The Anatomy of the Cranial Nerves

There are 12 pairs of cranial nerves. They each emerge separately from the brain stem, which is the lower part of the brain that links the brain with the spinal cord.

Each cranial nerve has a specific set of functions. Some of the cranial nerves control sensation, some control muscle movement, and some have both sensory and motor effects.

Several of the cranial nerves run through bones in the skull. The cranial nerves can become temporarily or chronically impaired as a result of illness, infection, inflammation, or head trauma.

This article gives an overview of each pair of cranial nerves, including its anatomy, location, functions, and related conditions.

Olfactory Nerve (Cranial Nerve One)

Your olfactory nerve controls your sense of smell. This nerve detects odors and sends messages to the brain.

Location

Many tiny olfactory nerve fibers are embedded in your nose. The fibers merge together in the olfactory bulb, which is located above your nose. The nerve sends messages to the olfactory cortex, a region in the brain that distinguishes odors. The sense of smell also plays a major role in mediating the perception of taste.

Conditions 

The function of your olfactory nerve can become temporarily impaired when you have an infection, swelling, or inflammation in the region of your olfactory nerve fibers (such as when you have an upper respiratory infection or nasal allergies). 

Due to its location and length, the olfactory nerve can become injured as a result of head trauma. An injury of this nerve can result in a diminished sense of smell and decreased appreciation for taste.

Toxins can interfere with the function of your olfactory nerve, potentially resulting in a permanent deficit of the sense of smell. A tumor in the area around your nose can also impair the function of your olfactory nerve.

Optic Nerve (Cranial Nerve Two)

Your optic nerve controls your sense of vision. This nerve detects light, colors, and shapes and sends the messages to the occipital lobe in your brain so that you can make sense of the things that you see. 

Location and Structure 

Each of your optic nerves runs from the corresponding retina (vision receptor cells in your eye) into the brain through the optic canal (a tunnel formed of bone). The right optic nerve comes from your right eye, and the left optic nerve comes from your left eye.

Within the brain, your optic nerves merge at the optic chiasm, an area right below your pituitary gland. The nerves divide and send messages to the right and left occipital lobes (visual cortex) in the back of the head. 

Conditions 

Your optic nerve can become swollen due to a condition called papilledema. This condition causes headaches and blurred vision. It results from pressure in the back of the eye pushing on the optic nerve.

It can occur due to a brain tumor or obstruction of the cerebrospinal fluid (CSF) in the brain, and the symptoms can improve once pressure on the optic nerve is relieved.

Your optic nerve can also be affected by demyelination, which is loss of the protective myelin coating around nerves. Demyelination occurs in multiple sclerosis (MS) and causes vision loss. A tumor or a traumatic injury to the optic nerve can affect your vision as well.

Oculomotor Nerve (Cranial Nerve Three)

Your oculomotor nerve controls many of your eye movements. This nerve provides stimulation to the muscles that control your pupils.

It also controls muscles that move your eyes up and down, muscles that turn your eyes up and away from your nose, and muscles that move your eyes toward your nose. Additionally, your oculomotor nerve stimulates the muscles that open your eyelids.

Location and Structure 

Each of your oculomotor nerves emerges from the midbrain, which is the upper region of the brainstem. Each oculomotor nerve travels through the cavernous sinus (a tunnel formed of bone) to the eye on the same side as the nerve. The oculomotor nerve divides into small branches, each of which sends messages to an individual muscle.

Conditions 

If your oculomotor nerve becomes impaired, you would have a limitation in your eye movements, a droopy eyelid, and/or enlarged pupils. This can cause diplopia (double vision) or nystagmus (eye jerking movements), and your eye may deviate out and down.

Your oculomotor nerve function can become impaired due to MS, a brainstem stroke, a brain tumor, or a brain aneurysm.

Trochlear Nerve (Cranial Nerve Four) 

Your trochlear nerve controls eye movement so that your eyes can move down and away from your nose. This nerve sends motor stimulation to the superior oblique muscle.

Location and Structure 

Your trochlear nerve emerges from your midbrain, below the level of your oculomotor nerve. This nerve travels to your ipsilateral (on the same side) eye to power the superior oblique muscle.

Conditions

Generally, trochlear nerve impairment affects only one eye, although in rare instances it can affect both eyes. You may experience double vision, blurred vision, or nystagmus as a result of trochlear nerve impairment.

You may also develop a lazy eye—the eye on the same side of the damaged trochlear nerve would deviate up and away from your nose.

Trochlear nerve damage can occur due to trauma, MS, a stroke, a brain tumor, or a brain aneurysm.

Trigeminal Nerve (Cranial Nerve Five)

This is a large nerve that mediates sensations of the face and eye, and controls some of the muscle movements involved in chewing and swallowing.

Location and Structure 

Your trigeminal nerve has three sensory nerve branches—the ophthalmic nerve, the maxillary nerve, and the mandibular nerve.

The ophthalmic nerve detects sensation on the upper part of the face, the maxillary nerve detects sensation on the middle region of the face, and the mandibular branch detects sensation from the lower part of the face and has motor function as well. 

The trigeminal nerve emerges from the pons of the brainstem, which is below the midbrain. 

Conditions 

Trigeminal neuralgia, a painful condition characterized by severe pain in one side of the face, is the most common condition that affects the trigeminal nerve.

This nerve can become impaired due to traumatic injury or a brain tumor, which would result in diminished sensation and/or problems chewing.

