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The muscles of mastication are a group of muscles that enable the movement of the jaw. There are four muscles of mastication: 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 assisting in grinding food. The masseter muscle is rectangular in shape and has three layers: superficial, deep, and intermediate. The temporalis muscle has anterior, mid, and posterior fibres, with the latter functioning to retract the mandible. The medial pterygoid muscle is thick and rectangular, with a superficial head and a deep head, while the lateral pterygoid muscle is triangular with superior and inferior heads. While the masseter, medial pterygoid, and lateral pterygoid muscles assist in jaw movement, the focus here is on the role of the temporalis muscle in retracting the jaw.
| Characteristics | Values |
|---|---|
| Muscles that retract the jaw | Temporalis, Masseter |
| Number of muscles of mastication | 4 |
| Names of muscles of mastication | Temporalis, Medial Pterygoid, Lateral Pterygoid, Masseter |
| Shape of Masseter muscle | Rectangular |
| Number of layers of Masseter muscle | 3 (Superficial, Deep, Intermediate) |
| Function of Superficial fibres of Masseter muscle | Protrude the mandible |
| Function of Intermediate and Deep fibres of Masseter muscle | Retract the mandible |
| Function of Lateral Pterygoid muscle | Protract the mandible, depress the chin |
| Function of Medial Pterygoid muscle | Assist with elevation and protrusion of the mandible |
| Function of Temporalis muscle | Elevate the mandible, retract the mandible, grind food |
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What You'll Learn
The temporalis muscle
The contraction of the posterior fibres of the temporalis muscle results in the backward movement of the mandible, or retrusion. The contraction of its anterior fibres moves the mandible dorsocranially, resulting in the elevation of the mandible and the closing of the mouth. In unison, these actions facilitate the closing of the mouth and the approximation of the teeth. The unilateral contraction of the temporalis muscle also contributes to the side-to-side movement of the jaw, assisting in the grinding of food.
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The masseter muscle
The word "masseter" originates from the Greek word "masasthai," which means "to chew." This is indicative of the muscle's function in the chewing or mastication process. The masseter muscle works in conjunction with the other muscles of mastication, including the temporalis, medial pterygoid, and lateral pterygoid muscles, to produce chewing movements of the mandible at the temporomandibular joint. These movements include elevation, depression, protrusion, retraction, and side-to-side grinding motions, all of which aid in grinding food and approximating the teeth.
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The medial pterygoid muscle
The superficial head of the medial pterygoid 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 deep head also arises from the pyramidal process of the palatine bone. The outer surface of the medial pterygoid muscle is related to the process of the parotid gland, while the inner surface is connected to the tensor veli palatini, styloglossus, and salpingopharyngeus muscles.
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The lateral pterygoid muscle
The primary function of the lateral pterygoid muscle is to depress the mandible and open the jaw. It is the only muscle among the four muscles of mastication that participates in this action. 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 chewing, masticating, and clenching.
When both bellies of the lateral pterygoid muscle contract bilaterally, the mandible moves forward (protrusion). The lateral pterygoid muscle also contributes to the rotation of the mandible when contracting unilaterally. The concerted effort of the lateral pterygoid muscles helps in lowering the mandible and opening the jaw. Additionally, the unilateral action of a lateral pterygoid muscle causes contralateral excursion, a form of mastication, usually performed in conjunction with the medial pterygoids.
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Jaw retraction and mastication
The muscles of mastication are a group of muscles that enable the chewing movement of the mandible at the temporomandibular (TMJ) joint. They enhance the process of eating by assisting in grinding food and bringing the teeth together. The four main muscles of mastication are the temporalis, medial pterygoid, lateral pterygoid, and masseter muscles. These muscles originate from the surface of the skull and attach onto the rami of the mandible at the TMJ.
The temporalis muscle is responsible for elevating the mandible, with its anterior vertical fibres opposing gravity when the mouth is closed. The contraction of the posterior fibres of the temporalis muscle results in the retraction of the mandible, pulling the jaw backward. Additionally, the temporalis muscle contributes to side-to-side grinding movements.
The masseter muscle, a strong quadrangular muscle, covers the lateral aspect of the ramus of the mandible. It is composed of three layers: superficial, deep, and intermediate. The intermediate and deep muscle fibres of the masseter function to retract the mandible, while the superficial fibres protrude the mandible.
The medial pterygoid muscle is a thick rectangular muscle with a superficial head and a deep head. The deep head is larger and originates from the medial surface of the lateral pterygoid plate of the sphenoid bone. The medial pterygoid muscle fibres converge to insert on the medial ramus of the mandible, forming a tendon along with the insertion of the masseter.
Mastication muscle pain disorders, such as masticatory myofascial pain and myalgia, can cause a dull, persistent ache in the jaw and temple muscles. Myospasm, or muscle cramp, can lead to acute conditions like lockjaw, severely limiting the range of motion of the mandible.
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Frequently asked questions
The temporalis muscle, specifically the posterior fibres, functions to retract the mandible.
The other muscles of mastication are the masseter, medial pterygoid, and lateral pterygoid.
The masseter muscle is responsible for elevating the mandible and approximating the teeth. The intermediate and deep muscle fibres of the masseter also function to retract the mandible.
