The cranial nerves represent a set of twelve peripheral nerves that stem from the brain. Each cranial nerve has a different function related to either sensory or motor stimuli and a name that reflects its function. They are numbered according to their exit location in the skull. For this, neuroscientists use Roman numerals from I–XII in order to label the cranial nerves in the brain.
What Is the Anatomy, Location, and Structure of the Cranial Nerves?
There are three general types of cranial nerves, emerging above the level of the first vertebrae of the vertebral column. Each one of the cranial nerves has its pair in the other brain hemisphere.
Two of the cranial nerves (the olfactory and the optic nerves) arise from the cerebrum, another two of them (the oculomotor and trochlear) stem from the mesencephalon; the trigeminal, abducens, facial, and acoustic-vestibular cranial nerves are located in the pons, while the last four – the glossopharyngeal, vagus, accessory, and hypoglossal cranial nerves can be found in the medulla oblongata.
According to the function they perform, the cranial nerves can be divided into:
- Sensory cranial nerves
- Motor cranial nerves
- Mixed type (performing both sensory and motor function)
According to their direction and modalities, the cranial nerves can be labeled as:
- General somatic afferent (GSA)
- General somatic efferent (GSE)
- General visceral afferent (GVA)
- General visceral efferent (GVE)
- Special somatic afferent (SSA)
- Special visceral afferent (SVA)
- Special visceral efferent (SVE)
What Is the Function of the Cranial Nerves?
The cranial nerves innervate the structures of the head, neck, thorax, and the abdomen, so they have sensory, motor, as well as mixed sensory and motor functions. While the sensory cranial nerves support vision, smell, and hearing, the motor cranial nerves control and regulate the muscle movements in the head and the neck.
What Are the Cranial Nerves and Their Respective Functions?
Conventionally, there are 12 pairs of cranial nerves. These brain structures are labeled with Roman numbers from I-XII, respectively:
- Olfactory nerve (CN I) – Conveys the sense of smell.
- Optic nerve (CN II) – Conveys vision.
- Oculomotor nerve (CN III) – Coordinates eye movement.
- Trochlear nerve (CN IV) – Coordinates eye movement.
- Trigeminal nerve (CN V) – Conveys the sensations on facial skin and controls the muscles responsible for chewing.
- Abducens nerve (CN VI) – Coordinates eye movement.
- Facial nerve (CN VII) – Controls facial expressions.
- Vestibulocochlear nerve (CN VIII) – Conveys both hearing and balance.
- Glossopharyngeal nerve (CN IX) – Conveys salivation, oral sensation, and taste.
- Vagus nerve (CN X) – Controls heart rate and digestion.
- Accessory nerve (CN XI) – Provides motor functions to the sternocleidomastoid muscle.
- Hypoglossal nerve (CN XII) – Controls tongue movement.
Olfactory nerve (CN I)
The olfactory nerve is the first of the 12 cranial nerves. It relays only special sensory information and is responsible for our sense of smell.
The branches of the olfactory nerve (or: fila olfactoria) pass from the nasal cavity through the cribriform plate of the ethmoid bone and stem right into the olfactory bulb. The information on the odor is perceived in the epithelium of the nasal cavity, after which it is relayed to the brain via the cranial nerve 1 (CN I) and the olfactory pathway.
Optic nerve (CN II)
The cranial nerve 2 (Cn II) is a special somatic afferent nerve that relays visual information to the brain. Its neural fibers stem from the photoreceptors located in the retina of the eyes. They cross at the optic disc, creating the optic nerve. This nerve leaves the ocular orbit through the optic canal.
Oculomotor nerve (CN III)
The oculomotor nerve (CN III) exits the brain in the middle of the cerebral peduncle and supports 4 of the 6 eyeball muscles. It controls the movement of the eyes, as well as the dilatation of the pupils.
In addition, this nerve supports the automatic constriction of the pupil in order to allow less light into the eye when the light is strong. In the dark, it allows more light to enter the eyes.
The oculomotor nerve is also involved in the accommodation of the lens to the distance of the objects we see in our field of vision.
Trochlear nerve (CN IV)
The trochlear nerve (CN IV) is a general somatic motor nerve that also participates in eye movement. Originating in the mesencephalon, it supports the downward and inward movements of the eyes.
Trigeminal nerve (CN V)
The trigeminal nerve (Cn V) performs some of the most important functions out of all the nerves, as it relays the sensory input from the face, cornea, mouth, nose, and the temporomandibular joint to the dedicated processing areas of the cerebral cortex.
It divides into 3 additional nerves that leave through a different opening of the skull:
- Ophthalmic nerve (CN V1) – Exits via the superior orbital fissure.
- Maxillary nerve (CN V2) – Exits via the foramen rotundum.
- Mandibular nerve (CN V3) – Exits via the foramen ovale.
All 3 branches of the trigeminal nerve are responsible for the sensation perceived via the facial skin.
Furthermore, the trigeminal nerve is also comprised of several nuclei:
- Motor nucleus of the trigeminal nerve (SVE)
- The principal sensory nucleus of the trigeminal nerve (GSA)
- Spinal nucleus of the trigeminal nerve (GSA)
- The mesencephalic nucleus of the trigeminal nerve (GSA)
The motor functions of this cranial nerve support the chewing and clenching of the teeth. It also provides a sensation to the muscles located in the tympanic membrane of the ear.
