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The muscles of mastication, or the chewing muscles, are a group of four muscles that work in pairs and are primarily responsible for mandibular movement and TMJ function. The TMJ, or temporomandibular joint, is formed by the lower jaw (mandible), upper jaw (maxilla), and temporal bone. The masseter, temporalis, medial pterygoid, and lateral pterygoid muscles are the four primary muscles of mastication. The masseter muscle, which has three layers, is the major muscle responsible for jaw protrusion. The superficial fibres of the masseter and the lateral pterygoid muscles work together to protrude the mandible.
| Characteristics | Values |
|---|---|
| Muscle that protrudes the jaw | Masseter, Lateral pterygoid, Medial pterygoid |
| Type of muscle | Involuntary |
| Other functions | Elevation, depression, retraction, side-to-side movement, grinding of food, closing of the mouth |
| Origin of the masseter muscle | Inferior zygomatic arch, anterior two-thirds of the zygomatic arch, posterior aspect of the zygomatic bone |
| Blood supply | Masseteric artery, a branch of the maxillary artery |
| Nerve supply | Masseteric nerve, a branch of the mandibular nerve |
| Main bones of the masticatory system | Maxilla (upper jaw), Mandible (lower jaw), Temporal bone |
| Other bones | Hyoid, Sphenoid |
| Innervation | Motor fibres of the mandibular branch of the trigeminal nerve (CN V3) |
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What You'll Learn
The masseter muscle
The word "masseter" originates from the Greek word "μασασθαι" ("masasthai"), which means "to chew." The masseter muscle's positioning is a distinguishing feature of hystricognathous creatures, such as mole-rats, where it passes partially through the infraorbital foramen and connects to the bone on the opposite side. In toothed whales, the masseter muscle has become redundant due to a shift from chewing to swallowing, and it now provides the tissue for acoustic fat bodies used for echolocation.
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The temporalis muscle
The contraction of the posterior fibres of the temporalis muscle results in the backward movement of the mandible (retrusion). The contraction of its anterior fibres moves the mandible dorsocranially (elevation). In unison, these actions facilitate the closing of the mouth and the approximation of the teeth. The unilateral contraction of the temporalis muscle also plays an important role in the side-to-side movements of the jaw.
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The lateral pterygoid muscle
When the lateral pterygoid muscle contracts unilaterally, it rotates the mandible, and when it contracts bilaterally, it protrudes and depresses the mandible. Unlike the other three muscles of mastication, the lateral pterygoid is the only muscle that participates in depressing the mandible. The unilateral contraction of the lateral pterygoid muscle with the ipsilateral medial pterygoid muscle results in lateral mandibular movement to the contralateral side. This movement is observable during functional and parafunctional lateral excursive movements, such as during the chewing stroke, mastication, and clenching.
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The medial pterygoid muscle
The function of the medial pterygoid muscle includes elevating the mandible (closing the mouth), protruding the mandible, mastication (especially when the maxillary and mandibular teeth are close together), and excursing the mandible. Unilateral contraction of the medial pterygoid muscle causes a slight medial rotation of the mandible, while bilateral contraction elevates and protrudes it. These movements are crucial for the act of chewing.
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Accessory muscles of mastication
The muscles of mastication are a group of paired muscles that are responsible for the movement of the lower jaw at the temporomandibular joints (TMJs). They are involved in chewing food and speech, controlling the opening and closing of the mouth. They also play a role in the configuration of the face. The four main muscles of mastication are the masseter, temporalis, lateral pterygoid, and medial pterygoid muscles. These muscles attach to the rami of the mandible and function to move the jaw. The masseter muscle, for example, functions to elevate the mandible and approximate the teeth. The intermediate and deep muscle fibres of the masseter retract the mandible, while the superficial fibres protrude it.
The muscles of mastication can be subdivided into two groups: primary (main) and secondary (accessory) muscles. All primary masticatory muscles originate from the bones of the skull and insert into various features of the mandible. The primary muscles include the temporalis, masseter, and pterygoids (medial and lateral). The medial pterygoid, lateral pterygoid (inferior head), digastric, and temporalis muscles are responsible for protruding the mandible.
The accessory muscles of mastication support the primary muscles in jaw movement and mastication. They include the platysma muscle, a superficial muscle of the neck that assists with depressing the mandible against resistance. The strap muscles, composed of the suprahyoid and infrahyoid muscles, are also accessory muscles that function to raise and depress the hyoid bone and larynx.
Disorders of the muscles of mastication include bruxism, tetanus, and myofascial pain disorders. Bruxism is an involuntary, repetitive oral activity characterised by teeth clenching or grinding. It can occur during sleep, disrupting the sleep of a bedroom partner, or during wakefulness, arising from various psychological, neurological, and orodental conditions. Myofascial pain disorders are characterised by a dull ache with trigger points that produce referred pain, similar to other skeletal muscle disorders.
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Frequently asked questions
The masseter, lateral pterygoid, and medial pterygoid muscles all contribute to the protrusion of the mandible or jaw.
The masseter muscle is a rectangularly or quadrangularly shaped muscle with three layers: superficial, deep, and intermediate. It is responsible for elevating the mandible and approximating the teeth.
The function of the lateral pterygoid muscle depends on the degree of its contraction. When contracted bilaterally, it protrudes and depresses the mandible. When contracted unilaterally, it moves the mandible to the opposite side.
The medial pterygoid muscle is a quadrangular muscle that assists in the protrusion of the mandible when contracted unilaterally along with the lateral pterygoid muscle.
