The Cranial Nerves

The cranial nerves are made up of twelve pairs of nerves with the origin at the back of the brain while passing the spinal cord (1). The cranial nerves serve as a connection between the brain and other parts of the body performing sensory and motor functions. The sensory functions are responsible for the sense of touch, smell by the nose, taste by the tongue, and others. The motor function performed by the cranial nerves is responsible for the movement done by the muscles, and glands they supply (2).

The Types Of Cranial Nerves

There are twelve cranial nerves in total, and each of them is in pairs that divide to supply the right, and left parts of the body, starting from the brain. For instance, the first cranial nerve named the olfactory nerve is a pair. One of it is supplying the left side of the brain, while the other supply the right side of the brain (3).

The nerves are labeled using the roman numerals figure from I to XII, that is one to twelve, according to how they are located in the brain, and the name gives a clue to the function they may be performing. The twelve nerves are:
I. Olfactory nerve
II. Optic nerve
III. Oculomotor nerve
IV. Trochlear nerve
V. Trigeminal nerve
VI. Abducens nerve
VII. Facial nerve
VIII. Vestibulocochlear nerve
IX. Glossopharyngeal nerve
X. Vagus nerve
XI. Spinal accessory nerve
XII. Hypoglossal nerve (1).
This article will provide an overview of each of the twelve cranial nerves, and also answer some of the frequently asked questions on cranial nerves.

I. Olfactory nerve

The olfactory nerve is the first cranial nerve and it is responsible for the sense of smell. It helps to send the signal of whichever smell is perceived to the brain, and thereafter, it is interpreted. The nasal mucosa is innervated by this nerve, and once fragrant molecules are inhaled, they are transmitted right to the olfactory bulb, from the bulb, it is taken to the olfactory tract. The olfactory tract is responsible for sending the signal to the different locations of the brain, such as the frontal lobe which will help in interpreting the kind of smell it is (1).

II. Optic nerve

The optic nerve is the second cranial nerve, and it is the nerve responsible for vision. When the light gets into the eyes, there are sensitive receptors in the eyes known as the cones and rods. The receptors receive the light upon entering the eyes. The cones are sensitive to colored vision, with low light sensitivity, and the quantity is not as more as the rods. Rods on the other hand are more in number, and more sensitive to light, and they help with the black and white vision.

The retina transmits information from these sensitive receptors to the optic nerve, and the two optic nerves right and left communicate at a point to form the optic chiasm, and from the chiasm, the optic tracts are formed. The optic tracts take visual information to the brain’s visual cortex located behind the brain where the image information is processed and interpreted (2).

III. Oculomotor nerve

The oculomotor nerve is also responsible for vision. It performs the motor function modality by supplying the muscles surrounding the eyes, to aid in their movement and ability to focus well on objects. The oculomotor nerve also helps in controlling pupil size when it comes in contact with light, to dilate or constrict.
The nerve took its origin from the midbrain, at the brain stem, from where it finds its way to the eye sockets where it performs its motor function (4).

IV. Trochlear nerve

The trochlear nerve most times functions alongside the oculomotor and abducens nerve. The trochlear also performs a motor function on the superior oblique muscle responsible for moving the eyeballs inward, outward, and downward. It took its origin from behind the midbrain, and just like the oculomotor nerve, it goes to the eye sockets where it functions to control the movement of the superior oblique (2).

V. Trigeminal nerve

The trigeminal nerve is the fifth cranial nerve and the largest of the twelve cranial nerves. It performs both sensory, and motor function modalities. The motor function is responsible for being able to clench the teeth and chew adequately, and it also supplies the middle ear tympanic membrane providing sensation to it.

The trigeminal sensory portion is further divided into three parts, namely the ophthalmic, maxillary, and mandibular parts.
⦁ The ophthalmic part supplies the eyes, the nose, and the forehead. It provides sensation to the cornea of the eyes, the skin and nasal mucosa, the eyelid, and also the forehead.
⦁ The maxillary part of the ophthalmic nerve provides sensation to the lateral sides of the nose, the low eyelid, as well as the teeth’ upper layer.
⦁ The mandibular portion of the ophthalmic nerve is responsible for the lower portion of the face, tongue, and oral mucosa. Unlike the maxillary part, the mandibular part is responsible for supplying the lower teeth.
A common disorder associated with the trigeminal nerve is trigeminal neuralgia, which causes severe pain and tics on the face of the affected individual (1).

VI. Abducens nerve

Abducens is the sixth cranial nerve, it performs motor functions controlling one muscle of the eye called the lateral rectus. This nerve helps the lateral rectus in performing the series of outward movements, which also involve looking sideways (4).

The nerve took its origin from the pons, a portion of the brainstem, from the pons it moves to the Torello’s canal, and goes through the cavernous sinus before it finally enters the eye socket to control the muscle in the bony orbit (1).

