Dominic Stone
The muscles of mastication are a group of muscles that enable chewing, grinding food, and approximating the teeth. They are innervated by the trigeminal nerve, specifically the mandibular branch, and are embryologically derived from the first pharyngeal arch. These muscles include the masseter, temporalis, medial pterygoid, and lateral pterygoid, which work in pairs and are responsible for opening and closing the mouth. The masseter and temporalis muscles close the mandible, while the medial pterygoid aids in closing and lateral movements. The lateral pterygoid is unique in its ability to open the jaw, deviate it to either side, and facilitate anterior movement.
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What You'll Learn
The masseter muscle
The word "masseter" comes from the Greek "masasthai," meaning "to chew." The masseter muscle is well-conserved across the human population and the animal kingdom, although masseteric hypertrophy (enlargement of the masseter muscles) is characteristic in populations of Asian descent. This condition typically results in a square jawline shape. Masseter muscle rigidity, also known as "jaws of steel," can occur following a dose of succinylcholine, leading to jaw muscle tightness and potentially affecting intubation.
Singers often experience various kinds of masseter tension, which can be treated with transdermal massages or stretches as vocal warm-ups. In toothed whales, the masseter muscle has become redundant due to a shift from chewing to swallowing, and it now provides tissue for acoustic fat bodies used for echolocation.
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The temporalis muscle
The contraction of the temporalis muscle results in the backward movement of the mandible (retrusion). The anterior fibres of the muscle move the mandible dorsocranially (elevation), while the posterior fibres pull the mandible backward (retrusion). Together, these actions facilitate the closing of the mouth and the approximation of the teeth. The unilateral contraction of the temporalis muscle is also responsible for the side-to-side movements of the jaw.
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The medial pterygoid muscle
The primary function of the medial pterygoid muscle is to produce movements of the mandible at the temporomandibular joint. Unilateral contraction of the medial pterygoid causes a slight medial rotation of the mandible. When this occurs simultaneously with the contraction of the ipsilateral lateral pterygoid, the same-sided part of the mandible swings anteriorly and medially. Bilateral contraction of the medial pterygoid, in synergy with other masticatory muscles, elevates and protrudes the mandible, facilitating the act of chewing and closing the mouth.
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The lateral pterygoid muscle
The primary function of the lateral pterygoid muscle is to pull the head of the condyle out of the mandibular fossa along the articular eminence to protrude the mandible. When both bellies of the lateral pterygoid muscle contract bilaterally, the mandible travels anteriorly (protrusion). Unilateral action of the lateral pterygoid muscle causes contralateral excursion (a form of mastication), usually performed in concert with the medial pterygoids. When they work independently, they can move the mandible side to side.
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The four primary muscles work in pairs
The muscles of mastication are a group of muscles that enable the chewing movement of the mandible at the temporomandibular joint (TMJ). They enhance the process of eating by assisting in grinding food and approximating the teeth. The four primary muscles of mastication are the masseter, the temporalis, the medial pterygoid, and the lateral pterygoid. These muscles work in pairs: the masseter and medial pterygoid work together to elevate and protrude the mandible, while the temporalis and lateral pterygoid work together to depress and retract the mandible.
The masseter muscle is a strong, quadrangular muscle that covers the lateral aspect of the ramus of the mandible. It originates from the zygomatic arch and inserts along the angle and lateral surface of the mandible. The major function of the masseter muscle is to elevate the mandible, with a minor contribution to protrusion. The masseter muscle is supplied by the trigeminal nerve, specifically the mandibular branch (V3), and its blood supply comes from the masseteric artery, a branch of the maxillary artery.
The temporalis muscle originates from the floor of the temporal fossa and inserts onto the coronoid process of the mandible. Contraction of the temporalis muscle causes elevation and retraction of the mandible, with the anterior vertical fibres contributing to elevation and the posterior, more horizontal fibres producing retraction. The temporalis muscle is also supplied by the trigeminal nerve and assists in grinding movements by moving the mandible from side to side.
The medial pterygoid muscle has a quadrangular shape and originates from two heads: the medial surface of the lateral pterygoid plate and the tuberosity of the maxilla. It inserts along the medial surface of the mandibular ramus. Contraction of the medial pterygoid causes elevation and protrusion of the mandible, similar to the masseter muscle. The medial pterygoid is innervated by the mandibular nerve (V3).
The lateral pterygoid muscle has a triangular shape and originates from two heads: the infratemporal surface/crest of the great wing of the sphenoid and the lateral surface of the lateral pterygoid plate. It inserts at the joint capsule of the TMJ, the articular disc of the TMJ, and the condylar neck of the mandible. Contraction of the lateral pterygoid allows for the depression and protraction of the mandible, with unilateral contraction contributing to side-to-side movements during chewing. The lateral pterygoid is the only muscle that actively opposes the other muscles of mastication, allowing for the depression of the mandible.
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Frequently asked questions
The muscles of mastication are a group of muscles responsible for the chewing movement of the mandible at the temporomandibular (TMJ) joint. They enhance the process of eating and assist in grinding food.
The lateral pterygoid muscle is responsible for opening the mouth, as well as deviation to either side and anterior movement of the jaw.
The function of the lateral pterygoid depends on the degree of its contraction. Bilateral contraction of the lateral pterygoid muscles protrudes and depresses the mandible. A unilateral contraction on a particular side, in conjunction with the ipsilateral medial pterygoid muscle, moves the mandible to the opposite side.
The four primary muscles of mastication are the masseter, temporalis, medial pterygoid, and lateral pterygoid muscles.
