What Are the 12 Cranial Nerves and Their Functions?

Cranial nerves arise from the brain as 12 pairs and function as relays of information between the brain and other parts of the body. They are numbered using roman numbers from I-XII:

• Cranial Nerve I: Olfactory Nerve
• Cranial Nerve II: Optic Nerve
• Cranial Nerve III: Oculomotor Nerve
• Cranial Nerve IV: Trochlear Nerve
• Cranial Nerve V: Trigeminal Nerve
• Cranial Nerve VI: Abducens Nerve
• Cranial Nerve VII: Facial Nerve
• Cranial Nerve VIII: Vestibulocochlear Nerve
• Cranial Nerve IX: Glossopharyngeal Nerve
• Cranial Nerve X: Vagus Nerve
• Cranial Nerve XI: Accessory Nerve
• Cranial Nerve XII: Hypoglossal Nerve

1. What Is the Function of the Olfactory Nerve?

The olfactory nerve or cranial nerve I is the shortest cranial nerve and is involved in transmitting smell (olfaction) information to the brain for processing [1].

It originates from the olfactory epithelium covering the nasal cavities and where olfactory sensory neurons (bipolar neurons) are found and from which the olfactory fibers of the olfactory nerves originate and extend.

The olfactory nerves ascend and cross openings in the frontal bone of the skull known as olfactory foramina and connect with the olfactory bulb which sends olfactory information to the amygdala, hippocampus, and orbitofrontal cortex for further processing.

• What Happens When Your Olfactory Nerve Is Damaged?

Damage of the olfactory nerve can be caused by genetic defects, tumors, trauma, or surgery (iatrogenic) and can result in partial (hyposmia) or total loss (anosmia) of smell.

2. What Is the Function of the Optic Nerve?

The optic nerve is the 2nd cranial nerve responsible for the transmission of visual information from the retina to the brain such as colors, brightness, and contrast (acuity) [2].

The optic nerves are made of axons of the retinal ganglion cells that receive visual information from photoreceptors in the eye retina through intermediary neurons known as bipolar cells and retina amacrine cells.

Optic nerves (right and the left) leave the orbital sockets through openings in the middle front of the skull known as optic canals and reach the optic chiasma where they cross each other and continue their extension to the lateral geniculate nuclei.

At the optic chiasma, the optic nerve comes from the left eye extent towards the right hemisphere of the brain, while the optic nerve comes from the right eye extent towards the left hemisphere of the brain.

The lateral geniculate nuclei relay the visual information to other structures of the brain for further processing.

• What Happens When Your Optic Nerve Is Damaged?

Damage to the optic nerve results in a permanent loss of vision on the left, the right, or both fields of vision, depending on the type of damage which can affect one of the optic nerves or both.

Damages to the optic nerves can be due to the following conditions or disorders:

• Optic Neuritis
• Glaucoma
• Ischemic Optic Neuropathy
• Tumor Compression
• Inflammation
• Genetic or Congenital Diseases (e.g., Leber’s hereditary optic neuropathy)

3. What Is the Function of the Oculomotor Nerve?

The oculomotor nerve is the 3^rd cranial nerve involved in most movements of the eye and eyelids [3].

It is a motor nerve that originates from a cluster of neurons within the midbrain known as the nucleus and enters the orbit through the superior orbital fissure.

Once within the orbit, the oculomotor nerve branches into an inferior and superior branch.

The inferior branch innervates the medial rectus, the inferior rectus, and inferior muscles responsible for eye movements in different directions and plans.

An ending of the inferior branch also emerges into the ciliary ganglion which is a group of nerves found at the back of the eye and part of the parasympathetic nervous system.

The superior branch innervates the superior rectus (eye movement) and the levator palpebrae superioris (eyelid movement) muscles.

• What Happens When Your Oculomotor Nerve Is Damaged?

Damage of the oculomotor nerve can result in paralysis affecting eye and eyelids movements. The damage can be caused by trauma, demyelinating diseases, microvascular diseases, or compression of the nerve by a brain tumor.

