Victor Forge
The muscles of mastication are a group of muscles that enable the movement of the jaw. There are four primary muscles of mastication: the temporalis, masseter, medial pterygoid, and lateral pterygoid. These muscles originate from the surface of the skull and attach to the mandible, allowing for movements such as elevation, depression, protrusion, retraction, and side-to-side grinding. The focus of this discussion is on the muscles responsible for retracting the jaw, which include the posterior fibres of the temporalis muscle and the intermediate and deep muscle fibres of the masseter.
| Characteristics | Values |
|---|---|
| Number of muscles of mastication | 4 |
| Names of muscles of mastication | Temporalis, Masseter, Medial Pterygoid, Lateral Pterygoid |
| Masseter muscle shape | Rectangular |
| Masseter muscle layers | Superficial, Deep, Intermediate |
| Temporalis muscle shape | Fan-shaped |
| Temporalis muscle fibres | Anterior (vertical), Mid (oblique), Posterior (horizontal) |
| Medial Pterygoid muscle shape | Rectangular |
| Medial Pterygoid muscle heads | Superficial, Deep |
| Lateral Pterygoid muscle shape | Triangular |
| Lateral Pterygoid muscle heads | Superior, Inferior |
| Lateral Pterygoid muscle fibres | Horizontal |
| Functions of muscles of mastication | Elevation, Depression, Protrusion, Retraction, Side-to-side movement, Grinding, Approximation of teeth |
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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 primary muscles of mastication (chewing food). It is a powerful muscle of the temporomandibular joint and is the primary retractor of the mandible. The temporalis muscle is involved in jaw pain and headaches. Bruxism, or the habitual grinding of teeth, and clenching of the jaw while stressed can lead to overwork of the temporalis muscle and cause pain.
The temporalis muscle is covered by the temporal fascia, also known as the temporal aponeurosis. This muscle is accessible on the temples and can be seen and felt contracting while the jaw is clenched and unclenched. The muscle receives its blood supply from the deep temporal arteries, which are branches of the internal maxillary artery. The temporalis muscle is derived from the first pharyngeal arch in development.
The temporalis muscle can be divided into two functional parts: the anterior and posterior. The anterior portion runs vertically and its contraction results in the elevation of the mandible (closing of the mouth). The posterior portion has fibres that run horizontally, and the contraction of this portion results in the retrusion of the mandible. The middle portion, which contains oblique fibres, is used for both elevation and retraction of the mandible.
The unilateral contraction of the temporalis muscle is important for side-to-side movements of the jaw. The temporalis muscle is also used in reconstructive surgery of the mouth and face.
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The masseter muscle
The primary function of the masseter muscle is to elevate the mandible, causing a powerful jaw closure. The contraction of the superior part of the masseter muscle moves the mandible forward, a movement known as protrusion. In addition to this, the intermediate and deep muscle fibres of the masseter also function to retract the mandible. The masseter muscle is also involved in approximating the teeth and stabilising the tension of the articular capsule of the temporomandibular joint.
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Medial pterygoid muscle
The medial pterygoid muscle is a thick, rectangular or square-shaped muscle with two heads: a superficial head and a deep head. The deep head is larger than the superficial head. 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. The outer surface of the muscle lies against the inner surface of the mandible, from which it is separated by the lateral pterygoid muscle, sphenomandibular ligament, maxillary artery, mandibular nerve, and its lingual and inferior alveolar branches.
The medial pterygoid muscle is supplied by the medial pterygoid nerve, a branch of the mandibular nerve, which is itself a branch of the trigeminal nerve. The medial pterygoid nerve is a main trunk from the mandibular nerve, before the division of the trigeminal nerve. The medial pterygoid muscle receives blood supply from the pterygoid and buccal branches of the maxillary artery, as well as from the facial artery through its muscular branches.
The superficial head of the medial pterygoid has its origin 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 fibres of the medial pterygoid muscle converge inferiorly, forming a tendon that inserts on the medial ramus of the mandible posterior and inferior to the mylohyoid groove of the mandible. The medial pterygoid muscle functions to assist with elevation, protrusion, and rotation of the mandible, as well as mastication.
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Lateral pterygoid muscle
The lateral pterygoid muscle is a craniomandibular muscle that plays a crucial role in the inferior temporal region. It is one of the four muscles of mastication (chewing food), along with the medial pterygoid, temporalis, and masseter muscles. These muscles act upon the temporomandibular joint (TMJ) to enable chewing, biting, and mandibular movements.
The lateral pterygoid muscle is located deep to the temporalis and masseter muscles, spanning between the sphenoid bone and the TMJ. It is a two-headed, fan-shaped muscle, with the superior (upper) and inferior (lower) heads separated by a small horizontal fissure. The superior head is formed by the most superomedial fibres of the muscle, which originate from the infratemporal crest of the greater wing of the sphenoid bone. The inferior head, which is much wider than the superior one, originates from the lateral surface of the lateral pterygoid plate of the sphenoid bone.
The fibres from both heads converge to course posterolaterally in a predominantly horizontal plane. The superior fibres insert into the anteromedial part of the articular capsule and articular disc of the TMJ, while the inferior fibres insert into the pterygoid fovea on the neck of the condyloid process of the mandible. The superior attachment onto the TMJ enables the muscle head to produce gliding motions of the disc and mandibular condyle, while the inferior head acts on the inferior compartment of the TMJ, facilitating a hinge-like rotation between the mandibular condyle and the inferior surface of the articular disc.
The primary function of the lateral pterygoid muscle is to control the function of the jaw and TMJ. It protrudes and depresses the mandible when contracting bilaterally and rotates the mandible when contracting unilaterally. It is the only muscle of mastication that participates in depressing the mandible, which involves lowering the mandible and opening the jaw. Unilateral action of the lateral pterygoid muscle causes contralateral excursion (a form of mastication), usually performed in concert with the medial pterygoids. When they work independently, they can move the mandible side to side.
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Jaw movement and mastication
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 to the rami of the mandible at the TMJ. The masseter muscle, for example, originates from the zygomatic arch and the anterior two-thirds of the zygomatic bone, forming a tendon that inserts on the outer surface of the mandibular ramus.
The muscles of mastication enable various mandibular movements, including elevation, depression, protrusion, retraction, and side-to-side motion. The masseter muscle, for instance, functions to elevate the mandible and approximate the teeth. Meanwhile, the intermediate and deep muscle fibres of the masseter contribute to the retraction of the mandible.
The temporalis muscle also plays a crucial role in mandibular movements. Its anterior and mid-fibres elevate the mandible, while its posterior fibres retract it. Additionally, the temporalis muscle contributes to side-to-side grinding movements.
The medial pterygoid muscle, a thick rectangular muscle with a superficial head and a larger deep head, assists with the elevation and protrusion of the mandible. It originates from the medial surface of the lateral pterygoid plate of the sphenoid bone and the maxillary tuberosity of the inferior maxilla, respectively.
The lateral pterygoid muscle has a triangular shape with superior and inferior heads. Its bilateral contraction results in the protrusion and depression of the mandible, while unilateral contraction, along with the ipsilateral medial pterygoid muscle, moves the mandible to the opposite side, enabling side-to-side movements during chewing.
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Frequently asked questions
The muscles that retract the jaw are the temporalis, masseter, and medial pterygoid.
The temporalis muscle functions to elevate and retract the mandible, as well as contributing to side-to-side grinding movements.
The masseter muscle elevates the mandible, approximates the teeth, and also functions to retract the mandible.
The medial pterygoid muscle assists with the elevation and protrusion of the mandible.
