Logan Steele
The muscles of mastication are associated with movements of the jaw, allowing it to open and close. In humans, the mandible rotates forward during opening, and the temporomandibular joint allows for lateral movements. The muscles that open the jaw include the anterior digastric, mylohyoid, and inferior head of the lateral pterygoid. The lateral pterygoid muscle is the only muscle that actively opposes the other muscles of mastication, allowing for depression of the mandible. The masseter muscle is another muscle of mastication, and it is responsible for the elevation of the mandible.
| Characteristics | Values |
|---|---|
| Muscles that open the jaw | Anterior digastric, mylohyoid, inferior head of the lateral pterygoid, and cranial belly of the digastricus muscle |
| Muscles that close the jaw | Masseter, temporalis, medial pterygoid, superior head of the lateral pterygoid |
| Masseter muscle | Powerful, superficial quadrangular muscle with two divisions: superficial and deep |
| Temporalis muscle | Functions as an elevator of the mandible, opposing gravity when the mouth is closed |
| Pterygoid muscles | Examined by having the patient move the jaw from the side against resistance and protrude the jaw |
| Lateral pterygoid muscle | Triangular shape with two heads: superior and inferior |
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What You'll Learn
- The anterior digastric, mylohyoid, and inferior head of the lateral pterygoid are jaw-opening muscles
- The masseter is a powerful muscle that aids in jaw opening and closing
- The medial pterygoid is involved in jaw protrusion and elevation
- The temporalis muscle elevates and retracts the mandible
- The lateral pterygoid is the only muscle that actively opposes the other muscles of mastication
The anterior digastric, mylohyoid, and inferior head of the lateral pterygoid are jaw-opening muscles
The mandible rotates forward during jaw opening, and the temporomandibular joint allows lateral movements. The muscles that open the jaw are the anterior digastric, mylohyoid, and inferior head of the lateral pterygoid. These muscles depress the mandible, causing the jaw to open.
The lateral pterygoid muscle is a craniomandibular muscle that plays a crucial role in the inferior temporal region. It is active during mastication and mandibular movements, including protrusion (forward movement of the mandible), abduction (depression of the mandible), and mediotrusion (mandibular condyle movement towards the midline). The lateral pterygoid muscle has two heads or bellies: the inferior and the superior. The inferior belly is three times larger than the superior belly. The inferior belly may have little role in closing the mouth, but it is active during mouth opening, protrusion, and contralateral deviation, just like the superior belly.
The jaw-opening muscles work in conjunction with the jaw-closing muscles, which include the masseter, temporalis, medial pterygoid, and superior head of the lateral pterygoid. The jaw-closing muscles do most of the work during chewing, and they are larger with more motor units. Unlike the jaw-opening muscles, the jaw-closing motor neurons are centrally inhibited during the jaw-opening phase of mastication. This lack of inhibition on the jaw-opener motor neurons allows a protective jaw-opening reflex to be initiated by high-threshold nociceptors. For example, if the sharp teeth slide off a food bolus and make contact with the soft tissues of the mouth, a protective jaw-opening reflex is initiated.
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The masseter is a powerful muscle that aids in jaw opening and closing
The masseter muscle's main function is to elevate the mandible, with a minor contribution to the protrusion of the mandible. When the masseter contracts, it causes the elevation of the mandible, resulting in the closing of the mouth. This elevation occurs during the closing of the jaws, and the muscle's superficial fibres can cause protrusion. The masseter muscle is also involved in some protraction of the mandible, allowing for the anterior motion of the jaw.
The masseter muscle is relatively well-conserved across the human population and the animal kingdom. However, masseteric hypertrophy, or enlargement of the masseter muscles, is characteristic in populations of Asian descent. This condition is frequent targets for aesthetic treatments and Botox requests, as it can result in a square jawline shape. Singers often experience masseter tension, which can be treated with transdermal massages or stretches as vocal warm-ups.
The word "masseter" comes from the Greek word "masasthai," which means "to chew." The muscle's positioning is a distinguishing feature of hystricognathous creatures such as mole rats, where it 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, but it provides tissue for acoustic fat bodies used for echolocation.
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The medial pterygoid is involved in jaw protrusion and elevation
The jaw is opened by muscles that depress the mandible, including the anterior digastric, mylohyoid, and inferior head of the lateral pterygoid. The medial pterygoid muscle, a major elevator of the jaw, is involved in jaw protrusion and elevation. This muscle is located on the medial aspect of the lower jaw bilaterally and is also known as the internal pterygoid muscle. It lies deep to the masseter and temporalis muscles and medial to the lateral pterygoid muscle.
