Dominic Stone
The muscles that open the jaw are known as the muscles of mastication, which are associated with movements of the jaw (temporomandibular joint). The lateral pterygoid muscle, which has a triangular shape with two heads, is the major protractor of the mandible and is responsible for opening the jaw, in addition to deviation to either side, and anterior movement of the jaw. The jaw-opening movement is assisted by sets of paired muscles, including the digastric, geniohyoid, mylohyoid, omohyoid, and stylohyoid muscles, which coordinate the full integration of mandibular movement during opening and closing.
| Characteristics | Values |
|---|---|
| Muscles | Lateral pterygoid, anterior digastric, mylohyoid, geniohyoid, omohyoid, sternohyoid, sternothyroid, stylohyoid, thyrohyoid, medial pterygoid, masseter, temporalis |
| Shape | Lateral pterygoid is triangular, medial pterygoid is quadrangular |
| Parts/Heads | Lateral pterygoid has superior and inferior heads, medial pterygoid has deep and superficial heads |
| Masseter Parts | Deep and superficial |
| Masseter Shape | Quadrangular |
| Masseter Function | Elevate the mandible, minor contribution to protrusion of the mandible |
| Temporalis Function | Elevator of the mandible, moves the jaw backward, moves the mandible from side to side |
| Pterygoid Function | Deviation of the jaw to the opposite side |
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What You'll Learn
The lateral pterygoid muscle
The lateral pterygoid is a two-headed, fan-shaped or triangular muscle located laterally in the infratemporal fossa of the skull. It comprises the medial wall of the infratemporal fossa and is contained within the masticator space. The muscle is located deep to the temporalis and masseter muscles, spanning between the sphenoid bone and temporomandibular joint. Its muscle belly is separated by a small horizontal fissure into two heads: superior (upper) and inferior (lower). The superior head is formed by the most superomedial fibers of the muscle, originating from the greater wing of the sphenoid. The inferior head, which is much wider than the superior head, originates from the lateral pterygoid plate of the sphenoid bone.
Fibers from both heads of the lateral pterygoid converge to course posterolaterally in a predominantly horizontal plane. The superior fibers insert into the anteromedial part of the articular capsule and articular disc of the temporomandibular joint. The inferior fibers insert into the pterygoid fovea on the neck of the condyloid process of the mandible. The superior attachment onto the temporomandibular joint enables the muscle head to act on the superior compartment of the joint, producing gliding motions of the disc and mandibular condyle. The inferior head acts on the inferior compartment of the TMJ, facilitating the hinge-like rotation that occurs between the mandibular condyle and the inferior surface of the articular disc.
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The digastric muscle
The digastric is a small paired muscle located in the anterior compartment of the neck. It is part of a group of muscles called the suprahyoid muscles, which also includes the mylohyoid, geniohyoid, and stylohyoid muscles. The name "suprahyoid" implies that these muscles are found superior to the hyoid bone, and they form the floor of the mouth along with the adjacent tissue. The digastric muscle is derived from the Greek word "dis," meaning double or twofold, and the Latin "gaster," meaning belly, which accurately describes the composition of this muscle as having two muscle bellies.
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The mylohyoid muscle
Variations in the mylohyoid muscle are uncommon, but accessory mylohyoid muscles have been observed in some individuals. The muscle may be divided into anterior and posterior portions, with the sublingual gland occupying the space between them. In some cases, the mylohyoid raphe, a median tendon where the two muscles meet, may be absent. When this occurs, the mylohyoid muscle fibres blend with the anterior belly of the digastric muscle, or the left and right mylohyoid muscles fuse together.
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The geniohyoid muscle
The main function of the geniohyoid muscle is to elevate the hyoid bone and draw it anteriorly, which has the consequence of moving the attached larynx and pharynx anterosuperiorly. This action assists in deglutition, or swallowing, by widening the pharynx to receive food. It also plays a role in voice production by the vocal cords. Additionally, if the hyoid bone is fixed by other muscles, the geniohyoid muscle assists in opening the mouth by depressing the mandible and pulling it inwards.
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The masseter muscle
The superficial portion of the masseter muscle originates from the temporal process of the zygomatic bone and arch, with the anterior two-thirds of the inferior border of the latter. The deep portion of the muscle originates from the entire surface of the zygomatic arch, with its fibres running inferiorly and inserting along the mandibular ramus superior to the masseter muscle's superior portion. The masseter muscle is covered by the parotid gland, which surrounds its posterior and most of its superficial aspect.
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Frequently asked questions
The muscles that open the jaw are the anterior digastric, mylohyoid, and inferior head of the lateral pterygoid.
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 one side, along with the ipsilateral medial pterygoid muscle, moves the mandible to the opposite side.
Accessory muscles of mastication are directly associated with mandibular function. These include the digastric, geniohyoid, mylohyoid, omohyoid, sternohyoid, sternothyroid, stylohyoid, and thyrohyoid muscles.
