Cranial Nerves

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Cranial Nerve Overview

The cranial nerves are named and numbered, based on their location, from the front of the brain to the back.

There are twelve pairs of cranial nerves (CN) that lead directly from the brain to various parts of the head, neck, and trunk. Ten of the twelve cranial nerves originate in the brainstem; the two that do not are CN I (olfactory nerve) and CN II (optic nerve).

Once the cranial nerves leave the skull, they become part of the peripheral nervous system:

• some cranial nerves are involved in the special (or primary) senses (such as seeing, hearing, and taste), and others control muscles in the face or regulate glands

• cranial nerves are named and numbered, using Roman numerals, based on their anatomical organisation, starting from the front of the brain and progressing to the back
• "cranial nerves are composed of the neural processes associated with distinct brainstem nuclei and cortical structures"; this is different to spinal nerves whose roots are neural fibres from the grey matter of the spinal cord
• corticonuclear tracts act as bridges between upper motor neurons and cranial nerves to carry motor information from the cortex

The cranial nerves are listed below:

• CN I - Olfactory nerve
• CN II - Optic nerve
• CN III - Oculomotor nerve
• CN IV - Trochlear nerve
• CN V - Trigeminal nerve
• CN VI - Abducens nerve
• CN VII - Facial nerve
• CN VIII - Vestibulocochlear nerve (also known as the Auditory nerve, but this is a less inclusive name)
• CN IX - Glossopharyngeal nerve
• CN X - Vagus nerve
• CN XI - Spinal accessory nerve (also known as the Accessory nerve)
• CN XII - Hypoglossal nerve

If you would like more information on the structure of the human nervous system, please review this article: Introduction to Neuroanatomy.

Functions

The names of the cranial nerves sometimes correspond with their individual function. Some cranial nerves are purely sensory, others are purely motor, while the rest have both sensory and motor components.

Table 1. Cranial nerves and their primary functions

Cranial Nerve	Sensory Function	Somatic Motor Function	Autonomic (Parasympathetic Motor) Function	Function: Motor/Sensory/Both
CN I - Olfactory	Smell (olfaction)	-	-	Sensory
CN II - Optic	Vision	-	-	Sensory
CN III - Oculomotor	-	Innervates the inferior oblique muscle and medial, inferior, superior rectus muscles of the eye (move the eye) and the levator palpebrae superioris muscle (elevates eyelid)	Innervates the sphincter pupillae muscle (constricts the pupil), and the ciliary muscle (accommodates the eye for near vision)	Motor
CN IV - Trochlear	-	Innervates the superior oblique eye muscle (moves the eye inferiorly and laterally)	-	Motor
CN V - Trigeminal	Conducts touch, temperature and pain sensation from the face, nose, mouth, nasal and oral mucosa, anterior two-thirds of the tongue, anterior scalp and part of the auricle of the ear	Innervates the muscles of mastication, mylohyoid, digastric (anterior belly), tensor veli palatini, and tensor tympani	-	Both
CN VI - Abducens	-	Innervates the lateral rectus muscle of the eye (abducts the eye)	-	Motor
CN VII - Facial	Taste from anterior two-thirds of the tongue	Innervates muscles of facial expression, digastric (posterior belly), stylohyoid and stapedius muscle	Increases secretion from the lacrimal (tear glands) and nasal mucosal glands; submandibular and sublingual salivary glands	Both
CN VIII - Vestibulocochlear	Hearing (cochlear branch); linear and angular acceleration, or head position in space/equilibrium (vestibular branch)	-	-	Sensory
CN IX - Glossopharyngeal	Touch and taste from the posterior 1/3 of the tongue; visceral sensory from the carotid sinus and bodies	Innervates stylopharyngeus, a pharyngeal muscle	Increases secretion from the parotid salivary gland	Both
CN X - Vagus	Visceral sensation (excluding pain) from the heart, lungs, abdominal organs, bronchi, trachea, larynx, pharynx, and gastrointestinal tract to level of the descending colon. General sensation from the external acoustic meatus, eardrum, and pharynx. Also has a role in taste.	Innervates pharyngeal and laryngeal muscles and muscles at the base of the tongue	Innervates smooth muscles and glands of the heart, lungs larynx, trachea, and most abdominal organs	Both
CN XI - Spinal Accessory	-	Innervates the trapezius and sternocleidomastoid muscle	-	Motor
CN XII - Hypoglossal	-	Innervates intrinsic and extrinsic tongue muscles	-	Motor

Many different mnemonics can help with memorising or recalling the order, names, and functions of the twelve cranial nerves. These mnemonics use the first letter of each cranial nerve name to create a memorable expression and help you recall the cranial nerve's name.

Table 2. Mnemonics for cranial nerve names and order.

First letter of CN name	CN number	CN name	Mnemonic example one	Mnemonic example two
O	CN I	Olfactory nerve	On	Ooh
O	CN II	Optic nerve	Old	Ooh
O	CN III	Oculomotor nerve	Olympus's	Ooh
T	CN IV	Trochlear nerve	Towering	To
T	CN V	Trigeminal nerve	Top	Touch
A	CN VI	Abducens nerve	A	And
F	CN VII	Facial nerve	Fin	Feel
A or V	CN VIII	Auditory or Vestibulocochlear nerve	And	Very
G	CN IX	Glossopharyngeal nerve	German	Good
V	CN V	Vagus nerve	Viewed	Velvet
S	CN XI	Spinal accessory nerve	Some	Such
H	CN XII	Hypoglossal nerve	Hops	Heaven

This next set of mnemonics can help you memorise or recall the function of a cranial nerve, whether it is a sensory, motor, or mixed (both) nerve.

