Cranial Nerves

What are the cranial nerves?

Your cranial nerves are a set of 12 nerves that send electrical signals between your brain and different parts of your head, face, neck and torso. These signals help you see, smell, taste, hear and move your facial muscles. Your cranial nerves begin toward the back of your brain. They’re a key part of your nervous system.

You might first think of your eyes, nose, ears and mouth when it comes to using your senses. But these body parts don’t work properly without healthy cranial nerves. You can also thank your cranial nerves for allowing you to make facial expressions and communicate.

How many cranial nerves are there?

You have 12 cranial nerve pairs. Each nerve pair splits to serve the two sides of your brain and body. For example, you have one pair of olfactory nerves. One olfactory nerve is on the left side of your brain, and one is on the right side of your brain.

What is the function of the cranial nerves?

Your cranial nerves play a role in relaying sensory and/or movement (motor) information.

Sensory nerves can help you:

• Feel touch and sense pain and temperature.
• Hear.
• See.
• Smell.
• Taste.

Motor nerves play a role in controlling specific muscles. Some cranial nerves have both sensory and motor functions.

Your 12 cranial nerves each have a specific function. Healthcare providers categorize the cranial nerves based on number and function:

• Olfactory nerve (CN I): Providing the sense of smell.
• Optic nerve (CN II): Providing vision.
• Oculomotor nerve (CN III): Opening and moving your eyes and adjusting pupil width.
• Trochlear nerve (CN IV): Looking down and moving your eyes toward your nose or away from it.
• Trigeminal nerve (CN V): Providing sensations in your eyes, most of your face and inside your mouth. It also allows you to chew food.
• Abducens nerve (CN VI): Moving your eyes from left to right.
• Facial nerve (CN VII): Controlling several facial muscles to make facial expressions and providing the sense of taste in part of your tongue.
• Vestibulocochlear nerve (CN VIII): Providing the sense of hearing and balance.
• Glossopharyngeal nerve (CN IX): Providing taste sensations to part of your tongue and controlling muscles for swallowing. It also has parasympathetic nerve fibers that play a role in blood pressure regulation and saliva (spit) production. 
• Vagus nerve (CN X): Regulating several automatic bodily processes, including your digestion, blood pressure, heart rate, breathing, mood, saliva production and more. It’s the main nerve of your parasympathetic nervous system.
• Accessory nerve or spinal accessory nerve (CN XI): Controlling shoulder and neck movement.
• Hypoglossal nerve (CN XII): Controlling tongue movement, which plays a role in speaking, eating and swallowing.

Where are the cranial nerves located?

Two of your cranial nerve pairs — your olfactory and optic nerves — begin in your cerebrum and branch out to their target tissues. The cerebrum is the largest portion of your brain that sits above your brainstem.

The other 10 pairs of cranial nerves start in your brainstem and then branch out. Your brainstem connects your brain and spinal cord.

What is the longest cranial nerve?

The longest cranial nerve is your vagus nerve. It runs from your brain to your large intestine, innervating several tissues along the way.

What conditions affect your cranial nerves?

When a cranial nerve experiences damage, it doesn’t function as it should. This may cause specific movement or sensory issues, depending on its underlying function. Several conditions and situations can affect the functioning of your cranial nerves, like:

• Head trauma or traumatic brain injury.
• Nervous system or brain conditions, such as neuropathy and amyotrophic lateral sclerosis (ALS).
• Pressure from nearby blood vessels, tumors or swelling.
• Circulatory issues, like microvascular disease and stroke.
• Inflammation from certain infections, such as the herpes zoster virus (the virus that causes shingles).
• Autoimmune conditions, like scleroderma and Sjögren’s syndrome.
• Accidental damage from brain, facial or dental surgery.

Cranial nerve palsies

When there’s an issue with a specific cranial nerve that has a motor (movement) function, healthcare providers often call it a palsy. “Palsy” means “paralysis.” A palsy results in muscle weakness and movement issues.

Examples of cranial nerve palsies include:

• Third nerve palsy (oculomotor palsy): This condition typically causes one of your eyes to stay positioned as though you’re looking down and out to the side.
• Fourth nerve palsy (trochlear nerve palsy): This condition typically causes double vision vertically and difficulty looking downward.
• Sixth nerve palsy (abducens nerve palsy): This condition typically causes eye misalignment (strabismus) and double vision.
• Bell’s palsy: This condition affects your facial nerve and is a type of facial nerve palsy. It usually causes temporary drooping on one side of your face.

Other cranial nerve conditions

Another specific type of cranial nerve condition is trigeminal neuralgia. This condition causes episodes of intense facial pain that can disrupt your everyday activities. It typically happens when a blood vessel puts pressure on your trigeminal nerve.

Some cranial nerves, like your optic nerve and vagus nerve, can have several types of dysfunction. For example, conditions that affect your optic nerve include glaucoma and optic neuritis. Conditions that can arise from vagus nerve dysfunction include gastroparesis and vasovagal syncope (fainting).

How can I keep my cranial nerves healthy?

Some cranial nerve conditions are unpreventable. But you can help keep your brain, cranial nerves and entire nervous system healthier with these steps:

• Reach and maintain a weight that’s healthy for you.
• Eat nutrient-dense foods like fruits, vegetables and whole grains.
• Get regular physical activity.
• Keep your blood pressure within a healthy range.
• Manage any conditions that increase your risk of nerve damage, like diabetes.
• Drink alcohol only in moderation.
• Quit smoking.

When should I see my healthcare provider?

The symptoms of cranial nerve injuries or conditions vary depending on which nerve is affected. Contact your healthcare provider if you experience:

• Drooping on one side of your face.
• Facial pain or numbness.
• Muscle weakness or paralysis.
• Tingling anywhere in your body.
• Vision issues.
• Taste or smelling issues.
• Slurred speech (dysarthria).
• Difficulty chewing or swallowing (dysphagia).

Is there an easy way to remember the cranial nerves with a mnemonic?

Cranial nerve mnemonics are memory devices to help you remember the names of the nerves in order of one through 12. They can also help you remember whether the nerves are sensory, motor or both.

Cranial nerve mnemonics to remember the names of the nerves in order include:

• On old Olympus’s towering top, a Finn and German viewed some hops.
• Ooh, ooh, ooh to touch and feel very good velvet. Such heaven!

It’s important to note that in these mnemonics, “some” and “such” refer to another name for the accessory nerve: spinal accessory nerve.

To remember cranial nerve functions, the words in the mnemonic start with:

• S for sensory.
• M for motor.
• B for both.

The word order mirrors the numerical order of nerves one through 12:

• Some say marry money, but my brother says big brains matter more.
• Some say my mother bought my brother some bad beer, my, my.