).
Indeed, the optic nerve is a central nervous system limb (brain). The initial sensory bipolar cell body is placed peripherally in the retina and is not surrounded by Schwann cells.
Their central processes connect with ganglion cells, a particular type of neuron found close to the inner surface of the retina, on the vitreous surface of the retina, and their central processes exit the globe via the optic disc to create the actual optic nerve.
The optic nerve is distinctive due to a few characteristics. The optic nerve is the only cranial nerve in the central nervous system (CNS) to leave the cranial cavity. Moreover, it is the only cranial nerve that can be clinically seen. It is divided into fascicles by glial septa and connective structures, and cerebrospinal fluid surrounds it (2).
The Anatomical Course Of The Optic Nerve
The transfer of specific sensory stimuli from the retina to the primary brain region responsible for vision is described by the optic nerve’s anatomical route. It can be separated into intracranial and extracranial (found outside the cranial cavity) components.
a) Extracranial
The axons from cells of the retinal ganglion converge to form the optic nerve. The photoreceptors in the eye send impulses to these cells, which in turn receive them (the rods and cones).
The nerve emerges from the bony orbit through the optic canal, which runs through the sphenoid bone, after forming there. It enters the cranial cavity and travels along the middle cranial fossa’s surface (very close to the pituitary gland).
a) Intracranial (The visual pathway)
The optic chiasm is made up of the optic nerves out of each eye, which joins together in the middle cranial fossa. Fibers from the temporal (lateral) halves of each retina remain ipsilateral at the chiasm, while fibers from the nasal (medial) halves cross across to the contralateral optic tract:
⦁ Left optic tract: Right nasal (medial) and left temporal (lateral) retinal fibers are found in the left optic tract.
⦁ Right optic tract: Right temporal and left nasal retina fibers are found in the right optic tract.
The lateral geniculate nuclei, a transmission system found in the thalamus, is where each optic tract travels to reach its appropriate cerebral hemisphere, where the fibers synapse. Then, via optic radiation, fibers from the LGN transmit visual information. The pathway can be broken down into:
⦁ Upper optic radiation: Fibers from the superior retinal quadrants are carried by upper optical radiation (corresponding to the inferior visual field quadrants). To get to the visual cortex, it passes through the parietal lobe.
⦁ Lower optic radiation: Fibers from the inferior retinal quadrants are carried by lower optical radiation. To get to the visual cortex, it passes through the temporal lobe through the Meyers’ loop circuit.
The brain analyses the sensory information and reacts accordingly once it reaches the visual cortex (1).
Branches Of The Optic Nerve
There are branches from each optic nerve that go to the brain or connect to other fibers. At the optic chiasm, where the two optic nerves converge:
⦁ The left eye’s nerve fibers proceed towards the left side of the brain in half.
⦁ The right side of the brain is served by half of the nerve fibers in the right eye.
⦁ The remaining nerve cells splice. The brain gets messages from both eyes simultaneously to assemble a coherent visual representation (binocular vision) (3).
Embryology Of The Optic Nerve
In the fourth week of pregnancy, the optic stalk becomes visible and develops into the optic nerve. The lumen of the optic stalk is connected distally with the optic vesicle and proximally with the forebrain’s diencephalon.
The retina develops from the optic cup, which is formed when the lateral walls of the optic vesicle invade. The choroidal fissure is created by invasion on the inferior surface of the optic stalk and vesicle. The hyaloid artery and vein are situated within this fissure, and after the union of the fissure, they develop into the central retinal artery and vein.
A CNS structure that develops from the diencephalon is the optic nerve. The optic nerve is myelinated by oligodendrocytes and is covered in the three meningeal layers as a CNS component.
The myelination process starts in the center and, as was already mentioned, ends in the lamina cribrosa. About the majority of the optic nerve fibers will have myelinated during or shortly after birth.
The three meningeal layers of the brain and the three meningeal layers that surround the optic nerve form one continuous structure. Hence, papilledema seen on fundoscopy may occur from increased intracranial pressure propagating from the cerebral subarachnoid space to the perineural subarachnoid space (5).
Blood Supply Of The Optic Nerve
The first stem of the internal carotid artery, the ocular artery, is a significant artery that supplies blood to the optic nerve. Just distal to the cavernous sinus, the OA splits off from the internal carotid artery and travels via the optic canal.
The Ophthalmic artery runs lateral and inferior to the optic nerve within the optic canal. Although the ophthalmic artery has several branches, the central retinal artery and posterior ciliary arteries are its two main branches.
Inside the optic nerve’s dura mater, the central retinal artery is the first branch of the Ophthalmic artery. At about 12 mm posterior to the globe, the central retinal artery then penetrates the optic nerve and travels anteriorly to perfuse the inner layers of the retina.
The ophthalmic artery leads to several Posterior ciliary arteries. The optic nerve and posterior uveal tract are perfused by the Posterior ciliary arteries, which travel anteriorly and pierce the sclera (5).
Optic Nerve Function
The primary job of the optic nerve is to transmit impulses from the retina in the eye to the visual regions of your brain, where one’s brain can then transform these impulses into images in the head.
The retina’s rod and cone cells, which detect various light wavelengths and cause the cells to send nerve currents to the optic nerve through the use of the retinal ganglia, gather up visual information.
