Dominic Stone
The movement of the jaw is made possible by the muscles of mastication. These muscles are involved in biting and chewing and originate from the surface of the skull, attaching onto the rami of the mandible at the temporomandibular joint. There are four main muscles of mastication: the masseter, the temporalis, the medial pterygoid, and the lateral pterygoid. These muscles work together to produce movements such as elevation, depression, protrusion, retraction, and side-to-side grinding of the mandible.
| Characteristics | Values |
|---|---|
| Number of muscles involved in jaw movement | 4 |
| Names of muscles | Masseter, temporalis, medial pterygoid, lateral pterygoid |
| Functions | Elevation, depression, protrusion, retraction, side-to-side movement |
| Conditions associated with the muscles | Masticatory myalgia, myositis, myospasm, bruxism, MMR or "jaws of steel" |
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What You'll Learn
The masseter muscle
The medial, or inner, surface of the masseter muscle is mostly related to the ramus of the mandible, almost completely covering its superficial surface. The medial aspect of the masseter muscle forms the lateral wall of the facial space, called the submasseteric space. The posterior aspect of the muscle is located anteriorly to the deep part of the parotid gland, which surrounds the entire posterior and most of the superficial aspects of the muscle.
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The temporalis muscle
The unilateral contraction of the temporalis muscle is important for the side-to-side movement of the jaw. The anterior fibres move the mandible dorsocranially, while the posterior fibres pull the mandible posteriorly. Together, these actions facilitate the closing of the mouth and the approximation of the teeth. The temporalis muscle is the strongest muscle of the temporomandibular joint.
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The medial pterygoid muscle
The superficial head of the medial pterygoid originates from the maxillary tuberosity of the inferior maxilla and the pyramidal process of the palatine bone. The deep head originates from the medial surface of the lateral pterygoid plate of the sphenoid bone and the pyramidal process of the palatine bone. Both heads attach to the ramus of the mandible near the angle of the mandible, inserting onto the triangular impression located on the medial surface of the mandible.
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The lateral pterygoid muscle
The two bellies of the lateral pterygoid muscle were traditionally believed to have opposing functions due to their different anterior insertions. Recent studies, however, suggest that they may function similarly. The inferior belly was thought to depress, protrude, and deviate the mandible to the contralateral side, while the superior belly was believed to be active during the elevation of the mandible to close the mouth. Newer research indicates that the superior belly may have little role in closing the mouth and may instead be active during mouth opening, protrusion, and contralateral deviation, similar to the inferior belly.
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Jaw-opening and closing
The movement of the jaw is made possible by the muscles of mastication. These are a group of muscles that originate from the surface of the skull and attach to the mandible at the temporomandibular joint (TMJ). There are four muscles of mastication: the masseter, temporalis, medial pterygoid, and lateral pterygoid. These muscles work together to produce the movements of the jaw, including opening and closing.
The masseter muscle is a powerful, quadrangular muscle that originates from the zygomatic arch and inserts along the angle and lateral surface of the mandible. It is responsible for the elevation of the mandible, causing the mouth to close. It also aids in the protrusion of the mandible, allowing for the anterior motion of the jaw. The masseter muscle is the most powerful muscle of mastication and is involved in biting and chewing.
The temporalis muscle is also involved in elevating the mandible. Its anterior vertical fibres are continuously opposing gravity when the mouth is closed. The contraction of the posterior, more horizontal fibres of the temporalis muscle produces a retraction of the mandible, pulling the jaw backward. Additionally, the temporalis muscle contributes to grinding movements by moving the mandible from side to side.
The medial pterygoid is a thick, rectangular muscle with a superficial head and a deep head. It originates on the pterygoid process of the sphenoid bone and inserts on the medial ramus of the mandible. The medial pterygoid is involved in the protrusion of the mandible and contributes to side-to-side grinding movements.
The lateral pterygoid is a triangular muscle with two heads: superior and inferior. It has horizontally oriented fibres that allow it to act as the major protractor of the mandible. Bilateral contraction of the lateral pterygoid depresses the mandible, opening the mouth. Unilateral contraction on one side, along with the ipsilateral medial pterygoid muscle, moves the mandible to the opposite side, enabling side-to-side movements during chewing.
The coordination of these four muscles allows for the complex movements of the jaw during chewing, grinding, and speaking. The jaw-closing muscles, including the masseter, temporalis, medial pterygoid, and superior head of the lateral pterygoid, are larger and more powerful than the jaw-opening muscles, as they perform most of the work during chewing.
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Frequently asked questions
The muscles that move the jaw are called the muscles of mastication. There are four of them: the masseter, temporalis, medial pterygoid, and lateral pterygoid.
The masseter muscle is a powerful, quadrangular muscle that covers the lateral aspect of the ramus of the mandible. It is one of the muscles of mastication and is responsible for the elevation of the mandible.
The temporalis muscle functions as an elevator and retractor of the mandible. It also contributes to side-to-side grinding movements.
The medial pterygoid is a thick, rectangular muscle with a deep and superficial head. It contributes to the protrusion of the mandible and is involved in side-to-side movements during chewing.
The function of the lateral pterygoid muscle depends on the degree of its contraction. Bilateral contraction depresses and protrudes the mandible, while unilateral contraction moves the mandible to the opposite side, enabling side-to-side movements during chewing.
