What are cranial nerves?
Your cranial nerves are pairs of nerves that connect your brain to different parts of your head, neck, and trunk. There are 12 of them, each named for their function or structure.
Each nerve also has a corresponding Roman numeral between I and XII. This is based off their location from front to back. For example, your olfactory nerve is closest to the front of your head, so it’s designated as I.
Their functions are usually categorized as being either sensory or motor. Sensory nerves are involved with your senses, such as smell, hearing, and touch. Motor nerves control the movement and function of muscles or glands.
Keep reading to learn more about each of the 12 cranial nerves and how they function.
The olfactory nerve transmits sensory information to your brain regarding smells that you encounter.
When you inhale aromatic molecules, they dissolve in a moist lining at the roof of your nasal cavity, called the olfactory epithelium. This stimulates receptors that generate nerve impulses that move to your olfactory bulb. Your olfactory bulb is an oval-shaped structure that contains specialized groups of nerve cells.
From the olfactory bulb, nerves pass into your olfactory tract, which is located below the frontal lobe of your brain. Nerve signals are then sent to areas of your brain concerned with memory and recognition of smells.
The optic nerve is the sensory nerve that involves vision.
When light enters your eye, it comes into contact with special receptors in your retina called rods and cones. Rods are found in large numbers and are highly sensitive to light. They’re more specialized for black and white or night vision.
Cones are present in smaller numbers. They have a lower light sensitivity than rods and are more involved with color vision.
The information received by your rods and cones is transmitted from your retina to your optic nerve. Once inside your skull, both of your optic nerves meet to form something called the optic chiasm. At the optic chiasm, nerve fibers from half of each retina form two separate optic tracts.
Through each optic tract, the nerve impulses eventually reach your visual cortex, which then processes the information. Your visual cortex is located in the back part of your brain.
The oculomotor nerve has two different motor functions: muscle function and pupil response.
- Muscle function. Your oculomotor nerve provides motor function to four of the six muscles around your eyes. These muscles help your eyes move and focus on objects.
- Pupil response. It also helps to control the size of your pupil as it responds to light.
This nerve originates in the front part of your midbrain, which is a part of your brainstem. It moves forward from that area until it reaches the area of your eye sockets.
It emerges from the back part of your midbrain. Like your oculomotor nerve, it moves forward until it reaches your eye sockets, where it stimulates the superior oblique muscle.
The trigeminal nerve is the largest of your cranial nerves and has both sensory and motor functions.
The trigeminal nerve has three divisions, which are:
- Ophthalmic. The ophthalmic division sends sensory information from the upper part of your face, including your forehead, scalp, and upper eyelids.
- Maxillary. This division communicates sensory information from the middle part of your face, including your cheeks, upper lip, and nasal cavity.
- Mandibular. The mandibular division has both a sensory and a motor function. It sends sensory information from your ears, lower lip, and chin. It also controls the movement of muscles within your jaw and ear.
The trigeminal nerve originates from a group of nuclei — which is a collection of nerve cells — in the midbrain and medulla regions of your brainstem. Eventually, these nuclei form a separate sensory root and motor root.
The sensory root of your trigeminal nerve branches into the ophthalmic, maxillary, and mandibular divisions. The motor root of your trigeminal nerve passes below the sensory root and is only distributed into the mandibular division.
The abducens nerve controls another muscle that’s associated with eye movement, called the lateral rectus muscle. This muscle is involved in outward eye movement. For example, you would use it to look to the side.
This nerve, also called the abducent nerve, starts in the pons region of your brainstem. It eventually enters your eye socket, where it controls the lateral rectus muscle.
The facial nerve provides both sensory and motor functions, including:
- moving muscles used for facial expressions as well as some muscles in your jaw
- providing a sense of taste for most of your tongue
- supplying glands in your head or neck area, such as salivary glands and tear-producing glands
- communicating sensations from the outer parts of your ear
Your facial nerve has a very complex path. It originates in the pons area of your brainstem, where it has both a motor and sensory root. Eventually, the two nerves fuse together to form the facial nerve.
Both within and outside of your skull, the facial nerve branches further into smaller nerve fibers that stimulate muscles and glands or provide sensory information.
Your vestibulocochlear nerve has sensory functions involving hearing and balance. It consists of two parts, the cochlear portion and vestibular portion:
- Cochlear portion. Specialized cells within your ear detect vibrations from sound based off of the sound’s loudness and pitch. This generates nerve impulses that are transmitted to the cochlear nerve.
- Vestibular portion. Another set of special cells in this portion can track both linear and rotational movements of your head. This information is transmitted to the vestibular nerve and used to adjust your balance and equilibrium.
The cochlear and vestibular portions of your vestibulocochlear nerve originate in separate areas of the brain.
The cochlear portion starts in an area of your brain called the inferior cerebellar peduncle. The vestibular portion begins in your pons and medulla. Both portions combine to form the vestibulocochlear nerve.
The glossopharyngeal nerve has both motor and sensory functions, including:
- sending sensory information from your sinuses, the back of your throat, parts of your inner ear, and the back part of your tongue
- providing a sense of taste for the back part of your tongue
- stimulating voluntary movement of a muscle in the back of your throat called the stylopharyngeus
The glossopharyngeal nerve originates in a part of your brainstem called the medulla oblongata. It eventually extends into your neck and throat region.
The vagus nerve is a very diverse nerve. It has both sensory and motor functions, including:
- communicating sensation information from your ear canal and parts of your throat
- sending sensory information from organs in your chest and trunk, such as your heart and intestines
- allowing motor control of muscles in your throat
- stimulating the muscles of organs in your chest and trunk, including those that move food through your digestive tract (peristalsis)
- providing a sense of taste near the root of your tongue
Out of all of the cranial nerves, the vagus nerve has the longest pathway. It extends from your head all the way into your abdomen. It originates in the part of your brainstem called the medulla.
Your accessory nerve is a motor nerve that controls the muscles in your neck. These muscles allow you to rotate, flex, and extend your neck and shoulders.
It’s divided into two parts: spinal and cranial. The spinal portion originates in the upper part of your spinal cord. The cranial part starts in your medulla oblongata.
These parts meet briefly before the spinal part of the nerve moves to supply the muscles of your neck while the cranial part follows the vagus nerve.
Your hypoglossal nerve is the 12th cranial nerve which is responsible for the movement of most of the muscles in your tongue. It starts in the medulla oblongata and moves down into the jaw, where it reaches the tongue.
Explore this interactive 3-D diagram below to learn more about the 12 cranial nerves.