Which cranial nerve is primarily associated with mastication (jaw movement and clenching)?

• A. Facial nerve (CN VII)
• B. Glossopharyngeal nerve (CN IX)
• C. Trigeminal nerve (CN V)
• D. Vagus nerve (CN X)

Answer: (C)

The key idea is that the muscles used for chewing are controlled by a motor branch of a cranial nerve. The four muscles of mastication—the masseter, temporalis, and the two pterygoids—receive their main motor commands from the mandibular division of the trigeminal nerve. This division is unique among the trigeminal branches because it carries motor fibers in addition to sensory fibers, and it supplies the muscles that close and move the jaw.

Fibers from the trigeminal motor nucleus travel with the mandibular division through the foramen ovale to reach the muscles of mastication. When these nerves fire, the jaw can elevate to bite (masseter and temporalis), elevate and then move side-to-side or protrude (medial and lateral pterygoids), enabling clenching and grinding.

The other cranial nerves listed have different roles: the facial nerve mainly controls muscles of facial expression; glossopharyngeal and vagus are more involved in swallowing, taste, and autonomic functions rather than chewing. Because chewing hinges on the motor control of the jaw muscles, the trigeminal nerve (CN V), specifically the mandibular division, is the primary nerve involved.