VII. Facial nerve

The facial nerve is the seventh cranial nerve with both the motor and sensory function modalities.
⦁ The motor function of the facial nerve is responsible for facial expressions as a result of muscle movements on the face and jaw.
⦁ It also helps in tasting, supplying the tongue to serve as the sense organ of taste.
⦁ It also supplies the salivary gland and lacrimal glands, responsible for the production of saliva, and tears respectively, and all located at the head or neck region. Sensations are present in the outer ear as a result of supply by the facial nerve (2).

The pathway of the facial nerve is complex and integrative, in that its origin is from the pons in the brainstem, where it has both the sensory and motor root, the two nerve portions go through the internal acoustic meatus passing through the facial canal. At the facial canal, they both fuse forming the facial nerve proper, and they exit the cranial cavity through another hole called the stylomastoid foramen. Once the nerve reaches the face, it gives rise to smaller nerve fibers that supply and enables the functions (5).

VIII. Vestibulocochlear nerve

The vestibulocochlear nerve is the eighth cranial nerve responsible for hearing and balancing. The nerve is a two-in-one nerve made up of the vestibular nerve, and the cochlear nerve. The vestibular nerve function to help position the head, and aid the body in sensing the position of the head, this is translated to the body maintaining balance. The cochlear nerve receives the signal from the inner hair cells to aid hearing.

There are hair cells in the inner ear that respond to sound, and also determine the magnitude and the frequency of transmitted sounds. The nerve fibers in the pons leave the cranium through the internal acoustic meatus located around the temporal bone (1).

Each of the vestibular, and cochlear nerve has different origins in the brain. The cochlear nerve took its origin from the brain portion called the ”inferior cerebellar peduncle”, while the vestibular nerve portion originates from the pons, and the medulla. These two portions thereafter come together to form the vestibulocochlear nerve (2).

IX. Glossopharyngeal nerve

The Glossopharyngeal nerve is the ninth cranial nerve, and it serves both sensory, and motor functions. The motor part of the nerve is responsible for muscle movements. It aids in the movement of the stylopharyngeus muscle present in the throat and is responsible for expanding and shortening the throat when necessary (1), and it also gets motor information from the parotid gland, one of the types of salivary gland (4).

The sensory portion of the glossopharyngeal nerve receives information from other parts like the upper part of the tongue at the back where it performs the sense of taste, the middle ear, and the throat (1). The glossopharyngeal nerve took its origin from the brain stem, a part there called the medulla oblongata, from where it proceeds to the neck and the throat (2).

X. Vagus nerve

Our favourite, the vagus nerve is the tenth of the cranial nerves. It is an interesting, and diverse nerve with both sensory and motor functions. Some of its functions include:
⦁ It takes charge of the control of muscles in the throat, through its motor function
⦁ It causes muscles stimulation of organs during digestion, allowing easy flow of food through the digestive tract
⦁ It stimulates organs located in the chest and the trunk.
⦁ It sends sensory information from the ear canal to the brain
⦁ It supplies the root of the tongue to provide a sense of taste (2).
⦁ It is also an important nerve in phonation (5).
Interestingly, of the twelve cranial nerves, the vagus nerve possesses the longest pathway measuring from the head to the abdomen. It took its origin from the medulla oblongata in the brain stem (2).

XI. Spinal accessory nerve

The spinal accessory nerve, otherwise called the accessory nerve provides only the motor modality function. It provides motor function to muscles located in the neck.
It supplies the sternocleidomastoid and the trapezius group of muscles thereby allowing for rotation of the neck, flexion and extension of the neck, and the shoulder.

The spinal accessory nerve is further divided into the spine and the cranial part. The spinal part of the nerve has its root in the spinal cord and goes to the skull through the foramen magnum, from where it comes in contact with the cranial part of the nerve, and they joined together, and leave the skull via the internal carotid artery. The cranial part joins the vagus nerve too (1).

XII. Hypoglossal nerve

The hypoglossal nerve is the twelfth nerve of the cranial nerves, it originates from the brainstem, and passes through the hypoglossal foramen to exit the skull. It serves a motor function, which is mainly to aid the movements of the tongue. The hypoglossal is very unique in its involvement in the daily stability and functioning of individuals, in that it is important for speech and swallowing, and these are two important features important to have a smooth day (5).

Frequently asked questions on cranial nerves (FAQs)

Haven read the above article on cranial nerves, are you curious about certain things on cranial nerves? Below are the frequently asked questions on cranial nerves, and the best answers:

References:

1. https://www.medicalnewstoday.com/articles/326621#olfactory-nerve
2. https://www.healthline.com/health/12-cranial-nerves#cranial-nerves
3. https://my.clevelandclinic.org/health/body/21998-cranial-nerves
4. https://www.simplypsychology.org/12-cranial-nerves.html
5. https://www.kenhub.com/en/library/anatomy/the-12-cranial-nerves