The medial pterygoid muscle has two heads: the superficial head and the deep head. The deep head is larger than the superficial head. The muscle fibres converge inferiorly, forming a tendon that inserts into the medial ramus of the mandible. The outer surface of the medial pterygoid muscle is related to the parotid gland, while the inner surface is connected to the tensor veli palatini, styloglossus, and salpingopharyngeus muscles.
The medial pterygoid muscle has three main functions. Firstly, bilateral contraction of the medial pterygoid muscle with the lateral pterygoid muscle results in protrusion of the mandible. Secondly, unilateral contraction of the medial pterygoid muscle with the lateral pterygoid muscle on the same side results in lateral movement of the mandible towards the opposite side. Lastly, the medial pterygoid muscle functions together with the masseter and temporalis muscles to elevate the mandible.
The medial pterygoid muscle is involved in chewing and food grinding between the maxillary and mandibular teeth. It can be injured during an inferior alveolar nerve block due to its proximity to the nerve, leading to potential complications such as haemorrhage and trismus.
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The temporalis muscle elevates and retracts the mandible
The temporalis muscle is a fan-shaped muscle situated within the temporal fossa of the skull. It is the strongest muscle of the temporomandibular joint and is the primary retractor of the mandible. The temporalis muscle is a masticatory muscle that facilitates the movements of the mandible. Its anterior portion moves the mandible dorsocranially (elevation), while its posterior fibres pull the mandible posteriorly (retrusion).
The temporalis muscle is divided into anterior and posterior parts. The anterior fibres run inferiorly in a vertical direction, while the posterior fibres are directed horizontally. The contraction of the anterior fibres moves the mandible upwards, facilitating the closing of the mouth. The contraction of the posterior fibres results in the backward movement of the mandible. In unison, these actions facilitate the closing of the mouth and the approximation of the teeth.
The temporalis muscle also contributes to grinding movements by moving the mandible from side to side. The unilateral contraction of the muscle plays an important role in the side-to-side movements of the jaw. The masseter muscle is another muscle that contributes to the elevation of the mandible. It is a strong, quadrangular muscle that covers the lateral aspect of the ramus of the mandible.
The medial pterygoid muscle also assists with the elevation of the mandible. It is a thick, rectangular muscle with a superficial head and a deep head. The medial pterygoid muscle fibres converge inferiorly, forming a tendon that inserts on the medial ramus of the mandible. The lateral pterygoid muscle is the primary muscle responsible for causing depression of the mandible, which opens the jaw.
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The lateral pterygoid is the only muscle that actively opposes the other muscles of mastication
The muscles of mastication are a group of muscles that enable functions such as chewing and grinding by producing movements of the lower jaw at the temporomandibular joint. These muscles include the temporalis, masseter, medial pterygoid, and lateral pterygoid. The temporalis and masseter muscles function to elevate the mandible and close the mouth, while the medial pterygoid assists with elevation and protrusion of the mandible.
The lateral pterygoid is unique among these muscles as it is the only muscle that actively opposes the other muscles of mastication. It is the major protractor of the mandible, responsible for depressing and protruding the mandible, as well as assisting with side-to-side movements. This muscle has a triangular shape with two heads: the superior and inferior. The inferior head of the lateral pterygoid is one of the muscles that depress the mandible and thus open the jaw, along with the anterior digastric and mylohyoid muscles.
The function of the lateral pterygoid depends on the degree of its contraction. Bilateral contraction of this muscle depresses and protrudes the mandible, while unilateral contraction on one side, along with the ipsilateral medial pterygoid muscle, moves the mandible to the opposite side. This allows for the side-to-side movements necessary for chewing. The lateral pterygoid is located on the medial side of the mandible, along with the cranial belly of the digastricus muscle, which is the only muscle devoted to opening the jaw.
The jaw-opening phase of mastication serves a protective function. Unlike the jaw-closing motor neurons that are centrally inhibited during this phase, the jaw-opening motor neurons are not subject to inhibition during jaw closing. This allows for a quick, protective jaw-opening reflex to be initiated by high-threshold nociceptors if the teeth make contact with the soft tissues of the mouth, for example.
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Frequently asked questions
The muscles that open the jaw include the anterior digastric, mylohyoid, and inferior head of the lateral pterygoid.
The lateral pterygoid muscle is the only muscle that actively opposes the other muscles of mastication, allowing for depression of the mandible. It also allows for the protraction of the mandible and the anterior motion of the temporomandibular joint, allowing the mouth to open.
The masseter muscle is one of the four muscles responsible for the action of mastication (chewing). It is a powerful, superficial quadrangular muscle that covers the lateral aspect of the ramus of the mandible. Its major function is to elevate the mandible, with a minor contribution to protrusion of the mandible.
