What are the cranial nerves?

Anatomy

The 12 cranial nerves form a set of nerves that originate in the brain.

Their functions are sensory, motor or both at the same time:

  • Cranial sensory nerves help a person see, smell, and hear.
  • Cranial motor nerves help control muscle movement in the head and neck.

Each nerve has a name that reflects its function and a number based on its location in the brain.

Scientists use Roman numerals from I to XII to identify the cranial nerves in the brain.

Doctors can identify neurological or psychiatric disorders by assessing the functions of the cranial nerve.

I Olfactory nerve

The olfactory nerve transmits information about a person’s smell to the brain.

When a person inhales scent molecules, olfactory receptors located in the nasal passage send the impulses to the cranium and then travel to the olfactory bulb.

Olfactory neurons merge with other nerves, which pass through the olfactory tract.

The olfactory tract then travels to the frontal lobe and other areas of the brain that are involved in memory and detection of different smells.

II Optic nerve

The optic nerve transmits information about a person’s vision to the brain.

When light enters the eye, it hits the retina, which contains rods and cones. These are photoreceptors that translate signals from light into visual information for the brain.

The cones are located in the central retina and participate in color vision. The rods are located in the peripheral retina and participate in uncolored vision (night vision).

These photoreceptors carry nerve impulses along nerve cells.

Most of the optic nerve fibers intersect in a structure called the optic chiasm. Then, they project to the primary visual cortex located in the occipital lobe at the back of the brain. The occipital lobe is where the brain handles visual information.

III Oculomotor nerve

The oculomotor nerve is a cranial nerve that helps control muscle movement in the eyes.

The oculomotor nerve moves most of the muscles that move the eyeball and upper eyelid, called extraocular muscles.

The oculomotor nerve also contributes to the involuntary functions of the eye:

The pupillary muscle automatically constricts the pupil to allow less light to pass into the eye when the light is bright. When it is dark, the muscle relaxes to let in more light.

Ciliary muscles help the lens adapt to short and long distance vision. This happens automatically when a person is looking at near or far objects.

IV Trochlear nerve

The trochlear nerve is also involved in eye movement.

The trochlear nerve, like the oculomotor nerve, originates in the midbrain. It supplies the contralateral superior oblique muscle which allows the eye to point downward and inward.

V Trigeminal nerve

The trigeminal nerve is the largest cranial nerve and has motor and sensory functions.

Its motor functions help a person chew and clench their teeth and contract the muscles of the eardrum.

It consists of three parts that connect to sensory receptors on the face:

The ophthalmic part transmits sensation to certain parts of the eyes, including the cornea, the lining of the nose, and the skin of the nose, eyelid and forehead.

The jawbone transmits sensation to part of the face, the side of the nose, the upper teeth and the lower eyelid.

The mandibular part transmits sensation to the lower part of the face, tongue, mouth and lower teeth.

Trigeminal neuralgia can cause severe pain and facial tics.

VI Nerf Abducens

The abducens nerve also helps control eye movements.

It helps the lateral rectus muscle, which is one of the extraocular muscles, to look outward.

The abducens nerve originates in the brainstem and ends in the lateral muscle within the bone orbit.

VII Facial nerve

The facial nerve works to produce facial expressions.

The facial nerve also has motor and sensory functions.

It is made up of four cores which perform different functions:

  • movement of the muscles that produce facial expression;
  • movement of the lacrimal, submandibular and submandibular glands;
  • sensation of the outer ear;
  • sensation of taste;

Bell’s palsy is a common disorder of the facial nerve, which causes paralysis on one side of the face and possibly loss of taste sensation.

VIII Vestibulocochlear (or auditory) nerve

The vestibulocochlear nerve is involved in a person’s hearing and balance.

The vestibulocochlear nerve contains two parts:

The vestibular nerve helps the body detect changes in the position of the head relative to gravity. The body uses this information to maintain balance.

The cochlear nerve helps to hear. Specialized internal hair cells and the basilar membrane vibrate in response to sound and determine the frequency and magnitude of sound.

IX Glossopharyngeal nerve

The glossopharyngeal nerve has motor and sensory functions.

  • Sensory function receives information from the throat, tonsils, middle ear, and the back of the tongue. It is also involved in the sensation of taste for the back of the tongue.
  • The motor function which is associated with the movement of the throat.

X vagus nerve

The vagus nerve is a cranial nerve that has motor, sensory, and parasympathetic functions.

  • The sensory part provides sensation in the outer part of the ear, throat, heart and organs of the abdomen. It also plays a role in taste sensation.
  • The motor part controls the movement of the throat and palate.
  • Parasympathetic function regulates the heart rate and innervates smooth muscles in the airways, lungs, and gastrointestinal tract.

The vagus nerve is the longest cranial nerve as it extends to the abdomen.

Doctors use vagus nerve stimulation therapy to treat a variety of conditions, including epilepsy, depression, and anxiety.

XI Accessory nerve

The accessory nerve provides motor function to the neck.

The accessory nerve separates into a spinal (which begins in the spinal cord and travels through the skull) and cranial part.

XII Hypoglossal nerve

The hypoglossal nerve is a motor nerve that supplies the muscles of the tongue. It takes its source in the marrow.

Disorders of the hypoglossal nerve can cause paralysis of the tongue, most often on one side.