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The mandible is the only bone in the body that moves during mastication (chewing) and other activities such as talking. The muscles of mastication are a group of muscles that are responsible for the movement of the jaw (mandible). There are four main muscles of mastication: the masseter, temporalis, medial pterygoid, and lateral pterygoid. These muscles originate from the surface of the skull and attach onto the rami of the mandible at the temporomandibular joint. They perform movements such as elevation, depression, protrusion, retraction, and side-to-side movement.
| Characteristics | Values |
|---|---|
| Number of muscles | 4 |
| Names of muscles | Masseter, temporalis, medial pterygoid, lateral pterygoid |
| Functions | Elevation, depression, protrusion, retraction, side-to-side movement |
| Innervation | Mandibular nerve (V3), branches of the trigeminal nerve |
| Blood supply | Masseteric artery, branches of the maxillary artery |
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What You'll Learn
The role of the temporalis muscle
The temporalis muscle is a fan-shaped muscle that is one of the four muscles of mastication (the other three being the medial pterygoid, lateral pterygoid, and masseter muscles). It is a powerful muscle of the temporomandibular joint and is the primary retractor of the mandible. The muscle is involved in the movement of the jaw, specifically in the elevation and retraction of the mandible.
The temporalis muscle is a thin, broad muscle that occupies most of the temporal fossa, a shallow depression on the lateral aspect of the skull. The origin of the muscle spans from the temporal fossa to the inferior temporal line of the lateral skull. The muscle fibres converge inferiorly, forming a tendon that exits the temporal fossa, passing underneath the zygomatic arch and inserting on the coronoid process of the mandible. The anterior fibres of the muscle run vertically, while the posterior fibres run horizontally.
The function of the anterior fibres of the temporalis muscle is to elevate the mandible, resulting in the closing of the mouth. The contraction of the posterior fibres of the muscle results in the backward movement of the mandible, or retrusion. The unilateral contraction of the temporalis muscle also plays an important role in the side-to-side movements of the jaw.
The temporalis muscle is innervated by the third (mandibular) branch of the trigeminal nerve, specifically the deep temporal nerves. The muscle receives its blood supply from the deep temporal arteries, which anastomose with the middle temporal artery. The temporalis muscle is also used in reconstructive surgery of the mouth and face, particularly in the reconstruction of the paralyzed face and skull base defects.
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The role of the masseter muscle
The masseter muscle is one of the four muscles of mastication (the other three being the temporalis, medial pterygoid, and lateral pterygoid muscles). It is a large, rectangular muscle with three layers (superficial, deep, and intermediate) and is found on both sides of the face, extending from the cheekbone down to the angle of the mandible. The masseter muscle is involved in mastication, or the process of chewing food. Its specific functions are elevation and protrusion of the mandible, as well as providing support to the articular capsule of the temporomandibular joint. The superficial part of the masseter functions to move the mandible forward, bringing the lower front teeth in front of the upper front teeth. This allows for the mouth to close. The deep part of the masseter muscle allows the mandible or lower jaw to elevate, helping with mandibular movements.
The origin of the masseter muscle is the inferior zygomatic arch and the anterior two-thirds of the zygomatic arch, with a connection to the posterior aspect of the zygomatic bone. The masseter muscle is innervated by the anterior division of the mandibular division (V3) of the trigeminal nerve. It is believed that the function of the masseter muscle is to retract the mandible and stabilize the mandibular coronoid process. The masseter muscle can become enlarged in patients who habitually clench or grind their teeth, or even in those who constantly chew gum. This condition is called masseteric hypertrophy and can lead to asymmetry of the face due to unequal enlargement of the muscles.
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The role of the medial pterygoid muscle
The medial pterygoid muscle is one of the four muscles of mastication, along with the lateral pterygoid, masseter, and temporalis muscles. These muscles are responsible for the movement of the mandible, or jaw, at the temporomandibular joint, enhancing the process of eating and assisting in grinding food. The medial pterygoid muscle specifically functions to elevate the mandible, closing the mouth, and protrude the mandible, which is crucial for mastication, especially when the maxillary and mandibular teeth are close together.
The medial pterygoid muscle is a thick, quadrilateral muscle with two heads: a superficial head and a deep head. The deep head is larger than the superficial head. The superficial head originates from the maxillary tuberosity of the inferior maxilla, while the deep head originates from the medial surface of the lateral pterygoid plate of the sphenoid bone. The muscle fibres run posteroinferiorly and laterally, surrounding the lower fibres of the lateral pterygoid muscle. The two heads of the medial pterygoid muscle converge and insert onto the triangular impression located on the medial surface of the mandible, forming an axis for a strong pull of this bone.
