Optic Nerve

What is the optic nerve?

The optic nerve is comprised of millions of nerve fibers that send visual messages to your brain to help you see. You have an optic nerve at the back of each eye that connects directly to your brain. Each optic nerve is a one-way connection, and it only carries signals from your eyes to your brain. Some of the signals in the optic nerve also contribute to other abilities and brain processes.

What is the purpose of the optic nerve?

The optic nerve is a critical part of your vision. Your eyes are like cameras, with the retinas at the back of each eye detecting light and converting what they pick up into electrical signals. The optic nerve is like the cable that carries those signals to the computer that is your brain.

The optic nerve is the second of 12 cranial nerves, which get their name from how they all directly connect to your brain. Each cranial nerve is a left- and right-sided pair (but experts commonly refer to them like it’s just a single nerve).

The optic nerve (also known as Cranial Nerve II or CN II) is extra special among the cranial nerves because of how it forms. It’s the only cranial nerve that’s also part of your central nervous system (CNS), along with your brain and spinal cord. The other 11 cranial nerves are part of your peripheral nervous system (PNS).

Where is the optic nerve?

You have an optic nerve at the back of each eye that connects directly to your brain. To get there, the optic nerves extend out from your retina and travel a route that includes the following:

• The optic canal: This is a bony opening that your optic nerves pass through to enter your skull and reach your brain.
• The optic chiasm (pronounced “KY-az-mm”): This Y-shaped junction is where the nerve fibers meet up to travel together. Some of the nerve fibers from both optic nerves also switch sides. That crossover is part of how your brain organizes left- and right-sided input from both eyes so it can merge them into the single, seamless picture that you see.
• The brain: Just after the separate optic nerves join at the optic chiasm, they enter your brain. Once in your brain, they head straight to the visual cortex, part of your occipital lobe at the back of your brain. That’s where most of the visual processing happens.

As it travels through your brain, a small fraction of the nerve fibers branch off to other places in your brain. Those fibers don’t go through processing in the visual cortex. Instead, they support abilities like the following:

• Pupil reflexes. The pupils of your eyes automatically adjust to let more or less light in. That reflex needs to be fast, which is why it involves nerve fibers that branch off before they get to your visual cortex.
• Accommodation reflex. This reflex involves muscles in the ciliary body of each eye. Those muscles adjust the shape of each eye’s lens. That’s how your eyes automatically focus on whatever you’re looking at.
• Circadian rhythm. This is like your body’s internal clock. It manages your natural sleep/wake cycle and contributes to several processes like blood pressure, body temperature and blood sugar. The optic nerve fibers that branch off for this tell your brain about the light they detect. Most people’s brains use that to help anchor their circadian rhythms to daytime and nighttime.

What conditions and disorders affect the optic nerves?

These conditions can damage an optic nerve and affect your vision:

• Glaucoma: Fluid buildup near the front of your eyes can put pressure on and damage your optic nerves.
• Anterior ischemic optic neuropathy: The condition causes sudden vision loss and happens when there’s a disruption in blood flow to your optic nerve.
• Congenital abnormalities: These are differences in how your optic nerves developed. “Congenital” means you had them when you were born.
• Optic atrophy: A chronic lack of blood flow to your optic nerve can make the nerve shrink, similar to how an unwatered plant withers.
• Optic nerve coloboma: This is an inherited condition that affects and disrupts optic nerve development. It can happen to one or both eyes.
• Optic nerve drusen: These are deposits made of protein, calcium and fatty compounds. They can build up in various places near the backs of your eyes, including your optic nerve.
• Optic nerve gliomas: Gliomas are tumors (growths) that can form on your optic nerve. They’re usually benign (not cancerous).
• Optic nerve meningiomas: These slow-growing tumors are rare and benign, but they can sometimes cause severe vision loss.
• Optic neuritis: Infections and autoimmune diseases like multiple sclerosis (MS) can irritate or inflame your optic nerve, causing optic neuritis.
• Papilledema: Pressure around your brain from a traumatic brain injury, brain tumors, meningitis or other conditions can make your optic nerve(s) swell.
• Neuromyelitis optica spectrum disorder (NMOSD): Often known simply as NMO, this condition happens when your immune system mistakenly attacks your optic nerves and spinal cord.

What are some common symptoms of optic nerve disorders?

Optic nerve problems cause various symptoms depending on the underlying condition. The symptoms may be temporary or permanent. Some of the most common symptoms include (but aren’t limited to) the following:

• Blind spots (scotomas).
• Blurred vision.
• Color blindness.
• Double vision (diplopia).
• Eye irritation.
• Eye pain.
• Flashing lights in your eyes (photopsias).
• Headaches.
• Nausea and vomiting.
• Night blindness (nyctalopia).
• Partial vision loss (low vision) or complete vision loss (blindness).
• Peripheral (side) vision loss.
• Sudden vision loss.
• Visual field defects.

How can I protect my optic nerves?

There are several things you can do that can maintain and protect the health of your optic nerves. They include:

• Getting regular eye exams (even if you don’t need glasses or contacts).
• Reaching and maintaining a weight that’s healthy for you.
• Making sure you get enough of the nutrients that your nerves need. Some important examples include vitamins B1, B6, B12 and the mineral copper.
• Managing conditions that affect circulation, brain health, vision and nerves, like diabetes and high blood pressure.
• Quitting tobacco products (including smokeless tobacco and vaping), or not using them to begin with.
• Protecting your head and eyes with protective gear like helmets and by wearing seatbelts in your car.
• Protecting your eyes from other hazards by wearing safety goggles when using tools or machinery, or wearing sunglasses to help reduce the impact of bright light on your eyes.

When should I talk to a doctor?

You should talk to an eye care specialist or healthcare provider if you notice gradual vision changes, increases in eye pain or vision loss. That includes things like blurred or double vision. You should seek medical attention immediately if you have sudden vision changes or vision loss, especially if you’ve never had them before.