Dominic Stone
The masseter is the primary muscle responsible for closing the jaw. It is one of four muscles of mastication, along with the temporalis, medial pterygoid, and lateral pterygoid muscles. The masseter is a powerful, quadrangular-shaped muscle that originates from the zygomatic arch and inserts along the angle and lateral surface of the mandibular ramus. It is a well-conserved muscle across the human population and animal kingdom, with the most noticeable difference being masseteric hypertrophy, which is characteristic of populations of Asian descent.
| Characteristics | Values |
|---|---|
| Name of the muscle that closes the jaw | Masseter muscle |
| Number of muscles responsible for the action of mastication (chewing) | Four |
| Other muscles responsible for the action of mastication | Temporalis, medial pterygoid, lateral pterygoid |
| Masseter muscle shape | Quadrangular |
| Masseter muscle layers | Superficial, intermediate, deep |
| Masseter muscle origin | Zygomatic arch |
| Masseter muscle insertion | Angle and lateral surface of the mandibular ramus |
| Masseter muscle rigidity (MMR) | Jaws of steel |
| Masseter muscle treatment for cosmetic reasons | BoNT-A |
| Masseter muscle treatment for pain and hyperfunctional reasons | BoNT-A |
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What You'll Learn
The masseter muscle is one of the four muscles of mastication
The masseter muscle is one of the four muscles responsible for the action of mastication (chewing). It is a powerful, fan-shaped, superficial quadrangular muscle with three layers: the superficial, intermediate, and deep layers. The masseter originates from the zygomatic arch and inserts along the angle and lateral surface of the mandibular ramus. The masseter receives its motor innervation from the mandibular division of the trigeminal nerve. Its blood supply is primarily from the masseteric artery, a branch of the internal maxillary artery.
The masseter muscle is responsible for the elevation of the mandible and some protraction of the mandible, allowing for the anterior motion of the jaw. When the masseter contracts, it causes a powerful elevation of the mandible, causing the mouth to close. The intermediate and deep muscle fibres of the masseter function to retract the mandible, while the superficial fibres function to protrude the mandible. The deep fibres are important stabilisers of the temporomandibular joint (TMJ).
The masseter muscle is relatively well conserved across the human population and the animal kingdom. However, masseteric hypertrophy, a benign condition of enlargement of the masseter muscles, is characteristic in populations of Asian descent. This condition results in a square jawline shape and is often a target for aesthetic treatments and Botox requests.
The masseter reflex, also known as the mandibular reflex or jaw-jerk reflex, involves opening the mouth and placing a finger on the chin of the patient and striking the finger with a reflex hammer. This reflex action in the masseter muscles can be used to evaluate the volume and firmness of the muscles.
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The temporalis, medial pterygoid and lateral pterygoid are the other three
The masseter muscle is one of four muscles responsible for the action of mastication (chewing). When the masseter contracts, it causes the mandible to elevate, closing the mouth. The masseter muscle is relatively well conserved across the human population and the animal kingdom. It is a powerful, quadrangular muscle with two divisions: superficial and deep. The superficial portion of the masseter muscle originates from a thick aponeurosis on the temporal process of the zygomatic bone and the anterior two-thirds of the inferior border of the zygomatic arch.
The other three muscles of mastication are the temporalis, medial pterygoid, and lateral pterygoid. The temporalis muscle functions as an elevator of the mandible, opposing gravity when the mouth is closed. The contraction of the posterior, more horizontal fibres of the muscle produces a retraction of the mandible, pulling the jaw backward. The temporalis muscle also contributes to grinding movements by moving the mandible from side to side.
The medial pterygoid muscle is a quadrangular muscle situated in the infratemporal fossa. It is composed of two heads that have two sets of 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 medial pterygoid muscle is innervated by the medial pterygoid branch of the mandibular nerve. Its principal blood supply stems from the pterygoid branches of the maxillary artery. The major functions of this muscle are the elevation of the mandible and side-to-side movements when grinding and chewing. The medial pterygoid is also involved in protrusion of the mandible.
The lateral pterygoid muscle has a triangular shape with two heads: superior and inferior. It has horizontally oriented muscle fibres, and thus is the major protractor of the mandible. The superior head originates from the greater wing of the sphenoid. The inferior head originates from the lateral pterygoid plate of the sphenoid. The two heads converge into a tendon that attaches to the neck of the mandible. The lateral pterygoid muscle is a triangular muscle that lies in the infratemporal fossa. Like the medial pterygoid muscle, it has two heads with distinct origins. The lateral pterygoid is the fourth masticatory muscle and causes jaw protrusion and jaw opening when activated.
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The masseter muscle is quadrangular and has two parts
The masseter muscle is one of the four muscles responsible for the action of mastication (chewing). When the masseter contracts, it causes a powerful elevation of the mandible, causing the mouth to close. It is a powerful, thick, and rectangular muscle that originates from the zygomatic arch and extends down to the mandibular angle. It is a superficial quadrangular muscle with two divisions: the superficial and deep parts.
The superficial portion of the masseter muscle originates from a thick aponeurosis on the temporal process of the zygomatic bone and the anterior two-thirds of the inferior border of the zygomatic arch. The deep part consists of vertically directed muscle fibres. The entire superficial (or lateral) aspect of the muscle is covered with thin but very strong masseteric fascia. The masseter muscle is relatively well conserved across the human population as well as the animal kingdom.
The masseter muscle is supplied with blood by the masseteric artery, a branch of the maxillary artery. The maxillary artery comes off the external carotid artery behind the neck of the mandible. The masseteric nerve, a branch of the mandibular nerve, innervates the masseter muscle. The masseter reflex, or jaw-jerk reflex, involves opening the mouth and placing a finger on the chin of the patient and striking the finger with a reflex hammer. This can be used to assess the status of a patient's trigeminal nerve.
The masseter muscle can be easily tested by having the patient clench their jaw and evaluating the volume and firmness of the muscles. The masseter muscle is also associated with several disorders, such as masticatory myofascial pain, myospasm, and masseter muscle rigidity (MMR) or "jaws of steel". Therapeutic interventions for masseter muscle dysfunction include therapeutic exercises, manual therapy, and dry needling.
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It is the most powerful muscle of mastication
The masseter muscle is one of the four muscles responsible for the action of mastication (chewing). It is a powerful, strong, superficial quadrangular muscle that covers the lateral aspect of the ramus of the mandible. Its insertion along the angle and lateral surface of the ramus also allows it to aid in the protrusion of the mandible, allowing for the anterior motion of the jaw. The masseter muscle is relatively well conserved across the human population as well as the animal kingdom.
The masseter muscle is composed of two layers that differ slightly in their attachments. Its larger, superficial layer arises from the maxillary process of the zygomatic bone and the anterior two-thirds of the zygomatic arch. From this origin, the muscle fibres run inferiorly and posteriorly to attach to the lateral surface of the angle and the lower half of the ramus of the mandible. The deep layer of the masseter muscle arises from the medial surface and the inferior margin of the zygomatic arch. These fibres run vertically downwards to insert onto the upper part of the ramus of the mandible and the coronoid process.
The masseter muscle is primarily responsible for the elevation of the mandible and some protraction of the mandible. Its major function is to elevate the mandible, with a minor contribution to protrusion of the mandible. The muscle fibres converge anteriorly to form a tendon that exits the temporal fossa, passing underneath the zygomatic arch and inserting on the coronoid process of the mandible. The normal response of the masseter reflex, also known as the mandibular reflex or jaw-jerk reflex, is very little to no reflex activity.
The other three muscles of mastication are the temporalis, medial pterygoid, and lateral pterygoid muscles. The temporalis muscle is a fan-shaped muscle with anterior fibres that have a vertical orientation, mid fibres with an oblique orientation, and posterior fibres with a more horizontal orientation. The origin of the temporalis muscle spans from the temporal fossa to the inferior temporal line of the lateral skull. The medial pterygoid muscle is a thick rectangular muscle with a superficial head and a deep head. The lateral pterygoid muscle is the sole muscle of mastication that causes depression of the mandible.
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Masseter muscle rigidity (MMR) is known as jaws of steel
The masseter muscle is a complex muscle in the jaw region that is responsible for elevating the mandible and closing the jaw. It is one of the four muscles that enable the action of mastication (chewing). The muscle is thick and strong and can exert a tremendous bite force.
Masseter muscle rigidity (MMR) is a condition that occurs following a dose of succinylcholine and is characterised by limb muscle flaccidity with jaw muscle tightness. It is also known as "jaws of steel", a term coined by Kaplan et al. to emphasise the severity of the condition. MMR can range from a tight jaw to a rigid jaw to severe spasticity or trismus, which is the inability to open the mouth.
This condition can interfere with medical procedures such as laryngoscopy and tracheal intubation, as the patient's mouth cannot be opened sufficiently. The risk of malignant hyperthermia (MH) is also increased in patients with MMR. MH is a hypermetabolic reaction to certain drugs, such as suxamethonium, and stress, and can have varied clinical presentations, making diagnosis challenging.
MMR can be managed by halting the triggering agent and the surgical procedure if possible. The patient should be admitted to the hospital for close observation, and creatine kinase levels should be monitored. If creatine kinase levels are significantly elevated, treatment for rhabdomyolysis is necessary to prevent acute kidney damage.
In summary, masseter muscle rigidity (MMR) or "jaws of steel" is a condition characterised by increased tension and tightness in the jaw muscles, often occurring after the administration of certain drugs. It can have serious implications for patient care and requires careful management to prevent complications such as malignant hyperthermia.
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Frequently asked questions
The masseter muscle is the primary muscle responsible for closing the jaw.
The temporalis and medial pterygoid muscles also play a role in closing the jaw.
The masseter is one of the four muscles of mastication, which is the action of chewing. It is a powerful, quadrangular-shaped muscle that originates from the zygomatic arch.
The masseter muscle has two parts: a deep part and a superficial part. The superficial part is larger and arises from the maxillary process of the zygomatic bone.
The main function of the masseter muscle is to elevate the mandible, causing the mouth to close. It also aids in the protrusion of the mandible, allowing for the forward movement of the jaw.
