The masseter reflex, also known as the mandibular reflex or jaw-jerk reflex, involves opening the mouth and placing a finger on the chin of the patient and striking the finger with a reflex hammer. There is a variable presentation from complete trismus and severe spasticity to a tight jaw response, which becomes concerning as the patient is unable to be adequately intubated, leading to an increased risk of malignant hyperthermia in the patient. Masseter muscle rigidity (MMR), also known as "jaws of steel," occurs following a dose of succinylcholine and is defined as limb muscle flaccidity with jaw muscle tightness. The lateral pterygoid is the only muscle to actively oppose the other muscles of mastication and allow for depression of the mandible. Contracting this muscle allows for the protraction of the mandible and anterior motion of the TMJ allowing for the mouth to open. The lateral pterygoid also originates from two heads, the infratemporal surface/crest of the great wing of the sphenoid and the lateral surface of the lateral pterygoid plate, and inserts at the joint capsule of the temporomandibular joint (TMJ), the articular disc of the TMJ, and the condylar neck of the mandible. Contraction of the medial pterygoid is similar to the masseter muscle causing elevation and protrusion of the mandible. The medial pterygoid originates from two heads, the medial surface of the lateral pterygoid plate and the tuberosity of the maxilla, and inserts along the medial surface of the mandibular ramus. Contraction causes elevation of the mandible and retraction of the mandible. The temporalis muscle originates from the floor of the temporal fossa and inserts onto the coronoid process of the mandible. The masseter muscle provides powerful elevation and protrusion of the mandible by originating from the zygomatic arch and inserting along the angle and lateral surface of the mandible. The four muscles separate into a superficial group involving the masseter and temporalis muscles, and a deep group involving the medial and lateral pterygoids. The actions of the muscles of mastication open and close the mouth by influencing motion on the mandible. The classification of the muscles of mastication refers to four main muscles including the masseter, temporalis, medial pterygoid, and lateral pterygoid. The masseteric nerve branches off of the mandibular nerve and crosses over the mandibular notch with the masseteric artery to the deep portion of the masseter muscle where it supplies motor innervation. The mandibular nerve (V3) splits into anterior and posterior trunks, with the anterior portion supplying motor innervation to the muscles of mastication, and sensory innervation to the buccal (cheek) area. The motor fibers of the trigeminal nerve do not synapse within the trigeminal ganglion, but rather bypass the trigeminal ganglion via the portio minor, a small branch of the trigeminal nerve which passes inferior to the trigeminal ganglion and through the foramen ovale, where it then rejoins the mandibular nerve. Sensory fibers travel along the trigeminal nerve, enter the trigeminal ganglion and travel to their final destinations via V1 (ophthalmic), V2 (maxillary) and V3 (mandibular) divisions. The trigeminal nerve carries both sensory and motor fibers. The masseteric nerve is a branch of the mandibular division (V3) of the trigeminal nerve (cranial nerve V). The masseteric nerve innervates the masseter muscle. Its insertion along the angle and lateral surface of the ramus also allows it to aid in the protrusion of the mandible allowing for the anterior motion of the jaw. ![]() When the masseter contracts it causes powerful elevation of the mandible causing the mouth to close. The masseter muscle is one of the four muscles responsible for the action of mastication (chewing). ![]() ![]() Anteriorly, the deep portion is covered by the superior portion of the masseter, while posteriorly, the parotid gland covers the deep portion. The fibers run inferiorly and insert along the mandibular ramus superior to the masseter muscle’s superior portion. The deep portion of the masseter muscle originates from the entire surface of the zygomatic arch. The superficial masseter muscle has a quadrangular shape appearance on gross examination due to its origins and insertions. The fibers of the superficial portion pass inferior-posteriorly over the deep portion and insert onto the angle of the mandible (masseteric tubercle) and the inferior portion of the lateral surface of the mandibular ramus. The superficial portion of the masseter muscle originates from a thick aponeurosis on the temporal process of the zygomatic bone and the anterior two-thirds of the inferior border of the zygomatic arch. It is a powerful superficial quadrangular muscle with two divisions: superficial and deep. The masseter muscle is one of the muscles of mastication.
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