Logan Steele
Masticatory muscles are a group of muscles that enable functions such as chewing and grinding. They are also referred to as the muscles of mastication. The primary masticatory muscles are the masseter, temporalis, medial pterygoid, and lateral pterygoid. The masseter muscle is the most prominent and powerful masticatory muscle. It is composed of two parts with differently directed fibres. The temporalis muscle is the largest among the masticatory muscles and is responsible for producing biting force. The medial and lateral pterygoid muscles are situated on the inner side of the mandible and play a crucial role in enabling the full range of lower jaw motion.
| Characteristics | Values |
|---|---|
| Definition | Muscles involved in the chewing process |
| Number of muscles | 4 |
| Names of muscles | Temporalis, masseter, medial pterygoid, lateral pterygoid |
| Location of temporalis muscle | Temporal fossa |
| Location of masseter muscle | Cheek area |
| Location of medial and lateral pterygoids | Infratemporal fossa |
| Innervation | Motor fibres of the mandibular branch of the trigeminal nerve (CN V3) |
| Blood supply | Branches of the maxillary artery |
| Function | Enable movements of the lower jaw at the temporomandibular joint (TMJ) for functions such as chewing and grinding |
| Movements | Protrusion, retraction, elevation, depression, rotation |
| Masticatory muscle disorders | Myospasm, myofibrotic contracture, myositis, myofascial pain and dysfunction, neoplasms |
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What You'll Learn
The four main masticatory muscles
The temporalis muscle is the largest of the masticatory muscles and is situated in the temporal fossa. It functions as an elevator of the mandible, with its anterior vertical fibres opposing gravity to keep the mouth closed. The contraction of its posterior fibres pulls the mandible backwards. The temporalis also contributes to grinding movements by moving the mandible from side to side.
The masseter muscle is the most prominent and powerful muscle of mastication. It is quadrangular in shape and covers the lateral aspect of the ramus of the mandible. The masseter is composed of two parts: a superficial layer that arises from the maxillary process of the zygomatic bone and an anterior portion of the zygomatic arch; and a deeper layer formed by vertical fibres. The masseter's primary function is to elevate the mandible, with a minor contribution to protrusion, or forward movement, of the mandible.
The medial pterygoid muscle is situated in the infratemporal fossa and is composed of two heads with distinct origins. The larger, deep head arises from the medial surface of the lateral pterygoid plate of the sphenoid bone and the adjacent pyramidal process of the palatine bone. The smaller, superficial head originates from the tuberosity of the maxilla. The pterygoid muscles are crucial for enabling the full range of lower jaw motion and controlling the articular disc's position in the temporomandibular joint.
The lateral pterygoid muscle is also located in the infratemporal fossa. It is the only masticatory muscle with almost horizontally aligned fibres. Anatomically, it is divided into two heads: upper and lower. The upper head originates from the greater wing of the sphenoid, while the lower head originates from the lateral pterygoid plate of the sphenoid. The two heads converge into a tendon that attaches to the mandible.
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Masticatory muscle disorders
Masticatory muscles are a group of muscles involved in the chewing process. These include the temporalis, masseter, medial pterygoid, and lateral pterygoid muscles, with the temporalis muscle being the largest in volume. The masseter muscle is the most prominent masticatory muscle and is responsible for elevating the mandible, while the temporalis muscle functions as an elevator of the mandible, opposing gravity to keep the mouth closed. The pterygoid muscles are crucial for enabling the full range of lower jaw motion and controlling the articular disc's position in the temporomandibular joint (TMJ).
Myospasm (Masticatory Muscle Cramp)
Myospasm is characterised by the acute and involuntary contraction of a single muscle, often the masseter or temporalis muscle. It can lead to lockjaw, pain on movement, and a severely limited range of motion.
Myofibrotic Contracture
Myofibrotic contracture involves the painless shortening of a muscle due to fibrosis after an infection or trauma. It results in limited mouth opening and resistance to jaw muscle stretch.
Myositis
Myositis is an inflammatory condition caused by trauma or infection, leading to swelling, redness, and increased temperature in the affected area. It can result in jaw dysfunction and limited jaw movement.
Bruxism
Bruxism is an involuntary condition characterised by teeth clenching or grinding and can occur during sleep or wakefulness. It exerts powerful forces on the teeth, TMJ, and masticatory muscles, causing tooth wear, joint pain, and headaches.
Masticatory Myalgia
Masticatory myalgia is a persistent dull ache affecting the jaw and temple muscles, sometimes radiating to the head, neck, ear, and teeth. It can also cause restricted opening, fatigue, stiffness, and tenderness of the muscles.
Myofascial Pain (MFP)
MFP is characterised by the presence of trigger points within taut bands of muscle that refer pain to other areas. It can affect both the masticatory and cervical muscles.
Treatment for masticatory muscle disorders typically involves addressing the underlying cause, managing pain with analgesics, and restoring normal jaw function through physiotherapy.