The 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 muscle, it results in a noticeable movement where the same-sided part of the mandible swings anteriorly and medially. On the other hand, bilateral contraction of the medial pterygoid muscle elevates and protrudes the mandible. These movements are essential for the act of chewing.
The medial pterygoid muscle is innervated by the medial pterygoid branch of the mandibular nerve (CN V3), which is a branch of the trigeminal nerve (CN V). It receives its blood supply primarily from the pterygoid and buccal branches of the maxillary artery. Additionally, the ascending palatine artery and muscular branches of the facial artery also contribute to a lesser extent. The medial pterygoid muscle is located in the infratemporal fossa, lying deep to the masseter and temporalis muscles and medial to the lateral pterygoid muscle.
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The role of the lateral pterygoid muscle
The lateral pterygoid muscle is one of the four muscles of mastication, along with the medial pterygoid, temporalis, and masseter muscles. These muscles act on the temporomandibular joint (TMJ) to enable chewing and biting. The lateral pterygoid muscle is responsible for the depression, protrusion, and side-to-side movement of the mandible.
The lateral pterygoid muscle is a two-headed, fan-shaped muscle located in the infratemporal fossa of the skull. The superior head originates from the greater wing of the sphenoid bone, while the inferior head originates from the lateral surface of the lateral pterygoid plate. The fibres from both heads converge to form a tendon that attaches to the neck of the mandible.
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, causing protrusion of the mandible. When both lateral pterygoid muscles work together, they help in lowering the mandible and opening the jaw. Additionally, the unilateral action of a single lateral pterygoid muscle causes the mandible to move side to side. This movement is crucial for the act of chewing.
The lateral pterygoid muscle is innervated by the lateral pterygoid nerve, a branch of the mandibular nerve (CN V3), which is itself a branch of the trigeminal nerve (CN V). It receives its blood supply from the pterygoid branches of the maxillary artery and the ascending palatine branch of the facial artery.
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Jaw-openers vs jaw-closers
The muscles of mastication are a group of muscles that are responsible for the movement of the jaw (mandible). They are involved in biting, chewing, and talking. These muscles can be divided into two groups: jaw-openers (mandible depressors) and jaw-closers (mandible elevators). However, natural masticatory movements are rarely so simple. In humans, the mandible rotates forward during jaw opening, and the temporomandibular joint allows for lateral movements as well.
Jaw-openers
The lateral pterygoid muscle is the sole muscle of mastication that causes depression of the mandible. It assists with protrusion and side-to-side movement of the mandible. The function of the lateral pterygoid depends on the degree of its contraction. Bilateral contraction of the lateral pterygoid muscles depresses and protrudes the mandible. A unilateral contraction on a particular side, in conjunction with the ipsilateral medial pterygoid muscle, moves the mandible to the opposite side. This allows for alternating side-to-side movements during chewing. The lateral pterygoid muscle has a triangular shape with two heads: superior and inferior.
The anterior digastric, mylohyoid, and geniohyoid muscles also depress the mandible against resistance when the infrahyoid muscles fix or depress the hyoid bone.
Jaw-closers
The masseter, temporalis, medial pterygoid, and superior head of the lateral pterygoid are the muscles that elevate the mandible. The masseter muscle is a strong, quadrangular muscle that covers the lateral aspect of the ramus of the mandible. The temporalis muscle is fan-shaped, with anterior fibres that have a vertical orientation, mid fibres with an oblique orientation, and posterior fibres with a horizontal orientation. The medial pterygoid muscle is a thick rectangular muscle with a superficial head and a deep head.
Because the jaw-closing muscles do most of the work during the closing powerstroke of chewing, these muscles are larger with more motor units.
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Frequently asked questions
The muscles that move the mandible are the masseter, temporalis, medial pterygoid, and lateral pterygoid.
The masseter muscle is a powerful elevator of the mandible. It also has a minor role in protrusion of the mandible.
The temporalis muscle is also a powerful elevator of the mandible. Its posterior fibres retract the mandible, and it also contributes to side-to-side grinding movements.
The medial pterygoid is involved in protrusion of the mandible. The lateral pterygoid is the sole muscle that causes depression of the mandible, and it also assists with protrusion and side-to-side movement.
