Cranial Nerves

Original Editor - Tarina van der Stockt

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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 CN originate in the brainstem, which the exception of CN I (the olfactory nerve) and CN II (the optic nerve).
• Once CN leave the skull they become part of the peripheral nervous system.
  • cranial nerves consist of both afferent and efferent nerve fibres, with individual nerves carrying sensory information, motor information, or a combination of the two types of information.
  • Some of the CN are involved in the special (or primary) senses (such as seeing, hearing, and taste), and others control muscles in the face or regulate glands.
• The nerves are named and numbered (according to their location, from the front of the brain to the back). The cranial nerves are numbered using roman numerals.
• Unlike spinal nerves whose roots are neural fibers from the spinal grey matter, cranial nerves are composed of the neural processes associated with distinct brainstem nuclei and cortical structures.

The cranial nerves listed in order:

• 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, however 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 

For more information on the structure of the human nervous system, please review this article: Introduction to Neuroanatomy.

Function

The names of the cranial nerves sometimes correspond with their individual function. Some of the cranial nerves are purely sensory, others are purely motor, and 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, and superior rectus muscles of the eye (move the eye); levator palpebrae superioris muscle (elevate eyelid)	Innervates the sphincter pupillae muscle (constricts the pupil), and the ciliary muscle (accomodate the eye for near vision)	Both
CN IV - Trochlear	-	Innervate 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 tongue, and anterior scalp; part of auricle of the ear	Innervate the muscles of mastication, mylohyoid, digastric (anterior belly), tensor veli palatini, and tensor tympani	-	Both
CN VI - Abducens	-	Innervate the lateral rectus muscle of eye (abducts the eye)	-	Motor
CN VII - Facial	Taste from anterior two-thirds of tongue	Innervate muscles of facial expression, digastric (posterior belly), stylohyoid and stapedius muscle	Increase 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	Innervate the pharyngeal muscle Stylopharyngeus	Increase secretion from the parotid salivary gland	Both
CN X - Vagus	Visceral sensation (excluding pain) from heart, lungs, abdominal organs, bronchi, trachea, larynx, pharynx, gastrointestinal tract to level of descending colon. General sensation from the external acoustic meatus, eardrum, and pharynx	Innervates pharyngeal and laryngeal muscles and muscles at base of 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

There are many different mnemonics that can be helpful in 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 to aid in recall of 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	Olfactory 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 aid in the memorization or recall of 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
Olfactory 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

Dysfunction of Cranial Nerves

Dysfunction of certain CN may affect the eye, pupil, optic nerve, or extraocular muscles and their nerves; thus, they can be considered CN disorders, neuro-ophthalmologic disorders, or both.

CN disorders can also involve dysfunction of smell, vision, chewing, facial sensation or expression, taste, hearing, balance, swallowing, phonation, head turning and shoulder elevation, or tongue movements (see table below). One or more CN may be affected.

These disorders can result from tumours, inflammation, trauma, systemic disorders, and degenerative or other processes, causing such symptoms as vision loss, diplopia, ptosis, pupillary abnormalities, periocular pain, facial pain, or headache.

Treatment of neuro-ophthalmologic and cranial disorders depends on the cause.

Clinical Relevance and Assessment

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 odors 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 - Test pursuit eye movement 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 face (forehead, cheeks, jaw) - Open and close 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 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 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 placed in the middle on top of the head and check if the sound is equal or louder in one ear (Weber’s test) - Vibrating tuning fork place on mastoid bone, then near the ear canal and note hearing acuity (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 air - Sensorineural loss: sound heard longer through air
CN IX – Glossopharyngeal CN X - Vagus	- Listen to voice quality - Test for difficulty swallowing - Let the patient say “ah” and observe the soft palate elevating and that the uvula remain in midline - Examine the gag reflex	- Dysphonia - Dysphagia - With paralysis the palate does not elevate (lesion CN V), unilateral paralysis there is asymmetrical elevation - Absent gag reflex (lesion CN IX, possibly X)
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 weak side - Atrophy or tongue fasciculations - Dysartrhia (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