Cranial Nerves
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Cranial Nerve Overview[edit | edit source]
- 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.[1][2]
- Cortinuclear tracts act as bridges between upper motor neurons and cranial nerves to carry motor information from the cortex.
The cranial nerves listed in order:
- CN I - Olfactory nerve
- CN II - Optic nerve
- CN III - Oculomotor nerve
- CN IV - Trochlear nerve [3]
- 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 [4]
For more information on the structure of the human nervous system, please review this article: Introduction to Neuroanatomy.
Functions[edit | edit source]
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. [5]
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.[7]
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.[7]
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[edit | edit source]
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[9].
Treatment of neuro-ophthalmologic and cranial disorders depends on the cause.
Clinical Relevance and Assessment[edit | edit source]
"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."[10]-Taylor et al. 2021
The cranial nerve testing provides a clear understanding about the clinical implications and impairments along the cranial nerve pathway.[11][12]
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[edit | edit source]
- 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
References[edit | edit source]
- ↑ Sonne J, Lopez-Ojeda W. Neuroanatomy, cranial nerve. InStatPearls [Internet] 2019 Apr 3. StatPearls Publishing.
- ↑ Sonne J, Lopez-Ojeda W. Neuroanatomy, Cranial Nerve.[Updated 2021 Nov 14]. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. 2022. [1]
- ↑ Image: Wikipedia, https://en.wikipedia.org/wiki/Trochlear_nerve (accessed 12 Oct 2016)
- ↑ 4.0 4.1 McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York: McGraw-Hill, 2008.
- ↑ Butler A.B. Encyclopedia of the Human Brain, Four-Volume Set. Academic Press; 2002.
- ↑ Fuller KS, Introduction to Central Nervous System Disorders. Goodman CC, Fuller KS. Pathology: implications for the physical therapist. Elsevier Health Sciences; 2014. p1371-1404
- ↑ 7.0 7.1 Radiopaedia. Cranial nerves (mnemonic). Available from: https://radiopaedia.org/articles/cranial-nerves-mnemonic?lang=us (accessed 4 June 2024).
- ↑ Youtube Video: MEDZCOOL - How to Remember the Cranial Nerves (Mnemonic) https://youtu.be/6ENCJkXJvio (accessed 12 Oct 2016)
- ↑ MSD manauls Overview of Neuro-ophthalmologic and Cranial Nerve Disorders Available from: https://www.msdmanuals.com/professional/neurologic-disorders/neuro-ophthalmologic-and-cranial-nerve-disorders/overview-of-neuro-ophthalmologic-and-cranial-nerve-disorders (accessed 4.2.2021)
- ↑ Taylor A, Mourad F, Kerry R, Hutting N. A guide to cranial nerve testing for musculoskeletal clinicians. Journal of Manual & Manipulative Therapy. 2021 Nov 2;29(6):376-90.
- ↑ O'Sullivan SB, Neuromuscular Physical Therapy. In: O'Sullivan SB, Siegman RP. National Physical Therapy Examination Review & Study Guide. 19th edit. Evanston: TherapyEd, 2016. p121-184
- ↑ Chui KK, Schmitz TJ. Examination in Sensory Function. In: Physical Rehabilitation. 6th Edit. Philadelphia: O’Sullivan SB, Schmitz TJ, Fulk GD F.A. Davis Company, 2014. P87-121