The masseter muscle is a thick, rectangular muscle of mastication.
It consists of a superficial and a deep part, both originating from the zygomatic arch.
The superficial part inserts on the masseteric tuberosity at the outer surface of the mandibular angle, whereas the deep part runs further dorsally to the outer surface of the mandibular ramus. At that point, the muscle can be easily palpated from the oral cavity along the cheek. Furthermore, part of the deep fibers radiates into the anterior capsule and articular disc of the temporomandibular joint.
The parotid gland lies on the lateral side of the masseter with its duct coursing underneath the zygomatic arch across the muscle. As all muscles of mastication, the masseter is supplied by a branch of the mandibular nerve, the masseteric nerve.
The masseter is one of the four muscles of the masticatory apparatus. It elevates the mandible causing a powerful jaw closure. The contraction of the superior part which runs diagonally to the front moves the mandible forward (protrusion). Furthermore, the muscle helps stabilize tension of the articular capsule of the temporomandibular joint.
The routine neurological examination includes testing of the jaw jerk reflex, also known as the masseter or mandibular reflex. The examiner places his index finger on the chin of the patient and taps the finger with the reflex hammer. In healthy people the stretch of the masseter provokes an upward movement of the mandible.
In order to produce better results, it is helpful to ask the patient to relax and slightly open his or her mouth and close the eyes. A pathological jaw jerk reflex can indicate a lesion of the trigeminal nerve but if this seems rather unlikely (e.g. intact facial sensation, well developed masseter muscles on both sides etc.) one has to consider a brainstem lesion.