Abducens nerve (CN VI)
This nerve also contributes to the eye movements by supporting the turn of the eyes outward. The abducens nerve (CN VI) runs from the Dorello’s canal, i.e. at the pons of the brainstem, and exits the skull at the superior orbital fissure.
It contains the nucleus of the abducens nerve. Damage to this nerve can result in strabismus – an abnormal alignment of the eyes.
Facial nerve (CN VII)
The facial nerve (CN VII) has both a sensory and motor function, and is comprised of 4 nuclei that serve different functions:
- Superior salivatory nucleus (GVE) – Involved in the movement of the lacrimal, submaxillary, and submandibular glands.
- Motor nucleus of the facial nerve (SVE) – Supports the movement of muscles that are responsible for facial expression.
- Nuclei of the solitary tract (GVA, SVA) – Responds to stimuli from the respiratory, cardiovascular, and gastrointestinal systems.
- Spinal nucleus of the trigeminal nerve (GSA) – Receives information about deep/crude touch, pain, and temperature from the same side of the face it is located on.
Vestibulocochlear nerve (CN VIII)
The vestibulocochlear nerve participates in the processes of hearing and balance.
It comprises of 2 components:
- The vestibular nerve
- The cochlear nerve
While the cochlear component supports hearing, the vestibular nerve is involved in balance and motion. There are specialized inner hair cells that vibrate in response to sounds, thus determining both the frequency and magnitude of the perceived sound.
Glossopharyngeal nerve (CN IX)
The sensory function of the glossopharyngeal nerve (CN IX) gains the sensory data from the throat, tonsils, middle ear, and back of the tongue. It is also responsible for the sensation of taste occurring at the back of the tongue.
Vagus nerve (CN X)
The vagus nerve provides motor, sensory, and parasympathetic support to the brain and body structures. This is the longest cranial nerve – it starts from multiple nuclei in the medulla and extends to the abdomen.
By traveling through the thoracic and abdominal cavities, it provides parasympathetic supply to the visceral organs, making it the only one of the 12 cranial nerves to leave the head and the neck.
Accessory nerve (CN XI)
The accessory nerve (CN XI) regulates and controls the sternocleidomastoid and trapezius muscles. This supports the rotation, extension, and flexing of the neck and shoulders.
Hypoglossal nerve (CN XII)
The hypoglossal nerve is a motor nerve that supplies sensations for the tongue muscles and controls the movement of the tongue, so it has a significant role in the production of speech and in swallowing.
What Is the Difference Between the Cranial and the Spinal Nerves?
While the cranial nerves stem directly from the central nervous system, the spinal nerves emerge from the nerve roots of the spinal cord.
There are 12 cranial nerves and 31 spinal nerves. The role of the spinal nerves is to relay the motor, sensory, and autonomic signals between the spinal cord and the body. In addition, they are distributed throughout the skin, skeletal muscles, and blood vessels, whereas the cranial nerves are located in the head, neck, and in parts of the face.
While the cranial nerves are involved in hearing, vision, the sense of the smell, sense of the taste, and the movement of eyes, the spinal nerves participate in movement, sensations, as well as secretion of sweat.
How Can We Remember the Cranial Nerves More Easily in a Numerical Order?
Creating a mnemonic of the cranial nerves initials is an easy and fun way of memorizing them in numerical order. We can, for instance, try the following pattern:
- Olfactory nerve (CN I) Oh
- Optic nerve (CN II) Oh
- Oculomotor nerve (CN III) Oh
- Trochlear nerve (CN IV) To
- Trigeminal nerve (CN V) Touch
- Abducens nerve (CN VI) And
- Facial nerve (CN VII) Feel
- Vestibulocochlear nerve (CN VIII) Very
- Glossopharyngeal nerve (CN IX) Good
- Vagus nerve (CN X) Velvet is
- Accessory nerve (CN XI) A
- Hypoglossal nerve (CN XII) Heaven
Fun Facts
Did you know?
- The trigeminal nerve is the largest of the cranial nerves and has both sensory and motor functions.
- The terms such as afferent, efferent, mixed, general, visceral, special, and somatic refer to the modalities of the cranial nerves.
- A motor nerve is efferent if the information is relayed from the brain to the periphery.
- The motor nerve is afferent if the information is relayed from the periphery to the brain.
- The nerves that relay the sensory information in two ways (i.e. performing both afferent and efferent function) are labeled as mixed nerves.
- The spinal nerves are always of the mixed type, while the cranial nerves can be of sensory, motor, or mixed type.
- If the relayed information is received from our senses of smell, taste, hearing, sight, or touch, then it is referred to as a special type of information that is received from the stimuli.
- General information is the data collected from all other places of the body, apart from our senses.
- If the information travels to/from the skin and the skeletal muscles, it’s labeled as somatic information received from the stimuli.
- If the information travels to/from our internal organs, i.e. gastrointestinal tract, it’s labeled as visceral information received from the stimuli.