Facial Nerve Anatomy

Facial Nerve Roots

• The facial nerve contains motor, sensory, and parasympathetic roots.

Course of the Facial Nerve

• The facial nerve passes through the internal acoustic meatus.
• It then travels through the facial canal in the medial wall of the tympanic cavity (middle ear), forming the sensory geniculate ganglion.
• The facial nerve exits the skull through the stylomastoid foramen.
• The extracranial course involves entering the parotid gland, where it gives rise to five terminal motor branches.

Branches Within Petrous Bone

• Greater petrosal nerve: Mainly contains preganglionic parasympathetic fibers that relay in the pterygopalatine ganglion, along with sensory taste afferents from the soft palate.
• Nerve to stapedius: Supplies the stapedius muscle of the middle ear.
• Chorda tympani: Contains preganglionic parasympathetic fibers to submandibular and sublingual glands, as well as sensory taste afferents from the anterior 2/3 of the tongue.

Branches Below Stylomastoid Foramen

• Posterior auricular nerve: Supplies the occipital belly of the occipitofrontalis muscle.
• Digastric nerve: Supplies the posterior belly of the digastric muscle.
• Stylohyoid nerve: Supplies the stylohyoid muscle.

Branches Within the Parotid Gland

• The facial nerve runs forward within the parotid salivary gland and divides into five terminal branches:
  • Temporal
  • Zygomatic
  • Buccal
  • Marginal Mandibular
  • Cervical
• Temporal branch supplies the frontal belly of occipitofrontalis and orbicularis oculi.
• Zygomatic branch supplies the orbicularis oculi.
• Buccal branch supplies the orbicularis oris, buccinator, and elevators of the upper lip.
• Marginal Mandibular branch supplies the muscles of the lower lip.
• Cervical branch supplies the platysma muscle.

Important Note

• All sensory and parasympathetic nerve fibers leave the facial nerve within the petrous bone. At the stylomastoid foramen, the facial nerve is purely motor.
• Along its course, the facial nerve forms two parasympathetic ganglia: pterygopalatine and submandibular ganglia.

Clinical Anatomy

• Upper motor neuron lesions:
  • Site of lesion: Above the facial nucleus (in the brain).
  • Only the lower part of the face is involved.
  • Affects the contralateral side of the face.
  • Taste is not affected.
  • No dry eye.
• Lower motor neuron lesions:
  • Site of lesion: In the nucleus or distal to the nucleus.
  • Both upper and lower parts of the face are involved.
  • Affects the ipsilateral side of the face.
  • Loss of taste sensation from the anterior 2/3 of the tongue.
  • Dry eye.
• The part of the nucleus that supplies the muscles of the upper part of the face receives cortico-nuclear fibers from both cerebral hemispheres.
• The part of the nucleus that supplies the muscles of the lower part of the face receives cortico-nuclear fibers from the opposite cerebral hemisphere only.

Bell's Palsy

• Bell's palsy is a lower motor neuron lesion of the facial nerve.
• Causes:
  • Damage to the facial nerve in: the internal acoustic meatus (e.g., by a tumor), the middle ear (e.g., by infection or operation), the facial nerve canal (perineuritis) or edema, the parotid gland (e.g., by a tumor), and lacerations of the face.
• Symptoms:
  • Inability to close the eye.
  • Inability to puff cheeks.
  • Loss of forehead wrinkles.
  • The angle of the mouth sags on the affected side.