4. What Is the Function of the Trochlear Nerve?

The trochlear nerve is the fourth cranial nerve involved in the innervation of the superior oblique muscle of the eye responsible for eye rotation [4].

It is originated from a nucleus found in the midbrain and enters the orbit through the superior orbital fissure.

• What Happens When Your Trochlear Nerve Is Damaged?

Damage of the trochlear nerve results in double vision (diplopia) and can be caused by head trauma, intracranial pressure (e.g., brain tumor, hemorrhage) that compress the nerve, viral infections, demyelinating diseases, or congenital defects.

5. What Is the Function of the Trigeminal Nerve?

The trigeminal nerve is the fifth cranial nerve and is divided into the ophthalmic nerve, maxillary nerve, and mandibular nerve [5].

The ophthalmic nerve (V1) is a sensory (afferent) nerve responsible for sensations from the eyeball, upper face, and anterior scalp.

The maxillary nerve (V2) is a sensory nerve responsible for sensations from the nasal cavity and sinuses, mid-face, palate, and maxillary teeth.

The mandibular nerve (V3) is both a sensory (afferent) and motor (efferent) nerve responsible for sensations from the chin, ear, temple, oral cavity, tongue, temporomandibular joint (TMJ), mandibular teeth, proprioception from muscles of mastication, and motor functions such as biting and chewing.

The ophthalmic nerve exits the skull through the superior orbital fissure

The maxillary nerve exits the skull through the foramen (hole) rotundum.

The mandibular nerve exits the skull through the foramen (hole) ovale.

After exiting the skull, the ophthalmic nerve, the maxillary nerve, and the mandibular nerve converge into the trigeminal ganglion.

From the trigeminal ganglion, a single root extends toward the pons of the brainstem where it branches toward the cerebellum and the medulla (part of the brainstem), and another toward the thalamus where the information is processed.

• What Happens When Your Trigeminal Nerve Is Damaged?

The most common manifestations of trigeminal nerve damage are trigeminal neuralgia, cluster headache, and Wallenberg syndrome.

Trigeminal neuralgia is caused by the mechanical compression of the trigeminal nerve such as compression by an aneurysm, a tumor, trauma, or arteriovenous malformation.

It mainly manifests with neuropathic pain with a burning or shock-like sensation. Trigeminal neuralgia is mainly treated with carbamazepine, but also with medications including lamotrigine, phenytoin, baclofen, gabapentin, pregabalin, and oxcarbazepine [6].

Cluster headache causes are unknown. It manifests with severe headaches on one side of the head.

Wallenberg Syndrome or Lateral Medullary Syndrome is due to the damage of the medulla (part of the brainstem) by a stroke (lack of blood supply). It manifests with a loss of sensations in one side of the body.

6. What Is the Function of the Abducens Nerve?

The abducens nerve is the sixth cranial nerve involved in the movement of the eyeball [7].

The abducens nerve originates from a cluster of neurons in the pons of the brainstem, known as the abducens nucleus, and enters the orbit through the superior orbital fissure to innervate lateral rectus muscle, responsible for lateral movements of the eye.

• What Happens When Your Abducens Nerve Is Damaged?

Damage to the abducens nerve can be caused by injury or compression (e.g., aneurysm) resulting in diplopia (double vision).

7. What Is the Function of the Facial Nerve?

The facial nerve is the seventh cranial nerve that has sensory and motor functions [8].

The sensory function is associated with conveying taste sensations (tongue), while the motor function involves the control of muscles responsible for facial expression (motor function) and the middle ear.

The facial nerve has also parasympathetic fibers that travel to the ganglia involved in the sensory innervation of the lacrimal, nasal, palatine, submandibular, sublingual glands.

The facial nerve originates from the pons of the brainstem and exits the skull through the stylomastoid foramen.

• What Happens When Your Facial Nerve Is Damaged?

Damage of the facial nerve can be caused by surgery, trauma, idiopathic Bell palsy, stroke, tumors, or granulomatous meningitis. It manifests with symptoms including facial paralysis, excessive salivation, dry eyes, and taste problems.

8. What Is the Function of the Vestibulocochlear Nerve?

The facial is the eighth cranial nerve involved in conveying sound (hearing) and balancing information to the brain [9].