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The role of the temporalis muscle
The masticatory muscles are a group of muscles that enable functions such as chewing and grinding. These include the temporalis, masseter, medial pterygoid, and lateral pterygoid muscles. The temporalis muscle is the largest among the masticatory muscles in terms of volume. It is a fan-shaped muscle that fills the temporal fossa, covering much of the temporal bone. The muscle originates from the temporal fossa and the deep part of the temporal fascia, and inserts on the coronoid process of the mandible.
The temporalis muscle has two functional parts: the anterior and the posterior. The anterior portion runs vertically and its contraction results in the elevation of the mandible (closing the mouth). The contraction of the posterior portion, which has more horizontal fibres, results in retrusion of the mandible. The middle portion, which has oblique fibres, is used for both elevation and retraction of the mandible. A unilateral contraction of the middle portion provokes a lateral movement of the mandible.
The temporalis muscle is involved in multiple actions, including elevating and retracting the mandible at the temporomandibular joint. It is also likely to be 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 result in pain. A myotendinous rupture of the muscle can occur during a seizure due to extreme clenching of the jaw.
The temporalis muscle has been used in the reconstruction of the paralyzed face and in defects involving the skull base, orbit, midface, and oropharynx. Inserting the muscle into the orbicularis oris allows for trigeminal-controlled lower face movement. The muscle is also used in skull base reconstruction and orbital cavity obliteration.
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The masseter muscle
The masseter is unique in that it has two distinct parts with different functions. The superficial part, closer to the skin, originates from the zygomatic arch and inserts on the angle of the mandible. This part is responsible for the elevation of the jaw, which is crucial for biting and chewing. On the other hand, the deep part of the masseter originates from the anterior half of the zygomatic arch and inserts on the mandibular notch, a groove just below the angle of the jaw. This part plays a role in depressing the jaw and is active during movements like opening the mouth widely.
Like other muscles, the masseter has a rich blood supply and nerve innervation. Several arteries provide blood to the muscle, including branches of the maxillary artery and deep temporal arteries. The nerve supply to the masseter is primarily through the masseteric nerve, a branch of the mandibular nerve (CN V3), which is responsible for motor function. Additionally, sensory innervation is provided by the buccal nerve, a branch of the trigeminal nerve (CN V).
In addition to its role in mastication, the masseter muscle also contributes to facial expression and maintaining jaw stability. Contraction of the masseter helps to stabilize the jaw during speech and other activities that involve the mouth. Moreover, the muscle's bulk and position at the side of the face contribute to the contour of the cheek, influencing facial aesthetics.
Finally, the masseter muscle is susceptible to various conditions, such as bruxism (teeth grinding) and masseteric hypertrophy (enlargement of the muscle). Bruxism can lead to pain and stiffness in the masseter muscle, as well as tooth wear and damage. Masseteric hypertrophy, often associated with bruxism, can result in a square-shaped jaw and a prominent masseter muscle that may be undesirable aesthetically. Treatment options for these conditions include botulinum toxin injections to relax the muscle, occlusal appliances, and in some cases, surgical procedures.
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The pterygoid muscles
The lateral pterygoid is a short, two-headed muscle, located in the infratemporal fossa of the skull. The smaller superior head arises from the infratemporal crest of the greater wing of the sphenoid bone, while the larger inferior head arises from the lateral surface of the lateral pterygoid plate. The fibres of both muscle heads merge and insert on the pterygoid fovea on the neck of the mandible. The lateral pterygoid is supplied by the pterygoid branches of the maxillary artery and 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). The primary function of the lateral pterygoid muscle is to protrude and depress the mandible when contracting bilaterally, and to rotate the mandible when contracting unilaterally.
The medial pterygoid muscle is a thick quadrilateral muscle that arises by two heads, a superficial and a deep head. The larger deep head originates from the medial surface of the lateral pterygoid plate of the sphenoid bone and the adjacent pyramidal process of the palatine bone. The smaller superficial head originates from the tuberosity of the maxilla. The bilateral contraction of this muscle elevates the mandible and closes the mouth. Because of the directions of its muscle fibres, this muscle also pulls the mandible forward and produces the movement known as protrusion. The medial pterygoid muscle is innervated by the medial pterygoid branch of the mandibular division of the trigeminal nerve (CN V3) and receives its blood supply from the pterygoid branches of the maxillary artery.
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Frequently asked questions
The masticatory muscles are a group of four muscles that enable functions such as chewing and grinding. These are the temporalis, masseter, medial pterygoid, and lateral pterygoid muscles.
The masticatory muscles attach to the mandible, allowing for movements of the lower jaw at the temporomandibular joint (TMJ). These movements include protrusion (protraction), retraction, elevation, depression, and rotation.
The masseter muscle is the most prominent and powerful masticatory muscle. Its major function is to elevate the mandible, with a minor contribution to protrusion.
