What is the correct nerve supply to the muscles labelled as A and B?

Explanation

## Correct Answer: B. A - facial nerve, B- Nerve to mylohyoid The question tests anatomical knowledge of nerve supply to muscles in the neck and oral region. Muscle A is the **platysma**, a superficial cervical muscle, which is innervated by the **facial nerve (CN VII)**. The facial nerve exits the stylomastoid foramen and gives off the cervical branch that supplies platysma—this is a classic anatomical relationship tested in NEET PG. Muscle B is the **mylohyoid**, an important muscle of the floor of mouth that forms the anterior belly of the digastric and supports the tongue during swallowing and mastication. The mylohyoid is innervated by the **nerve to mylohyoid**, which is a branch of the inferior alveolar nerve (itself a branch of the mandibular division of the trigeminal nerve). This nerve runs in the mylohyoid groove on the medial surface of the mandible. In Indian anatomy curricula (as per BDC Chaurasia and standard NEET PG anatomy texts), this distinction is critical: facial nerve → platysma (superficial cervical muscle), and mandibular nerve branch (nerve to mylohyoid) → floor of mouth muscles. The embryological origin also supports this: platysma derives from the second pharyngeal arch (facial nerve territory), while mylohyoid derives from the first pharyngeal arch (mandibular nerve territory). ## Why the other options are wrong **A. A - Spinal Accessory, B-Mandibular nerve** — This is incorrect because the spinal accessory nerve (CN XI) supplies the sternocleidomastoid and trapezius, not platysma. While the mandibular nerve does supply floor-of-mouth muscles, it does so via the nerve to mylohyoid (a specific branch), not the main trunk. This option conflates gross nerve territories without recognizing the specific branch anatomy. **C. A - facial nerve, B- spinal accessory nerve** — This is a trap answer that correctly identifies facial nerve for muscle A (platysma) but incorrectly assigns spinal accessory nerve to muscle B. The spinal accessory nerve does not innervate mylohyoid; it supplies neck muscles like sternocleidomastoid. This tests whether students confuse CN XI with CN V3 branches in the neck region. **D. A- Mandibular, B-Facial Nerve** — This reverses the correct answer and represents a common confusion between trigeminal and facial nerve territories. Mandibular nerve does not supply platysma (a facial nerve muscle). This trap exploits students who know both nerves are involved but assign them to the wrong muscles. ## High-Yield Facts - **Platysma** is the only muscle of facial expression supplied by the **cervical branch of CN VII** (facial nerve), not CN V. - **Mylohyoid** is innervated by the **nerve to mylohyoid**, a branch of the **inferior alveolar nerve** (CN V3), which runs in the mylohyoid groove. - **Pharyngeal arch 2** (Reichert's cartilage) derivatives are supplied by **CN VII**; **pharyngeal arch 1** (Meckel's cartilage) derivatives are supplied by **CN V3**. - The **nerve to mylohyoid** arises from the inferior alveolar nerve *before* it enters the mandibular foramen, making it a critical anatomical landmark in oral surgery. - **Platysma** is unique among facial muscles: it is the only one that extends below the mandible into the neck and is the only one supplied by the cervical branch of facial nerve. ## Mnemonics **Facial Nerve = Face + Neck Platysma** CN VII supplies all muscles of facial expression PLUS platysma (the only neck muscle it supplies). Platysma is the 'exception' that extends into the neck. **Mylohyoid = Mandibular (V3) Territory** Mylohyoid is a floor-of-mouth muscle derived from pharyngeal arch 1 → always CN V3 (mandibular), specifically the nerve to mylohyoid branch. ## NBE Trap NBE pairs "facial nerve" with "spinal accessory nerve" in option C to trap students who correctly identify CN VII for platysma but then confuse CN XI (which supplies sternocleidomastoid) with CN V3 branches in the neck region. The trap exploits incomplete knowledge of which cranial nerves supply the floor of mouth. ## Clinical Pearl In oral surgery and neck dissections performed in Indian cancer centers, the nerve to mylohyoid must be identified and preserved to maintain floor-of-mouth sensation and swallowing function. Facial nerve injury (Bell's palsy or post-parotidectomy) causes platysma paralysis, visible as loss of neck skin wrinkling—a bedside sign Indian clinicians use to confirm CN VII involvement. _Reference: BDC Chaurasia's Human Anatomy (Regional & Applied) - Neck & Head sections; Harrison Principles of Internal Medicine Ch. 379 (Cranial Nerves)_