The facial nerve has two nerve branches, the cochlear nerve, and the vestibular nerve.

The cochlear nerve is involved in transmitting auditory information from the cochlea to the cochlear nucleus in the medulla oblongata (part of the brainstem) for processing.

The vestibular nerve transmits sensory information received by the vestibular hair cells (inner ear) to the brain for the processing of body balance information.

• What Happens When Your Vestibulocochlear Nerve Is Damaged?

Damage of the vestibulocochlear nerve can be caused by direct trauma, tumors, infection, congenital malformations, and vascular injury.

It presents with symptoms including vertigo, tinnitus, nystagmus, false sense of motion, motion sickness, loss of balance, and sensorineural hearing loss.

9. What Is the Function of the Glossopharyngeal Nerve?

The glossopharyngeal nerve is the ninth cranial nerve that has sensory, motor, and parasympathetic functions [10].

It is originated from the medulla oblongata and leaves the skull through the jugular foramen (at the base of the skull) where it branches into the tympanic, tonsillar, carotid sinus nerve, stylopharyngeal, branches to the tongue, lingual branches, and a communicating branch to cranial nerve X (vagus nerve).

The sensory function is associated with conveying sensations from the oropharynx, posterior tongue, carotid body, and sinus.

The motor function is associated with the control of the stylopharyngeus muscle involved in elevating the pharynx and the larynx, and in the dilatation of the pharynx for swallowing.

The parasympathetic function of the glossopharyngeal nerve is involved in the innervation of the parotid gland (secretion of saliva).

What Happens When Your Glossopharyngeal Nerve Is Damaged?

Damage to the glossopharyngeal nerve can be caused by a tonsil tumor, glossopharyngeal neuralgia, injury, and surgery, resulting in a loss of taste at the posterior part of the tongue and difficulties swallowing.

10. What Is the Function of the Vagus Nerve?

The vagus nerve is the longest and tenth cranial nerve that has sensory, motor, and parasympathetic functions [11].

It originates from the medulla oblongata and leaves the skull through the jugular foramen where it divides into different branches that extend to innervate parts of the body.

The sensory function of the vagus nerve conveys sensations from the larynx, hypopharynx, heart, lungs, abdominal viscera, the epiglottic region, and the hypopharynx.

The motor function of the vagus nerve is involved in the control of the muscles of the pharynx and larynx responsible for phonation and swallowing.

The parasympathetic function is associated with the control of cardiac muscle, and muscles and glands of the anterior and middle parts of the gut.

• What Happens When Your Vagus Nerve Is Damaged?

Because the vagus nerve has many functions the effects are also multiple such as vasovagal syncope (sudden drop in blood pressure and fainting), gastroparesis (delayed gastric emptying), difficulty speaking, and swallowing.

11. What Is the Function of the Accessory Nerve?

The accessory nerve is the 11^th cranial nerve involved in the neck and shoulder movements through innervation of the cervical sternocleidomastoid and the trapezius muscles [12].

It is a motor nerve that has two parts, one part is originated from the nucleus ambiguous in the medulla and the other part known as the spinal accessory nerve is originated from neurons in the upper spinal cord [13].

The accessory spinal nerve exits the skull through the jugular foramen (base of the skull).

• What Happens When Your Accessory Nerve Is Damaged?

Damage of the accessory nerve can be caused by trauma, tumors, or surgery and results in neck and trapezius movement difficulties.

12. What Is the Function of the Hypoglossal Nerve?

The hypoglossal nerve is the 12^th cranial nerve involved tongue movement through innervation of the extrinsic and intrinsic muscles of the tongue [14].

It is originated in the hypoglossal nucleus in the medulla and leaves the skull through the hypoglossal canal.

• What Happens When Your Hypoglossal Nerve Is Damaged?

Damage to the hypoglossal nerve can be caused by trauma or tumors and results in weakness of the tongue muscles (tongue does not stick out straight).

Conclusion

Although cranial nerves have many functions associated with the sense, they have also parasympathetic functions such as their involvement in the control of the heart, lungs, and digestive organs.