The cranial nerves are a set of twelve nerves that originate in the brain. Each has a different function responsible for sense or movement. They include the olfactory nerve, which is essential for detecting smells and the optic nerve, which enables a person to see.
The functions of the cranial nerves are sensory, motor, or both. Sensory cranial nerves help a person see, smell, and hear. Conversely, motor cranial nerves help control muscle movements in the head and neck.
Each nerve has a name that reflects its function and a number according to its location in the brain. Scientists use Roman numerals from I to XII to label the cranial nerves in the brain.
The 12 cranial nerves include the:
- olfactory nerve
- optic nerve
- oculomotor nerve
- trochlear nerve
- trigeminal nerve
- abducens nerve
- facial nerve
- vestibulocochlear nerve
- glossopharyngeal nerve
- vagus nerve
- accessory nerve
- hypoglossal nerve
This article will explore the functions of each of the cranial nerves and provide a diagram.
The olfactory nerve transmits information regarding a person’s sense of smell to the brain.
When an individual inhales fragrant molecules, olfactory receptors within the nasal passage
Specialized olfactory neurons and nerve fibers meet with other nerves, which pass into the olfactory tract.
The olfactory tract then travels to the frontal lobe and other areas of the brain that have a role in memory and the notation of different smells.
The optic nerve transmits information to the brain regarding a person’s vision.
When light enters the eye, it hits the retina, which contains rods and cones. These are photoreceptors that
These cones sit within the central retina and have a role in color vision. Conversely, the rods in the peripheral retina are responsible for noncolor vision.
The photoreceptors carry signal impulses along nerve cells to form the optic nerve. Most of the fibers of the optic nerve cross into a structure called the optic chiasm. The optic tract then projects to the primary visual cortex in the occipital lobe at the back of the brain. The occipital lobe is where the brain handles visual information.
The oculomotor nerve helps control muscle movements of the eyes.
The oculomotor nerve
The oculomotor nerve also helps with involuntary functions of the eye. For example, the sphincter pupillae muscle automatically constricts the pupil to allow less light into the eye in bright light conditions. When it is dark, the muscle relaxes to allow more light to enter.
Another function is when the ciliary muscles help the lens adjust to short-range and long-range vision. This happens automatically when a person looks at near or far objects.
The trochlear nerve, like the oculomotor nerve, originates in the midbrain. It powers the contralateral superior oblique muscle that allows the eye to point downward and inward.
The trigeminal nerve is the largest cranial nerve and has both motor and sensory functions.
Its motor functions help a person to chew and clench the teeth. It also
Its sensory division has three parts that connect to sensory receptor sites on the face:
- The ophthalmic part gives sensation to parts of the eyes, including the cornea, mucosa in the nose, and skin on the nose, the eyelid, and the forehead.
- The maxillary part gives sensation to the middle third of the face, side of the nose, upper teeth, and lower eyelid.
- The mandibular part gives sensation to the lower third of the face, the tongue, mucosa in the mouth, and lower teeth.
Trigeminal neuralgia is a common disorder of the trigeminal nerve that can cause intense pain and facial tics.
The abducens nerve
It helps the lateral rectus muscle, one of the extraocular muscles, turn the gaze outward.
The abducens nerve starts in the pons of the brainstem, enters an area called Dorello’s canal, travels through the cavernous sinus, and ends at the lateral rectus muscle within the bony orbit.
The facial nerve also has both motor and sensory functions.
The facial nerve is
- movement of muscles that produce facial expression
- movement of the lacrimal, submaxillary, and submandibular glands
- the sensation of the external ear
- the sensation of taste
The four nuclei originate in the pons and medulla and join together to travel to the geniculate ganglion.
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.
The vestibulocochlear nerve
This nerve contains two components: the vestibular nerve and the cochlear nerve. The vestibular nerve helps the body sense changes in the position of the head with regard to gravity. The body uses this information to maintain balance.
The cochlear nerve helps with hearing. Specialized inner hair cells and the basilar membrane vibrate in response to sounds and determine the frequency and magnitude of the sound.
These fibers combine in the pons and exit the skull via the internal acoustic meatus in the temporal bone.
The glossopharyngeal nerve
The sensory function receives information from the throat, tonsils, middle ear, and back of the tongue. It also has a role in the sensation of taste on the back of the tongue.
The motor division provides movement to the stylopharyngeus, a muscle that allows the throat to shorten and widen.
The glossopharyngeal nerve starts in the medulla oblongata in the brain and leaves the skull through the jugular foramen, which leads to the tympanic nerve.
The vagus nerve
- The sensory part provides sensation to the outer part of the ear, throat, heart, and abdominal organs. It also plays a role in taste sensation.
- The motor part provides movement to the throat and soft palate.
- The parasympathetic function regulates heart rhythm and innervates the smooth muscles in the airway, lungs, and gastrointestinal tract.
The accessory nerve
It controls the sternocleidomastoid and trapezius muscles that allow a person to rotate, extend, and flex the neck and shoulders.
The accessory nerve separates into spinal and cranial parts.
The spinal component starts in the spinal cord and travels into the skull through the foramen magnum. From there, it meets the cranial component of the accessory nerve and exits the skull along the internal carotid artery.
The cranial part of the accessory nerve combines with the vagus nerve.
The hypoglossal nerve is a motor nerve that
Disorders of the hypoglossal nerve can cause paralysis of the tongue, most often occurring on one side.
The following diagram illustrates the different ways cranial nerves impact the brain:
The twelve cranial nerves are a group of nerves that start in the brain and provide motor and sensory functions to the head and neck.
Each cranial nerve has unique anatomical characteristics and functions.
Doctors can identify neurological or psychiatric disorders by testing cranial nerve functions.
Here are some common questions and answers about the 12 cranial nerves.
How do I remember the 12 cranial nerves?
There are many mnemonics a person can use to remember the 12 cranial nerves. One example is: “Oh Oh Oh To Touch And Feel Very Good Velvet — Ah, Heaven!”
Which cranial nerve is largest?
The vagus nerve is the longest cranial nerve as it starts in the medulla — the bottom part of the brain — and extends to the abdomen.
Which is the shortest nerve?
The shortest cranial nerve is the trochlear nerve, as it has the lowest number of axons.