Maximus Gage
The movement of the jaw is made possible by the muscles of mastication, which are a group of four muscles: the masseter, temporalis, medial pterygoid, and lateral pterygoid. These muscles are responsible for the chewing movement of the mandible at the temporomandibular joint, enhancing the process of eating and grinding food. The masseter muscle, in particular, is a powerful muscle that plays a crucial role in elevating the mandible, causing the mouth to close, while the temporalis muscle contributes to side-to-side grinding movements.
| Characteristics | Values |
|---|---|
| Number of muscles that move the jaw | 4 |
| Names of the muscles | Masseter, temporalis, medial pterygoid, and lateral pterygoid |
| Muscle functions | Elevation, depression, protrusion, retraction, and side-to-side movement of the jaw |
| Muscle disorders | Masticatory myalgia, myositis, myospasm, and bruxism |
| Muscle shape | Quadrangular, triangular |
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What You'll Learn
The masseter muscle
The superficial part of the masseter muscle originates from the maxillary process of the zygomatic bone and the anterior two-thirds of the inferior border of the zygomatic arch. The deep portion of the masseter muscle originates from the entire surface of the zygomatic arch. The fibres 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 masseter muscle is covered by tough and very strong masseteric fascia.
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The temporalis muscle
The contraction of the anterior fibres of the temporalis muscle results in the elevation of the mandible, or the closing of the mouth. On the other hand, the contraction of the posterior fibres leads to the retrusion of the mandible, or its backward movement. Working in unison, these actions facilitate the closing of the mouth and the approximation of the teeth. Additionally, the unilateral contraction of the temporalis muscle contributes to the side-to-side movements of the jaw, enabling grinding and chewing motions.
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The medial pterygoid muscle
Unilateral contraction of the medial pterygoid muscle causes a slight medial rotation of the mandible. When this occurs simultaneously with the contraction of the ipsilateral lateral pterygoid, the same side of the mandible swings anteriorly and medially. Bilateral contraction of the muscle, on the other hand, elevates and protrudes the mandible. This movement is crucial for the act of chewing.
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The lateral pterygoid muscle
The primary function of the lateral pterygoid muscle is to control the movement of the mandible, particularly protrusion (forward movement) and depression (opening) of the jaw. When contracting bilaterally, the muscle protrudes and depresses the mandible, while unilateral contraction results in side-to-side movement of the jaw. This unilateral action involves coordination with the medial pterygoid muscles, contributing to a form of mastication. The lateral pterygoid muscle is the only muscle among the four muscles of mastication that assists in depressing the mandible, which distinguishes it from the other jaw-closing muscles.
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Conditions affecting jaw movement
The muscles of mastication are a group of four muscles responsible for the movement of the jaw, or mandible, at the temporomandibular joint (TMJ). These muscles are the masseter, temporalis, medial pterygoid, and lateral pterygoid. The mandible is the only bone that moves during mastication and other activities such as talking.
When the mandible and the joints are properly aligned, smooth muscle actions such as chewing, talking, yawning, and swallowing can take place. However, when these structures are not aligned, several problems may occur, including conditions affecting jaw movement.
One such condition is temporomandibular disorder (TMD), which is a problem affecting the chewing muscles and the joints between the lower jaw and the base of the skull. TMD is a disorder of the jaw muscles, temporomandibular joints, and the nerves associated with chronic facial pain. Degenerative joint diseases such as osteoarthritis or rheumatoid arthritis in the jaw joint can cause TMD. Excessive strain on the jaw joints and the muscle group that controls chewing, swallowing, and speech can also lead to TMD. This strain may be caused by bruxism, or the habitual, involuntary clenching or grinding of the teeth. Trauma to the jaw, head, or neck may also cause TMD.
Another condition that can affect jaw movement is myositis, an inflammatory condition of the muscles caused by acute trauma or infection. It is characterized by swelling, redness, and increased temperature over the affected area, resulting in jaw dysfunction and a limited range of movement.
Masticatory myalgia is another condition that can affect jaw movement. It is characterized by a dull, persistent ache overlying the jaw and temple muscles, with symptoms including a restricted opening, fatigue, and stiffness. Masticatory myospasm, or muscle cramp, is an acute condition resulting from a sudden, involuntary, and continuous tonic contraction of the muscle, which may lead to trismus (lockjaw), pain on movement, and a severely limited range of motion of the mandible.
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Frequently asked questions
The muscles that move the jaw are the masseter, temporalis, medial pterygoid, and lateral pterygoid.
The masseter muscle is responsible for the elevation of the mandible and some protraction of the mandible. It is the most powerful muscle of mastication.
The function of the lateral pterygoid muscle 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, along with the ipsilateral medial pterygoid muscle, moves the mandible to the opposite side.
