masseter

The masseter is a thick, quadrilateral muscle of mastication located at the angle of the jaw. It is one of the primary muscles of chewing, alongside the temporalis, medial pterygoid, and lateral pterygoid. The muscle has superficial and deep parts; it originates from the inferior border and medial surface of the zygomatic arch and inserts on the lateral surface of the ramus of the mandible and the mandibular angle. The masseter is covered by a robust fascial sheath and lies adjacent to the parotid gland.

Functionally, the masseter’s main action is to elevate the mandible, producing jaw closure. The superficial fibers contribute to protrusion of the mandible, while the deeper fibers assist with elevation and, in coordination with other masticatory muscles, with lateral movement and fine control of bite force.

Innervation is via the masseteric nerve, a motor branch of the anterior division of the mandibular division (V3) of the trigeminal nerve. The primary arterial supply comes from the masseteric artery, a branch of the maxillary artery; venous drainage follows the accompanying veins into the pterygoid venous plexus and related veins.

Clinical significance includes masseteric hypertrophy, which can result from bruxism or malocclusion and may be treated cosmetically or functionally with botulinum toxin injections. The masseteric reflex (jaw jerk) is a monosynaptic reflex used in neurological examination to assess trigeminal nerve function. Developmentally, the masseter derives from the first pharyngeal arch.