Anatomy & Histology · patient case with 1 question
Facial nerve branches and the muscles of the lips
Patient: Male, 39 years old
Chief Complaint: “Since the surgery on my cheek I cannot purse my lips or whistle.”
Background: Excision of a benign parotid lesion four weeks ago. Sensation of the face is intact and equal bilaterally.
Vital Signs: BP 122/76 mmHg · HR 70 bpm
Findings: Unable to purse the lips or maintain an oral seal on the operated side. Smile and eye closure are preserved. Lower lip depression on the operated side is intact. No sensory deficit.
Question 1
Which nerve supplies the ORBICULARIS ORIS?
Show answer and explanation
Correct answer: B. The buccal branch of the facial nerve
- A. The ophthalmic division of the trigeminal nerve is sensory to the forehead, upper eyelid and cornea. It carries no motor fibres to the face.
- B. CORRECT. Orbicularis oris is a muscle of FACIAL EXPRESSION, so its innervation is motor from the FACIAL NERVE (CN VII) — principally through the BUCCAL branch, with a contribution from the marginal mandibular branch. The buccal branch also supplies buccinator, the zygomaticus muscles and levator anguli oris. Its loss gives exactly the deficit described: inability to purse the lips, whistle or maintain an oral seal, and difficulty controlling a bolus.
- C. The long buccal nerve is a SENSORY branch of V3 supplying the buccal mucosa and the buccal gingiva of the mandibular molars. It shares a name with the facial nerve's buccal branch and nothing else — a favourite examination trap.
- D. The marginal mandibular branch supplies depressor anguli oris, depressor labii inferioris and mentalis — the LOWER LIP DEPRESSORS. Its injury produces an asymmetric smile with failure to depress the lower lip, which this patient does not have.
Note: The rule that resolves nearly every question of this kind: muscles of FACIAL EXPRESSION are motor from CN VII; muscles of MASTICATION are motor from V3; and V as a whole is SENSORY to the face. When a stem describes weakness without numbness, the nerve is VII; numbness without weakness points to V. Facial nerve terminal branches from the stylomastoid foramen: Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical.