Logan Steele
The muscles of mastication are a group of muscles responsible for the movement of the jaw, including chewing. 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 mandible, allowing for various movements such as elevation, depression, protrusion, retraction, and side-to-side motion. This article will focus specifically on the muscles responsible for elevating the jaw, including the masseter, temporalis, and medial pterygoid muscles.
| Characteristics | Values |
|---|---|
| Muscles that elevate the jaw | Masseter, Temporalis, Medial Pterygoid, Lateral Pterygoid |
| Function of the Masseter Muscle | Elevate the mandible, approximate the teeth |
| Function of the Temporalis Muscle | Elevate the mandible, retract the mandible |
| Function of the Medial Pterygoid Muscle | Elevate the mandible, assist with protrusion of the mandible |
| Function of the Lateral Pterygoid Muscle | Major protractor of the mandible, depress the chin |
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What You'll Learn
Masseter muscle
The masseter muscle is one of the four muscles responsible for the action of mastication (chewing). It is a powerful, thick, and rectangular muscle with three layers: superficial, deep, and intermediate. The masseter muscle originates from the zygomatic arch and inserts along the angle and lateral surface of the mandibular ramus. The muscle fibres converge inferiorly, forming a tendon that inserts into the outer surface of the mandibular ramus and coronoid process of the mandible.
The primary function of the masseter muscle is to elevate the mandible, causing a powerful jaw closure. The contraction of the superior part, which runs diagonally to the front, moves the mandible forward (protrusion). The intermediate and deep muscle fibres of the masseter function to retract the mandible, while the superficial fibres protrude the mandible. The masseter muscle also helps to stabilise tension in the articular capsule of the temporomandibular joint.
The masseter muscle is supplied with blood by the masseteric artery, a branch of the maxillary artery. The maxillary artery divides into the mandibular, pterygoid, and pterygopalatine portions. The masseteric nerve, a branch of the mandibular division (V3) of the trigeminal nerve, innervates the masseter muscle. The trigeminal nerve carries both sensory and motor fibres.
The masseter muscle can become enlarged in patients who habitually clench or grind their teeth, a condition known as bruxism. This masseteric hypertrophy is typically asymptomatic and soft, and it can be unilateral or bilateral. Singers are also prone to experiencing masseter tension, which can be treated with transdermal massages or stretches as part of their vocal warm-up routine.
The word "masseter" comes from the Greek "masasthai," meaning "to chew." The masseter muscle's positioning is a distinguishing feature of hystricognathous creatures, such as mole-rats, and it serves a unique function in toothed whales, providing tissue for acoustic fat bodies used for echolocation.
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Temporalis muscle
The temporalis muscle, also known as the temporal muscle, is a fan-shaped muscle situated within the temporal fossa of the skull. It is one of the four primary muscles of mastication (chewing). The temporalis muscle is the strongest muscle of the temporomandibular joint and the primary retractor of the mandible. The muscle is involved in jaw pain and headaches. Bruxism, or the habitual grinding of teeth, especially while sleeping, and clenching of the jaw while stressed, can lead to overwork of the temporalis muscle, resulting in pain.
The temporalis muscle can be divided into 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 posterior portion has fibres that run horizontally, and their contraction results in the retrusion of the mandible. The middle portion, with fibres running in an oblique direction towards the inferior and anterior, is used for both elevation and retraction of the mandible. The unilateral contraction of the temporalis muscle is responsible for the side-to-side movement of the jaw.
The temporalis muscle receives its blood supply from the deep temporal branches of the maxillary artery and the middle temporal branches of the superficial temporal artery. It is supplied by the deep temporal nerves. The muscle is covered by the temporal fascia, also known as the temporal aponeurosis. This fascia is commonly used in tympanoplasty, or the surgical reconstruction of the eardrum.
The temporalis muscle is accessible on the temples and can be seen and felt contracting while the jaw is clenching and unclenching. The masseter and temporalis muscles are palpated on both sides. The temporalis muscle is palpated over the temple at the hairline, anterior to the ear and superior to the zygomatic bone.
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Medial pterygoid muscle
The medial pterygoid muscle is a thick, quadrilateral-shaped muscle that connects the mandible with the maxilla, sphenoid, and palatine bones. It is one of the four primary muscles of mastication (chewing food), along with the lateral pterygoid, masseter, and temporalis muscles. The medial pterygoid muscle has two heads: a superficial head and a deep head, with the deep head being larger than the superficial one.
The medial pterygoid muscle originates on the pterygoid process, which is a downward-pointing process extending from the sphenoid bone. 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 medial pterygoid muscle fibres 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 has several functions, including elevating the mandible (closing the mouth), protruding the mandible, mastication (especially when the maxillary and mandibular teeth are close together), and excursing the mandible. Unilateral contraction of the medial pterygoid muscle causes a slight medial rotation of the mandible, while bilateral contraction with the lateral pterygoid muscle results in the protrusion of the mandible and elevation of the jaw.
The medial pterygoid muscle is innervated by the medial pterygoid nerve, a branch of the mandibular nerve (CN V3), which is itself a branch of the trigeminal nerve (CN V). The muscle receives its vascular supply primarily from the pterygoid and buccal branches of the maxillary artery, with contributions from the ascending palatine artery and muscular branches of the facial artery.
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Lateral pterygoid muscle
The lateral pterygoid muscle is a craniomandibular muscle with a crucial role in the inferior temporal region. It 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 the only muscle of mastication that participates in depressing the mandible, which opens the jaw.
The lateral pterygoid muscle is a short, thick, fan-shaped muscle with two heads. The upper and lower heads originate from different parts of the sphenoid bone. The upper head arises from the infratemporal surface and infratemporal crest of the greater wing of the sphenoid bone. The lower head arises 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 temporomandibular joint.
The lateral pterygoid muscle is supplied by the lateral pterygoid nerve, a branch of the mandibular nerve (CN V3), itself a branch of the trigeminal nerve (CN V). 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 to protrude the mandible. Unilateral action of the lateral pterygoid muscle causes contralateral excursion, a form of mastication, and is usually performed in concert with the medial pterygoids. When they work independently, they can move the mandible side to side.
The lateral pterygoid muscle is located deep to the temporalis and masseter muscles, spanning between the sphenoid bone and temporomandibular joint. It is contained within the masticator space, which is enveloped by the deep cervical fascia. The masticator space also contains the temporalis tendon, masseter, and medial pterygoid muscles, and the pterygoid venous plexus. The lateral pterygoid muscle is particularly active during speaking, singing, and clenching.
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Trismus (muscle spasm)
Trismus, also known as lockjaw, is a condition that causes restricted mouth opening and is usually temporary, resolving within two weeks. However, permanent trismus can also occur, causing severe complications in everyday activities such as speaking, eating, and swallowing. It is often associated with muscle spasms in the masseter muscle, which is one of the four muscles responsible for elevating the jaw. These muscles are called the muscles of mastication and include the temporalis, medial pterygoid, lateral pterygoid, and masseter muscles. They are innervated by the motor branches of the mandibular division of the trigeminal nerve.
Trismus can be a symptom of a tumour or infection, with the most common cause being pericoronitis, an inflammation of the soft tissue around the third molar. Other causes include tonsillitis, meningitis, tetanus, and parotid abscess. Trismus may also occur as a complication of oral surgery, such as wisdom tooth extraction, or as a result of direct trauma to the temporomandibular joint (TMJ). In some cases, it can be a side effect of certain pharmacological agents, such as amphetamines.
The management of trismus involves treating the underlying condition and may require dental treatments, speech therapy, physical therapy, and passive range-of-motion devices. Pain medications, muscle relaxants, and warm compresses can be used to control symptoms. Physiotherapy treatments, including neck stretching, chin tucks, and massaging of the masticatory muscles, can help restore normal function. In certain cases, mandibular opening devices may be recommended.
Trismus can be diagnosed through a complete clinical examination, which includes checking for visible muscle fasciculation and performing radiographic investigations such as panoramic radiographs, CT scans, and MRI scans. A diagnosis of trismus is determined based on the clinical history, signs, and identification of any associated abnormalities.
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Frequently asked questions
There are four muscles that work together to elevate the jaw: the temporalis, masseter, medial pterygoid, and lateral pterygoid.
The anterior and mid fibres of the temporalis muscle function to elevate the mandible, while the posterior fibres retract the mandible.
The masseter muscle is a rectangularly shaped muscle with three layers: superficial, deep, and intermediate. The function of the masseter muscle is to elevate the mandible and approximate the teeth.
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