2.2 The Facial Nerve and Parotid Region

The Facial Nerve (Cranial Nerve VII)

The facial nerve is the nerve of the second pharyngeal arch. While its special sensory and parasympathetic components travel with the nervi intermedius, its motor root provides sole innervation to all muscles of facial expression, the posterior belly of the digastric, stylohyoid, and stapedius muscles.

Internal Acoustic Meatus ──> Facial Canal (Petrous Bone) ──> Stylomastoid Foramen
                                                                    │
                                                          Parotid Gland Substance
                                                        (Superficial to RMV & ECA)
                                                                    │
                                                              Pes Anserinus
                                                                    │
                        ┌──────────────┬──────────────┼──────────────┬──────────────┐
                        ▼              ▼              ▼              ▼              ▼
                     Temporal      Zygomatic        Buccal        Marginal       Cervical
                                                                 Mandibular

Extracranial Course & Parotid Anatomy

1. Exits the cranial base via the stylomastoid foramen, situated immediately posterior to the styloid process and anterior to the mastoid process.
2. Gives off three branches prior to entering the parotid:
   • Posterior auricular nerve (occipitalis and posterior auricular muscles).
   • Nerve to the posterior belly of the digastric.
   • Nerve to the stylohyoid muscle.
3. Enters the posteromedial surface of the parotid gland, passing forward and downward through the gland's substance. It divides at the pes anserinus (goose's foot) into two primary trunks: temporofacial and cervicofacial.
4. These trunks form the parotid plexus, which emerges from the superior, anterior, and inferior margins of the parotid gland as five terminal branches:
   • Temporal branches: Cross the zygomatic arch to supply frontalis, orbicularis oculi (superior portion), and corrugator supercilii.
   • Zygomatic branches: Cross the zygoma to supply orbicularis oculi (inferior portion) and zygomaticus muscles.
   • Buccal branches: Run superficial to the masseter below the parotid duct to supply the buccinator, orbicularis oris, and levator labii superioris.
   • Marginal mandibular branch: Runs forward along or 1–2 cm below the inferior border of the mandible, superficial to the facial vessels and submandibular gland, covered only by platysma and investing cervical fascia; turns superiorly across the mandibular body to supply mentalis, depressor labii inferioris, and depressor anguli oris.
   • Cervical branch: Descends into the neck behind the angle of the mandible to supply the platysma.

Spatial Hierarchy Within the Parotid Gland

Surgical dissection and exam questions heavily test the three-layered anatomical relationship traversing the parotid gland from superficial to deep:

$\text{Superficial} \longrightarrow \text{Deep: } \mathbf{Facial\ Nerve\ (CN\ VII)} \longrightarrow \mathbf{Retromandibular\ Vein} \longrightarrow \mathbf{External\ Carotid\ Artery}$

[!CAUTION] Clinical Trap — Transient Facial Palsy during IANB: If the needle is directed too far posteriorly during an inferior alveolar nerve block, it misses the medial surface of the ramus and passes behind its posterior border into the capsule of the parotid gland. Depositing local anaesthetic here bathes the branches of CN VII, producing instantaneous unilateral facial paralysis (inability to close the eye, drooping mouth, obliteration of nasolabial fold). Because this affects motor fibres, the patient retains intact mandibular sensory innervation. Management involves reassurance, eye protection (lubricating drops/taping the eyelid closed to prevent corneal drying), and waiting 2–3 hours for local anaesthetic clearance.

Upper vs Lower Motor Neuron Facial Lesions

• Lower Motor Neuron (LMN) Lesion (e.g., Bell's Palsy, parotid malignancy, surgical trauma): Complete flaccid paralysis of the entire ipsilateral half of the face, including the forehead (patient cannot wrinkle forehead, raise eyebrow, or close eye; Bell's phenomenon: globe rotates upward and outward on attempted closure).
• Upper Motor Neuron (UMN) Lesion (e.g., Stroke, cerebral infarction): Paralysis of the contralateral lower quadrant of the face only, with sparing of the forehead muscles (frontalis and orbicularis oculi) because the portion of the facial motor nucleus supplying the upper face receives bilateral corticonuclear innervation from both cerebral hemispheres.

---

Clinical Correlations Examiners Favour

Facial nerve anatomy is examined through consequences rather than through lists. Three scenarios recur. The first is transient facial palsy after an inferior alveolar nerve block: an over-inserted needle passes through the posterior border of the ramus into the body of the parotid gland and deposits solution around the facial nerve trunk. The patient develops unilateral weakness within minutes, cannot close the eye, and is distressed. The correct management is reassurance, protection of the cornea with an eye pad and artificial tears until the blink reflex returns, and documentation; the palsy resolves as the anaesthetic wears off, typically within two to three hours. The examinable trap is choosing urgent referral or steroids for what is a self-limiting iatrogenic event.

The second is distinguishing a lower motor neurone lesion from an upper motor neurone lesion, which turns entirely on the forehead. Bilateral cortical innervation of the frontalis means an upper motor neurone lesion — a stroke — spares the forehead, while a lower motor neurone lesion such as Bell's palsy paralyses the whole hemiface. A patient who cannot wrinkle the forehead has a lower motor neurone problem; a patient who can wrinkle the forehead but has a drooping mouth requires urgent stroke assessment.

The third is surgical risk during parotid and submandibular surgery. The marginal mandibular branch runs close to the inferior border of the mandible and is at risk during submandibular gland excision and during extraoral approaches to the mandible; injury produces an asymmetric lower lip on smiling. Because the facial nerve divides the parotid into surgical superficial and deep lobes, a tumour deep to the nerve plane requires a more extensive dissection and carries a higher risk of permanent weakness.

Remember also that the facial nerve carries more than motor fibres. The nervus intermedius contributes parasympathetic secretomotor supply to the lacrimal, submandibular and sublingual glands and special taste sensation from the anterior two-thirds of the tongue via the chorda tympani. A lesion proximal to the geniculate ganglion therefore causes dry eye and loss of taste in addition to motor weakness, which is how examiners ask you to localise the lesion along the nerve's course.