Table 3. Mnemonics for cranial nerve function.

CN name	CN Function: Motor, Sensory, Both	Mnemonic example one	Mnemonic example two
Olfactory nerve (CN I)	Sensory	Some	Some
Optic nerve (CN II)	Sensory	Say	Say
Oculomotor nerve (CN III)	Motor	Marry	My
Trochlear nerve (CN IV)	Motor	Money	Mother
Trigeminal nerve (CN V)	Both	But	Bought
Abducens nerve (CN VI)	Motor	My	My
Facial nerve (CN VII)	Both	Brother	Brother
Vestibulocochlear nerve (CN VIII)	Sensory	Says	Some
Glossopharyngeal nerve (CN IX)	Both	Big	Bad
Vagus nerve (CN X)	Both	Brains	Beans
Spinal accessory nerve (CN XI)	Motor	Matter	My
Hypoglossal nerve (CN XII)	Motor	More	My

Cranial Nerve Dysfunction

Cranial nerve dysfunction can be caused by a range of conditions, including tumours, inflammation, trauma, degenerative conditions, and systemic conditions. The exact presentation depends on the nerve or nerves impacted.

Clinical Relevance and Assessment

"There are many reasons for neural impairment, including insidious mechanisms such as local pressure from space occupying lesions, inflammation, infection, atrophy, or demyelination. For clinicians, gaining a working knowledge of the individual [cranial] nerve functions is key to understanding, performing and interpreting the testing." - Taylor et al. 2021

Cranial nerve testing highlights impairments / issues along the cranial nerve pathway.

Table 4. Cranial Nerve Integrity Testing.

Cranial Nerve	Examination	Involvement
CN I - Olfactory	- Test sense of smell by closing the other nostril and using non-irritating odours like coffee, lemon oil, etc.	- Inability to detect smells (anosmia) = temporal lobe lesions
CN II - Optic	- Test visual acuity using a Snellen chart, test central and peripheral vision	- Blindness, impaired vision: far (myopia) and near (presbyopia)
CN II – Optic CN III – Oculomotor	- Test pupil equality, size and shape - Test pupil constriction by shining a light in the eye	- Absence of pupil constriction - Unequal pupils (anisocoria) - Horner’s syndrome - CN III paralysis
CN III – Oculomotor CN IV – Trochlear CN VI - Abducens	- Test extraocular movements - Observe eye position, presence of strabismus (loss of ocular alignment) or ptosis of eyelid (eyelid droops over the eye) - Test smooth pursuit eye movement (slow linear movements of the eyes, without head movement)	- Strabismus and impaired eye movement - CN III: Ptosis, pupil dilation - CN IV: Eye cannot look down when adducted - CN VI: Eye pulled inward, eye cannot look out
CN V - Trigeminal	- Pain and light touch sensation of the face (forehead, cheeks, jaw) - Open and close the jaw against resistance - Test corneal and jaw jerk reflex	- Loss of facial sensation and numbness - Loss of ipsilateral corneal reflex - Weakness and wasting of mastication muscles - Jaw deviation when opened to the ipsilateral side
CN VII - Facial	- Test motor function of the facial muscles and look for asymmetry: raise eyebrows, frown, smile, close eyes tightly, puff cheeks, etc.	- Ipsilateral paralysis of the facial muscles: unable to close eye, mouth corner droops, difficulty with speech articulation = peripheral nerve injury (PNI), Bell’s Palsy (CN VII); or facial paralysis due to stroke
CN VIII - Vestibulocochlear	- Test balance - Gaze instability with head rotations - Test auditory acuity with a tuning fork. The fork is placed in the middle of the top of the head. Check if the sound is equal or louder in one ear (Weber’s test) - A vibrating tuning fork is placed on the mastoid bone, then near the ear canal. Hearing acuity is assessed (Rinne’s test)	- Vertigo and disequilibrium - Nystagmus - Deafness, tinnitus and hearing loss - Unilateral conductive loss - Sensorineural loss: sound heard in good ear - Conductive loss: sound heard through bone is longer or equal than sound heard through air conduction - Sensorineural loss: sound heard longer through air conduction
CN IX – Glossopharyngeal	- Test for difficulty swallowing - Let the patient say “ah” and listen for voice quality - Examine the gag reflex	- Dysphonia - Dysphagia - Absent gag reflex
CN X - Vagus	Let the patient say “ah” and observe the soft palate elevating and check if the uvula remains in the midline	- Uvula will deviate away from the side of a CN X lesion - With paralysis, the palate does not elevate, with unilateral paralysis, there is asymmetrical elevation
CN XI - Accessory	- Examine muscle bulk - Test trapezius and sternocleidomastoid muscles against resistance	- Atrophy, fasciculations, weakness = PNI - Shoulder droops and unable to shrug ipsilateral shoulder - Unable to turn the head to the contralateral side
CN XII - Hypoglossal	- Examine protruded tongue: rapid side-to-side movements - Examine the tongue’s resting position - Listen to the patient’s word articulations	- Movement impairment: deviation to the weak side - Atrophy or tongue fasciculations - Dysarthria (CN X or XII lesions)

See also

• Moebius Syndrome
• Bell's Palsy
• Temporomandibular Disorders
• Acoustic Neuroma
• Migraine Headache
• Ramsay Hunt Syndrome
• Trigeminal Neuralgia
• Cervicogenic Headache
• Sturge-Weber Syndrome
• Synkinesis
• Accessory Nerve Cranial Nerve XI
• Abducens Nerve Cranial Nerve VI