Abducens Nerve (Cranial Nerve Six)

Your abducens nerve controls certain eye movements that allow your eye to move outward away from your nose. Each of your abducens nerves stimulates the ipsilateral lateral rectus muscle. 

Location and Structure 

This nerve emerges from the lower pons and travels to the eye toward the lateral rectus muscle.

Conditions

Abducens nerve damage can cause diplopia or a lazy eye. If you have damage to this nerve, it can cause your eye to deviate inward toward your nose.

Your abducens nerve can become impaired due to MS, inflammation, an infection, a stroke, or a brain aneurysm. A tumor or trauma can damage this nerve as well.

Facial Nerve (Cranial Nerve Seven) 

Your facial nerve, or cranial nerve seven, controls most of your facial muscle movements and is involved in the production of tears from your eyes and saliva in your mouth. This nerve also has branches that help in the detection of taste, and it has a sensory branch that detects sensation behind your ear.

Location and Structure 

This is a large nerve with many branches. It arises from two roots at the pontomedullary junction and divides into branches throughout the face.

Conditions 

The facial nerve is most closely associated with Bell’s palsy, a condition in which you have weakness of one side of the face.

Herpes simplex virus activation has become widely accepted as the likely cause of Bell’s palsy in most cases. It can also be associated with herpes zoster, Lyme disease, diabetes-induced nerve disease, and pregnancy.

Bell’s palsy is usually self-limited (it improves on its own) and does not typically cause any serious health consequences.

If you develop signs and symptoms of Bell’s palsy, you may need neurological tests such as brain computerized tomography (CT), magnetic resonance imaging (MRI), or lumbar puncture (LP).

Vestibulocochlear Nerve (Cranial Nerve Eight)

Your eighth cranial nerve helps control your sense of hearing and balance. 

Location and Structure 

The vestibulocochlear nerve sensory fibers are located in the inner ear and combine to enter into the lower part of the pons. 

The vestibular (balance) and the cochlear (hearing) components of the vestibulocochlear nerve each receive information based on the movement of tiny hair cells in the inner ear.

This information is used to tell your body about your position (so that you can maintain balance) and to send sound signals to your brain (so you can make sense of the sounds you hear).

Conditions 

Toxins, infections, inflammation, a brain tumor, and trauma can damage this nerve. The vestibulocochlear nerve can be impaired due to congenital (from birth) conditions as well.

Impairment of this nerve can cause tinnitus (ringing in the ears), hearing loss, dizziness, and/or balance problems.

Glossopharyngeal Nerve (Cranial Nerve Nine)

This nerve is involved with swallowing, taste sensation, and production of saliva. It also detects sensation in the back of the throat and the ear.

Location and Structure 

The glossopharyngeal nerve emerges from the medulla, which is the lowest part of the brainstem, located above the spinal cord. This nerve travels down to the mouth and throat. 

Conditions 

You can have numbness of the ear and throat, diminished taste sensation, trouble swallowing, and/or a hoarse voice if you experience impairment of your glossopharyngeal nerve. Damage or impairment of this nerve can occur due to a tumor or traumatic nerve damage.

Vagus Nerve (Cranial Nerve 10)

Your vagus nerve controls vital functions such as breathing, and it plays a role in controlling your heart function and digestion. This nerve provides parasympathetic stimulation throughout your body, which involves the release of hormones that allow your body to carry out functions of survival.

Location and Structure 

The vagus nerve emerges from the medulla. This nerve travels outside the skull, alongside the carotid artery in the neck. The vagus nerve divides into branches that reach the heart, lungs, and gastrointestinal system.

Conditions 

The vagus nerve or any of its branches can be damaged due to a tumor or trauma. This can cause symptoms such as a hoarse voice, or parasympathetic deregulation such as breathing or heart issues.

Vagus nerve stimulation (VNS) therapy is a type of electrical therapy used to manage conditions such as epilepsy and pain. VNS therapy involves surgical placement of a stimulator near the vagus nerve which sends a signal to the cerebral cortex of the brain to achieve its therapeutic effect.

Accessory Nerve (Cranial Nerve 11)

The accessory nerve helps you raise your shoulders and turn your head and neck. The nerve stimulates the sternocleidomastoid and trapezius muscles in your upper back and neck.

Location and Structure 

This nerve arises from the medulla and travels outside the skull down toward the sternocleidomastoid and trapezius muscles.

Conditions 

Your accessory nerve can be damaged due to trauma. Damage to your accessory nerve would result in decreased ability to shrug your shoulders or turn your neck. It may result in a physical sign described as “winging,” which is a protrusion of the shoulder blade. 

Hypoglossal Nerve (Cranial Nerve 12)

This nerve controls the movement of your tongue to coordinate your ability to speak and swallow. 

Location and Structure 

The hypoglossal nerve emerges from the medulla and travels under the mouth to the muscles of the tongue.

Conditions

Your hypoglossal nerve can become damaged due to direct trauma or nerve damage. Signs of hypoglossal nerve damage include tongue weakness and tongue deviation toward the weak side (because the muscles in the weak side can’t push the tongue toward the middle).

Hypoglossal nerve damage can result in diminished swallowing ability, which may result in choking.

A Word From Verywell 

The 12 pairs of cranial nerves are each involved in highly specialized functions. Cranial nerve impairment can occur due to face, head, or neck trauma.

When you have impairment of one or more cranial nerves without a history of trauma, it can be a sign of a serious medical issue. You may need diagnostic tests such as imaging studies, hearing or vision examinations, or electromyography to help identify the problem. The results will guide your medical team in planning your treatment.