What Is the Optic Nerve?
The optic nerve is part of the eye, the central nervous system (CNS), and one of the 12 cranial nerve pairs.1 It’s also known as the second cranial nerve or cranial nerve II.
The optic nerve is responsible for transmitting visual information to the brain for interpretation. This enables clear vision.
Each eye consists of one optic nerve. Any damage to this nerve may cause vision issues, or even vision loss, depending on the extent of the damage. Optic nerve damage may affect one or both eyes.
Where is the Optic Nerve?
The optic nerve extends from the eye to the brain. It begins in the optic head (optic disc), runs through the optic canal and ends at the lateral geniculate nucleus.
The optic nerve exists as a bundle of about 1.2 million nerve fibers made up of retinal ganglion cells (RGCs) and glial cells.2 Glial cells produce and maintain the supportive tissues of the CNS that interact with the optic nerve. They include oligodendrocytes, astrocytes, and microglia.
The optic nerve has four parts:
- Intraocular part (optic nerve head). About 0.7 to 1.0 mm
- Intraorbital part. About 25 to 30mm
- Intracanalicular. About 4 to 10mm
- Intracranial. About 10mm
Although their length may vary even within the same person, the approximate length of an optic nerve is between 35 to 55 millimeters (mm).
The optic nerve head (optic disc) is attached to the back of the eye at the lamina cribrosa of the retina, a mesh-like structure through which the retinal ganglion cell axons and retinal blood vessels pass from the eye to the brain.
The section consisting of the intraorbital, intracanalicular, and intracranial parts extends towards the brain. It’s enclosed by a layer of glial cells known as oligodendrocytes, which are for support and insulation of brain tissues.
The optic nerves from opposite eyes join at an X-shaped area known as the optic chiasm or optic chiasma.3 This is an important area where visual information from both eyes is shared between both sides of the brain. It’s important for binocular vision (coordinated vision of both eyes).4
The nerve extends after the optic chiasm to form the lateral geniculate nucleus (LGN).5 The LGN is the point of origin for optic radiations (ORs), which connect the optic nerve to the brain. ORs relay visual information to the brain for processing.
The optic nerve receives its blood supply through the posterior ciliary artery (PCA) .
What are Optic Nerve Branches?
Optic nerve branches are sections of the optic nerve that travel to your brain or connect to other fibers responsible for vision. Three things happen when the two optic nerves meet at the optic chiasm are:
- Half of the nerve fibers from the left eye extend to the left side of your brain
- Half of the right eye’s nerve fibers extend to the right side of your brain
- The remaining halves of each eye cross over the chiasm to the opposite side
These nerve branches enable the brain to receive signals from both eyes, creating a cohesive visual image.
Optic Nerve Functions
The optic nerve plays a critical role in helping us understand the world around us by transmitting visual signals to the brain. When light enters the eye, the cornea and eye lens focus it on the retina. The retina creates impulses transmitted to the brain for processing via the optic nerve.
The following are health conditions that can damage the optic nerve and affect vision.
Glaucoma is a set of diseases that can cause vision loss due to optic nerve damage. According to statistics from the Center for Disease Control (CDC), glaucoma affects 3 million Americans and is one of the leading causes of vision loss globally.6
It’s not clear yet what causes the common types of glaucoma, but high eye pressure (ocular hypertension) is common among glaucoma patients.
Glaucoma is hardly noticeable during the early stages. However, as it progresses, one may notice considerable changes in side (peripheral) vision that worsen over time.
Glaucoma diagnosis is only through a comprehensive dilated eye exam. Unfortunately, there is no cure, but treatments such as those that lower eye pressure can stop further damage to the optic nerve.
Optic neuritis is characterized by the swelling of the optic nerve, causing damage.7 The inflammation often occurs in the optic nerve section before the optic chiasm. For this reason, the condition only affects one eye at a time.
The causes of optic nerve inflammation include:
- Multiple sclerosis (MS)
- Viral infections
- Sinus diseases
- Exposure to drugs and chemical toxins
Most people with optic neuritis recover without treatment, but medications may be necessary to speed up recovery and prevent vision loss.
Optic Nerve Atrophy
Optic nerve atrophy (ONA) is damage to the optic nerve. It may arise due to a tumor, eye trauma, ischemia, eye infection, exposure to chemicals, or certain degenerative disorders. Research also shows that parents can pass the condition to offspring.8
This condition affects central, peripheral, and color vision. If ONA occurs in a child, they may experience rapid and repetitive involuntary eye movements (nystagmus).
ONA diagnosis is through a comprehensive eye exam that includes an assessment of color and contrast vision. Optic nerve damage cannot be reversed, but treating the underlying cause can prevent further damage.
Drusen (singular: druse) are yellow-colored spots made up of protein and calcium. Drusen can be present in both eyes (bilateral), but it’s possible to have them in one (unilateral).
The condition often goes undiagnosed because most people with drusen have no visual symptoms. However, some people may experience peripheral vision loss. Visual field exams are necessary for determining the extent of drusen-associated vision loss.
Current research estimates the prevalence of optic nerve drusen at about 0.5 to 2.4%.9 Unfortunately, there is no treatment for optic nerve drusen.
Optic Nerve Coloboma
Colobomas are developmental defects that mostly occur before birth. They are characterized by missing tissues in some parts of the eyes, including the optic nerve.
Colobomas affecting the optic nerve often result in low vision. Low vision can’t be corrected by contact lenses or eyeglasses.
Gliomas are tumors that develop in the brain or spine. They start to form in the glial cells and are classified based on the type of cells affected and the tumors’ genetic features. Optic nerve gliomas are common in children, but the exact cause is unknown.
As the tumor grows, it presses on the optic nerve, affecting vision. According to studies, about 5% of people with glioma will develop blindness.10
Papilledema (papilloedema) refers to the swelling of the optic nerve head (optic disc) due to increased intracranial pressure (pressure inside the brain).11 In most cases, both eyes are affected. Causes may include head trauma, high blood pressure, blood clots, and brain infection.
Although visual problems are uncommon with papilledema, some people may experience short flickers, blurred or double vision, decreased field of vision, and decreased color vision. Treatment involves addressing the cause of intracranial pressure.
Underneath the optic chiasm sits the pituitary gland. Because the optic nerve fibers cross at the chiasm, any enlargement of the pituitary gland due to a tumor or growth may push on the optic chiasm. This can cause visual abnormalities in both eyes.
Signs of Optic Nerve Disorders
The symptoms of optic nerve issues may vary depending on the underlying condition. They may also be temporary or permanent. Common ones include:
- Loss of color vision
- Eye pain
- Flashing lights
- Nausea and vomiting
- A decline in brain function
- Night blindness
- Partial or complete vision loss
- Peripheral vision loss
- Seizures (in people without a history of seizures)
- Toxic amblyopia (nutritional amblyopia)
When to See an Eye Doctor
Call your health care provider if you experience severe eye pain, chronic headaches, vision changes, or vision loss. Early diagnosis can stop further damage or help manage the condition.
The optic nerve is an extension of the central nervous system (CNS), which is responsible for transmitting visual information to the brain. This enables clear vision.
The nerve exists as a bundle of about 1.2 million nerve fibers made up of retinal ganglion cells (RGCs) and glial cells. It’s enclosed in a layer of oligodendrocytes, a type of glial cells.
Conditions affecting the optic nerve include glaucoma, papilledema, adenoma, drusen, coloboma, optic neuritis, and atrophy. Some of these conditions have no cure, but early diagnosis is important.
In this article