Facial Nerve Anatomy

• The Facial Nerve is also known as cranial nerve VII.
• An understanding of the facial nerve function is relevant to dentistry.

Facial Nerve Anatomy

• The facial nerve emerges from the brain between the pons and medulla.
• It contains two roots.
• The larger root is the main motor root originating from the motor nucleus.
• The smaller root is known as the nervus intermedius.
• Both roots pass into the internal auditory meatus.

Motor Root Functions

• The motor root controls muscles of facial expression and the platysma.
• It also controls the stylohyoid muscle and posterior belly of the digastric.
• The motor root affects the stapedius muscle within the ear.

Nervus Intermedius Functions

• The nervus intermedius function includes the special sensory of taste to the anterior two thirds of the tongue.
• It governs sensory function to the external ear
• It provides Parasympathetic efferent innervation.
• The Parasympathetic efferent innervation includes sublingual and submandibular salivary glands.
• It also includes lacrimal, nasal, palatine and pharyngeal glands.

Anatomical Course

• The facial nerve course divides into intracranial and extracranial sections.
• The intracranial portion traverses the cranial cavity and cranium itself.
• The extracranial portion runs outside the cranium, through the face and neck.

Intracranial Section

• The facial nerve originates in the pons area of the brainstem.
• There are two roots: a large motor root and small sensory root.
• The two roots travel through the internal acoustic meatus into the facial canal.

Branches of the Facial Nerve Within the Facial Canal

• The greater petrosal nerve contains parasympathetic fibers.
• The greater petrosal nerve fibers go to the mucous glands of the nose, paranasal sinuses, palate, and lacrimal gland.
• The nerve to stapedius supplies motor fibers to the stapedius muscle of the middle ear.
• These motor fibers are important for dampening loud noises.
• The chorda tympani contains special sensory fibers for taste in the anterior 2/3 of the tongue.
• It also has parasympathetic fibers for the submandibular and sublingual glands.
• The facial nerve exits the facial canal through the stylomastoid foramen.

Chord Tympani Nerve

• Travels with the lingual nerve, a branch of the trigeminal nerve.
• Carries parasympathetic fibers to the submandibular and sublingual glands.

Extracranial Section

• The facial nerve exits the cranium through stylomastoid foramen.
• Extracranial branches include:
  • Posterior auricular nerve.
  • Posterior belly of the digastric muscle.
  • Stylohyoid muscle.
• The nerve runs through the parotid gland, branching into five main branches:
  • Temporal branch.
  • Zygomatic branch.
  • Buccal branch.
  • Marginal mandibular branch.
  • Cervical branch.

Posterior Auricular Nerve

• Motor nerve.
• Ascends in front of the mastoid process.
• Innervates both the intrinsic and extrinsic muscles of the outer ear.
• Supplies the occipital part of the occipitofrontalis muscle.

Facial Branch Mnemonics

• "Two Zebras Bit My Cat".
• "Ten Zebras Buggered My Car".

Temporal Branch

• Leaves the superior surface of the parotid gland.
• Crosses the zygomatic arch to reach the forehead.
• Supplies the auricular muscles, frontalis, orbicularis oculi, and corrugator supercilii.

Zygomatic Branch

• Usually comprised of two divisions: upper and lower.
• The upper branch passes above the orbit and supplies the frontal belly of the occipitofrontalis muscle and orbicularis oculi.
• The lower branch passes below to supply the lower part of the orbicularis oculi.
• Contributes to innervation of muscles in the upper lip and nose.

Buccal Branch

• Divides into two branches.
• Supplies the orbicularis oris, buccinator, and zygomaticus muscles.
• Contributes to the innervation of the muscles of the upper lip and nose.

Mandibular Branch

• Emerges from the lower border of the parotid gland.
• Runs near the inferior border of the mandible.
• Supplies the muscles of the lower lip, including the depressor labii inferioris, depressor anguli oris, and mentalis.

Cervical Branch

• Passes downwards from the lower border of the parotid gland.
• Supplies the platysma muscle.
• Raises the skin of the neck to form noticeable vertical and horizontal ridges and depressions.
• Pulls the corner of the mouth down.

Damage to the Facial Nerve - Intracranial Lesions

• Intracranial lesions occur during the intracranial course.
• Intracranial lesions are proximal to the stylomastoid foramen.
• Causes of Intracranial lesions:
  • Infection related to the external or middle ear.
  • Stroke.

Damage to the Facial Nerve - Extracranial Lesions

• Paralysis or severe weakness of the muscles of facial expression.
• Various causes:
  • Parotid gland pathology, such as a tumor, parotitis, or surgery.
  • Infection of the nerve, particularly by the herpes virus.
  • Local anaesthetic (rare).

Summary of Facial Nerve Function

• Provides motor supply to muscles of facial expression.
• Provides a minor sensory role.
• Controls taste to the anterior two-thirds of the tongue.
• Involved in parasympathetic innervation:
  • Submandibular and sublingual salivary glands.
  • Nasal, palatine, and pharyngeal mucous glands.
  • Lacrimal glands.