Facial Nerve in Dentistry

Facial Nerve

• The facial nerve (VII) is part of the oral and dental sciences module.
• GDC Learning Outcomes include describing relevant dental, oral, craniofacial, and general anatomy, and explaining their application to patient management.
• Intended learning outcomes include describing the facial nerve's function, outlining the anatomical regions it supplies, and explaining its relevance to dentistry.

Intended Learning Outcomes

• Describe the function of the facial nerve (VII).
• Outline the anatomical regions it supplies.
• Explain the relevance of the facial nerve to dentistry.

Why do I need to know this?

• Understanding the facial nerve is crucial for dental procedures and patient care.

Resources

• Anatomy.tv
• Acland's Video Atlas of Human Anatomy
• Netter's Head and Neck Anatomy for Dentistry
• TeachMe Anatomy

Intracranial

• The facial nerve emerges from the pons and medulla of the brainstem.
• It has two roots: a larger motor root and a smaller sensory root (nervus intermedius).
• The roots pass into the internal auditory meatus.

Motor Root Functions

• The motor root controls muscles of facial expression.

Function - Nervus intermedius

• Important for special sensory functions like taste.
• It controls the anterior two-thirds of the tongue.
• It manages parasympathetic efferents.
• It provides the parasympathetic output for salivary glands like the submandibular and sublingual glands, as well as tear glands.

Anatomical Course

• The facial nerve has intracranial and extracranial components.
• Intracranial refers to its path through the cranium.
• Extracranial refers to its subsequent course through the face and neck outside the cranium.
• The nerve leaves the cranium via the stylomastoid foramen.

Intracranial summary

• The nerve originates in the pons area of the brainstem.
• Two roots form: motor and sensory, which pass through the internal auditory meatus into the facial canal.

3 branches before leaving facial canal

• Greater petrosal nerve—parasympathetic fibers to mucous glands of the nose, paranasal sinuses, and palate and the lacrimal glands.
• Nerve to stapedius—motor fibers to the stapedius muscle for sound dampening in the middle ear.
• Chorda tympani—special sensory fibers to the anterior two-thirds of the tongue (taste) and parasympathetic fibers to submandibular and sublingual glands.

Chorda tympani

• Provides sensory innervation to the anterior two-thirds of the tongue, responsible for taste.
• Travels with the lingual nerve (a branch of the trigeminal nerve).
• Contains parasympathetic fibers that control the submandibular and sublingual salivary glands.

Extracranial

• The facial nerve exits the cranium via the stylomastoid foramen.
• Branches include:
• Posterior auricular nerve
• Posterior belly of the digastric muscle
• Stylohyoid muscle
• Temporal branch
• Zygomatic branch
• Buccal branch
• Marginal mandibular branch
• Cervical branch

Posterior Auricular Nerve

• Is a motor nerve.
• It elevates in front of the mastoid process.
• Innervates intrinsic and extrinsic muscles of the outer ear.
• Supplies the occipital part of the occipitofrontalis muscle.

Nerves to digastric and stylohyoid muscle

Facial branches

• A memory device to recall the branches is provided (e.g., "Two Zebras Bit My Cat").
• The branches are:
  • Temporal
  • Zygomatic
  • Buccal
  • Marginal mandibular
  • Cervical

Muscles of Facial Expression

• The facial nerve controls muscles responsible for facial expressions like smiling, frowning, and blinking

Temporal Branch

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

Zygomatic Branch

• Usually two parts: Upper and lower.
• The upper part passes above the orbit, innervating the frontal belly of occipitofrontalis and orbicularis oculi.
• The lower part runs below the orbit and innervates the lower part of orbicularis oculi, as well as upper lip and nasal muscles.

Buccal Branch

• Usually two parts.
• Supplies the orbicularis oris, buccinator, and zygomaticus muscles
• Contributes to innervation of 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 muscles of the lower lip: depressor labii inferioris, depressor anguli oris, and mentalis.

Cervical Branch

• Passes downward from the lower border of the parotid gland.
• Supplies the platysma muscle.
• Controls skin of the neck (noticeable vertical and horizontal ridges and depressions) and the pulling down of the mouth's corners during grimacing.

Damage to the facial nerve - Intracranial lesions

• Intracranial lesions occur during the intracranial course of the facial nerve, proximal to the stylomastoid foramen.
• Causes include:
  • 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 are symptoms.
• Causes can include:
  • Parotid gland pathology (e.g., tumor, parotitis, surgery).
  • Infection of the nerve (particularly by the herpes virus).
  • Local anaesthetic (rare).

Parotid gland tumour and surgery

• This is a specific cause of extracranial facial nerve damage.

Trigeminal and facial nerve examination

• A procedure to assess these nerves.

Summary

• The facial nerve provides motor function to facial muscles.
• The nerve has a minor sensory role related to tongue taste and parasympathetic control of salivary glands, glands in the nose, palate, and tear glands.

Quiz

• A quiz on the facial nerve will test the understanding of the material covered.

References

• A list of external websites (and a book) containing the material discussed in the slides.