Facial Nerve (CN VII)

Summary

The facial nerve, also known as cranial nerve number seven or the 7th cranial nerve (CN VII), carries both motor and sensory information. The facial nerve mediates most facial movements and eyelid closing, and is the primary motor component of the corneal or blink reflex. The nerve also controls volume modulation by innervating the stapedius muscle of the ear. Additionally, it carries outgoing parasympathetic signals to induce salivation and lacrimation, and is the main motor component of the lacrimation reflex. In terms of sensory roles, the facial nerve innervates the taste buds on the anterior 2/3rds of the tongue, and carries those taste signals back to our brain via a nerve branch called the chorda tympani. The pathway of the facial nerve starts from its emergence from the brainstem at level of the pons. The facial nerve then enters the skull through the internal acoustic meatus, travels inside the skull in the facial canal, before exiting the skull at the Stylomastoid Foramen. The facial nerve can be damaged in Bell’s Palsy and in advanced cases of Lyme disease. Finally, central lesions of the facial nerve, such as those caused by a stroke, spare muscle control of the forehead since the forehead receives innervation from both sides of the brain, while peripheral lesions of the nerve itself will cause loss of forehead muscle control.

Key Points

• Facial Nerve (Cranial Nerve VII)
  • Nerve Type
    • Motor
    • Sensory
  • Function
    • Facial movement
      • Key muscles innervated include frontalis, orbicularis ori, buccinator, platysma, posterior belly of digastric muscle
      • Tested by asking patient to exhibit a range of facial expressions (smile, eyebrow raise, tight eye closure)
      • Involved in most facial expressions
    • Eye closing
      • Via orbicularis oculi
    • Taste
      • Innervates taste buds for anterior ⅔ of tongue
      • Via chorda tympani
      • Taste tested with sweet, sour, or bitter solutions on cotton tip
    • Auditory volume modulation
      • Innervates stapedius muscle for sound dampening
      • Test for hyperacusis on tuning fork exam
      • Damage leads to hyperacusis (increased sensitivity to sound)
    • Parasympathetics
      • Salivation
        • Carries parasympathetic fibers to submandibular and sublingual glands
      • Lacrimation
      • Nasal secretion
  • Nerve Entry/Exit:
    • Pons in brainstem near the ventrolateral pontomedullary junction
      • Lateral to abducens nerve
      • May be affected by schwannomas affecting cerebellopontine angle (although CN VIII is more commonly affected)
      • May also be damaged by lateral skull fractures near temporal bone
      • Motor output and parasympathetics from nuclei in the pons
        • Motor output from facial motor nucleus in pons
        • Parasympathetics from superior salivatory nucleus in pons
      • Sensory input to medulla
        • Taste, visceral afferents to solitary nucleus (nucleus solitarius) in medulla
  • Cranial foramen
    • Skull entry internal acoustic meatus
    • Skull exit stylomastoid foramen
    • Travels within facial canal in temporal bone between entry and exit
  • Reflexes
    • Corneal reflex (efferent motor)
      • Sensory limb carried by CN V1
    • Lacrimal reflex (efferent motor)
  • Clinical Correlates
    • Bell’s palsy is an idiopathic neuropathy of the facial nerve
    • Lyme disease may also affect the facial nerve
    • Central vs. peripheral nerve lesions
      • Forehead motor function (wrinkling of forehead) spared with central lesion, due to redundant innervation by both left and right peripheral nerves