Cranial Nerves

Description

There are 12 pairs of cranial nerves and they are numbered according to their position of where they originate in the inferior surface of the brain.  The names of the cranial nerves (CN) are: CN I - olfactory, CN II - optic, CN III - oculomotor, CN IV - trochlear, CN V - trigeminal, CN VI - abducens, CN VII - facial, CN VIII - vestibulocochlear, CN IX - glossopharyngeal, CN X - vagus, CN XI - accessory, and CN XII - hypoglossal.  [1] Link: Introduction to Neuroanatomy

Cranial Nerves[2]

Function

The names of the cranial nerves sometimes correspond with their individual function.  The cranial nerves are comprised of axons that are either sensory, motor or both. [1]

Cranial nerves and their primary functions[1][3]
Cranial Nerve Sensory Function Somatic Motor Function Autonomic (parasympathetic motor) Function
CN I - olfactory
Smell (olfaction)
  -
  -
CN II - optic
Vision
  -
  -
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)
CN IV - trochlear
  -
Innervate the superior oblique eye muscle (moves the eye inferiorly and laterally)
  -
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
  -
CN VI - abducens
  -
Innervate the lateral rectus muscle of eye (abducts the eye)
  -
CN VII - facial
Taste from anterior two-thirds of tongue
Innervate muscles of facial expression, digastric (posterior belly) and stapedius muscle
Increase secretion from the lacrimal (tear glands) and nasal mucosal glands; submandibular and sublingual
salivary glands

CN VIII - vestibulocochlear
Hearing (cochlear branch); linear and angular acceleration, or head position in space/equilibrium
(vestibular branch)
  -
  -
CN IX - glossopharyngeal
Touch and taste from the posterior 1/3 of the tongue; visceral sensory from the carotid bodies
Innervate the pharyngeal muscle
Increase secretion from the parotid salivary gland
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 muscle an glands of the heart, lungs larynz trachea, and most abdominal organs
CN XI - accessory
  -
Innervates the trapezius and sternocleidomastoid muscle
  -
CN XII - hypoglossal
  -
Innervates intrinsic and extrinsic tongue muscles
  -


[4]

Clinical Relevance and Assessment [5][6]

Cranial Nerve Integrity
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 – trochlea
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

References

  1. 1.0 1.1 1.2 McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York: McGraw-Hill, 2008fckLRfckLRHill C. Practical guidelines for cystic fibrosis care. Edinburgh: Churchill Livingstone, 1998.
  2. Image: Wikipedia, https://en.wikipedia.org/wiki/Trochlear_nerve (accessed 12 Oct 2016)
  3. Fuller KS, Introduction to Central Nervous System Disorders. Goodman CC, Fuller KS. Pathology: implications for the physical therapist. Elsevier Health Sciences; 2014. p1371-1404
  4. Youtube Video: MEDZCOOL - How to Remember the Cranial Nerves (Mnemonic) https://youtu.be/6ENCJkXJvio (accessed 12 Oct 2016)
  5. 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
  6. 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