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Cranial nerves · Mixed

VII. Facial nerve

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The facial nerve (CN VII) is the motor nerve of the muscles of facial expression, and also carries taste from the anterior two thirds of the tongue and parasympathetic fibres to the lacrimal, submandibular and sublingual glands. The sensory and parasympathetic components form the nervus intermedius.

Terminologia Anatomica (Latin): Nervus facialis

Function

Special visceral efferent to the muscles of facial expression, stapedius, stylohyoid and posterior belly of digastric. Special visceral afferent (taste) from the anterior two thirds of the tongue via the chorda tympani. General visceral efferent (parasympathetic) to lacrimal, nasal, submandibular and sublingual glands. Small general somatic afferent component from the external ear.

Nuclei

Facial motor nucleus in the caudal pons (its upper part, supplying the forehead, receives bilateral corticonuclear input); superior salivatory and lacrimal nuclei (parasympathetic); and the nucleus of the solitary tract (rostral gustatory part) for taste.

Exit from the skull

It enters the internal acoustic meatus with CN VIII, runs in the facial canal of the temporal bone (giving the greater petrosal nerve, nerve to stapedius and chorda tympani), and exits the skull through the stylomastoid foramen, then divides within the parotid gland into temporal, zygomatic, buccal, marginal mandibular and cervical branches.

Testing

Ask the patient to raise the eyebrows, close the eyes tightly against resistance, show the teeth, smile and puff the cheeks. Compare the forehead (frontalis) with the lower face. Taste of the anterior tongue, lacrimation (Schirmer test) and hyperacusis (stapedius) can be assessed when localisation is needed.

Signs of a lesion

Lower motor neuron (nuclear or nerve) lesion paralyses the whole ipsilateral face, including the forehead. An upper motor neuron lesion spares the forehead because of bilateral cortical innervation of the upper face, producing contralateral weakness of the lower face only. Higher lesions in the facial canal add loss of taste, reduced lacrimation or hyperacusis.

Clinical correlations

Bell palsy is an acute idiopathic lower motor neuron facial palsy, a diagnosis of exclusion after other causes (Ramsay Hunt syndrome from herpes zoster, otitis media, parotid tumour, Lyme disease, stroke) are considered. Management follows local guidelines, with eye protection essential because of incomplete eye closure.

Common exam points

  • Exits via the stylomastoid foramen and divides in the parotid gland (temporal, zygomatic, buccal, marginal mandibular, cervical).
  • UMN lesion spares the forehead; LMN lesion paralyses the whole half of the face.
  • Chorda tympani carries taste from the anterior two thirds of the tongue.
  • Bell palsy is a diagnosis of exclusion; protect the eye.
  • Parotid surgery risks facial nerve injury.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)
  4. Bickley LS. Bates' Guide to Physical Examination and History Taking (Bickley). Wolters Kluwer, 13th edition (2020) — Tier 3 (textbook)

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