The optic chiasm of the brain receives these impulses up the optic nerve. The left cerebral hemisphere receives impulses from the left eye’s optic nerve, whereas the right cerebral hemisphere receives impulses from the right eye’s optic nerve.
The optic nerve not only sends impulses that govern the light reflex and the accommodation reflex, but it also carries visual data to the cerebral cortex, such as visual acuity, brightness, and color perception (4).
The light reflex
The pupillary light reflex also referred to as the light reflex, is the pupil’s response to changes in light level. This response is visible as a change in pupil diameter. For instance, the pupil will shrink if light is directed into the eye, allowing less light to enter.
On the other hand, if the light intensity is lower, the pupil will enlarge to let more light into your eye. One can appreciate why the pupil needs to respond to high light intensity the way it does after having the pupil dilated by a doctor during an eye exam and then having to go outside into the sun. This adaptation helps individuals see better in diverse light settings.
Accommodation reflex
The accommodation reflex is the eye’s response to focusing on a close object, looking at a distant galaxy, and vice versa. It is sometimes referred to as the accommodation-convergence reflex or close response.
It achieves this by causing the pupils to constrict, the lens to thicken, and the eyes to condense, bringing close objects into focus (inward rotation of the eyes).
An illustration of the transitional time during which the eyes must adjust and carry out these adjustments is the brief period when your vision is hazy when individuals suddenly have to focus on an image in front of them (4).
Optic Nerve Disorders
Optic nerve disorders are conditions that can lead to destruction or damage to the nerve, as well as disorders that can result from damage to the optic nerve. Some of these disorders are:
⦁ Optic neuritis
Optic nerve irritation is referred to as optic neuritis (cranial nerve II). An odd blind spot (a scotoma, typically in or near the center of the visual field), eyeball pain (frequently happening with eye movement), anomalous color vision, and unexpected flashes of light are all brought on by inflammation.
Young to middle-aged adults are more commonly affected, and women are more frequently affected than men.
Multiple sclerosis, a condition with an unknown origin that damages the optic nerve, central nervous system, and spinal cord, can cause optic neuritis in some cases.
These people could have or do not have a history of neurological issues, and additional testing is frequently carried out to look into the possibility of a multiple sclerosis diagnosis. Yet, if they happen at all, other multiple sclerosis symptoms might not become completely apparent until decades after the first signs of visual neuritis.
Infections like Lyme disease or syphilis as well as unidentified sources, in which instance the optic neuritis is referred to as idiopathic, are some additional causes of optic neuritis.
Optic neuritis may be located in the nerve shaft posterior to the eyeball or in the optic disk, which is where the nerve leaves the eye (papillitis) (retrobulbar neuritis)
In most cases, the optic nerve heals from inflammation, and eyesight gradually gets better, but there is frequently some residual nerve fiber damage and some recurrence of visual symptoms. Some people get optic neuritis attacks repeatedly (6).
⦁ Glaucoma
The death of retinal ganglion cells in glaucoma causes optic neuropathy and a structure of peripheral vision loss, first preserving the central vision. Increased intraocular pressure, which harms the optic nerve as it leaves the eye, is usually linked to glaucoma.
The trabecular meshwork aids in the aqueous humor fluid’s drainage. The excess aqueous fluid causes intraocular pressure to rise, which results in the diagnosis and glaucoma symptoms.
⦁ Anterior ischaemic optic neuropathy
The optic nerve head is affected by anterior ischemic optic neuropathy, sometimes described as a “stroke of the optic nerve”.
The optic nerve head typically experiences an abrupt decrease in blood flow and nutrients. Vision loss normally happens suddenly, and it usually happens in the morning. Diabetic patients between the ages of 40 and 70 are more likely to have this problem (7).
⦁ Optic atrophy
The optic nerve shrinks when it receives insufficient blood. Optic atrophy may be brought on by head injuries, strokes, hydrocephalus, infections, or brain tumors. Certain illnesses run in families (3).
⦁ Congenital abnormalities
Optic nerve coloboma is a congenital anomaly that occurs when the optic nerve would either incompletely developed or absent as a whole. It can happen to either or both eyes.
⦁ Nutritional amblyopia
Poor nutrition or exposure to hazardous substances like lead, methanol, antifreeze, or some medicines can harm the visual nerve (8).
What is the main function of the optic nerve?
Your eyes send electrical signals to your brain via the optic nerve. This sensory data is processed by your brain so you can see.
What are the clinical manifestations of optic nerve disorders?
⦁ Eye pain
⦁ Headache
⦁ Loss of vision
⦁ Nausea
⦁ Others are tinnitus, color blindness, and night blindness (3).
Who is at risk of optic nerve disorders?
References
- https://teachmeanatomy.info/head/cranial-nerves/optic-cnii/
- https://www.physio-pedia.com/Optic_Nerve
- https://my.clevelandclinic.org/health/body/22261-optic-nerve
- https://biologydictionary.net/optic-nerve/
- https://www.ncbi.nlm.nih.gov/books/NBK507907/#:~:text=The%20optic%20nerve%20is%20a,terminates%20at%20the%20lamina%20cribrosa.
- https://www.britannica.com/science/optic-neuritis
- https://en.wikipedia.org/wiki/Optic_nerve
- https://www.allaboutvision.com/eye-care/eye-anatomy/optic-nerve/
- https://www.barrowneuro.org/condition/optic-nerve-disorders/
