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The mandible is the only bone that moves during mastication (chewing) and other activities such as talking. The muscles of mastication are a group of muscles responsible for the chewing movement of the mandible at the temporomandibular joint (TMJ). They enhance the process of eating and assist in grinding food. There are four muscles of mastication: the masseter, temporalis, medial pterygoid, and lateral pterygoid. In this article, we will explore the role of these muscles in protracting the mandible.
| Characteristics | Values |
|---|---|
| Muscles responsible for protracting the mandible | Masseter, Medial Pterygoid, Lateral Pterygoid, and Temporalis |
| Type of muscle | The masseter muscle is rectangular with three layers (superficial, deep, and intermediate). The temporalis muscle is fan-shaped. The medial pterygoid is thick and rectangular with a superficial and deep head. The lateral pterygoid is short and thick with two heads. |
| Origin | The masseter originates from the inferior zygomatic arch and the anterior two-thirds of the zygomatic arch. The temporalis originates from the temporal fossa. The medial pterygoid originates from the maxillary tuberosity of the inferior maxilla, and its deep head originates from the medial surface of the lateral pterygoid plate of the sphenoid bone. The lateral pterygoid originates from the infratemporal surface and infratemporal crest of the greater wing of the sphenoid bone. |
| Function | The masseter elevates the mandible and assists in protrusion. The medial pterygoid assists in elevation and protrusion, as well as side-to-side motion for grinding food. The lateral pterygoid is the primary muscle for depression of the mandible and also assists with protrusion and side-to-side movement. The temporalis elevates the mandible and contributes to grinding movements. |
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What You'll Learn
The role of the masseter muscle
The masseter muscle is a large, paired, thick, and rectangular muscle with both a superficial and deep part. It is one of the four muscles of mastication, alongside the temporalis, lateral pterygoid, and medial pterygoid muscles. These muscles are responsible for the chewing movement of the mandible, enhancing the process of eating and grinding food. The masseter muscle is involved in elevating and protruding the mandible, allowing for the closure of the mouth and the protrusion of the lower jaw.
The masseter muscle originates from the zygomatic arch and extends down to the mandibular angle. It is located on both sides of the face, extending from the cheekbone down to the angle of the mandible or jaw. Its specific functions are dependent on its parts. The deep part of the masseter muscle allows for the elevation of the mandible, enabling the closure of the mouth. On the other hand, the superficial part of the masseter functions to move the mandible forward, bringing the lower front teeth in front of the upper front teeth.
The masseter muscle is innervated by the mandibular division of the trigeminal nerve. It receives its blood supply from the masseteric artery, a branch of the maxillary artery. The masseter muscle can become enlarged in individuals who habitually clench or grind their teeth or even in those who constantly chew gum. This condition, known as masseteric hypertrophy, is usually asymptomatic and soft, and it can be unilateral or bilateral. However, even if it is bilateral, asymmetry of the face may still occur due to unequal enlargement of the muscles.
The masseter muscle is also involved in other significant roles. It provides support to the articular capsule of the temporomandibular joint (TMJ). Additionally, the deep fibres of the masseter muscle play a crucial role in stabilising the TMJ. Furthermore, the masseter muscle can experience spasms under certain conditions, such as infection with the bacteria Clostridium tetani, leading to a condition called trismus, which interferes with normal feeding. The masseter muscle's positioning is also a distinguishing feature in hystricognathous creatures like mole-rats and has been observed to undergo adaptations in toothed whales due to their shift from chewing to swallowing.
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The temporalis muscle and its function
The temporalis muscle, also known as the temporal muscle, is one of the four muscles of mastication (chewing). It is a thin, fan-shaped convergent muscle on each side of the head that fills the temporal fossa, superior to the zygomatic arch, covering much of the temporal bone. The temporalis muscle is a powerful muscle of the temporomandibular joint and is the primary retractor of the mandible.
The muscle fibres converge inferiorly, forming a tendon that exits the temporal fossa, passing underneath the zygomatic arch and inserting on the coronoid process of the mandible. The temporalis muscle originates from the temporal fossa—a shallow depression on the lateral aspect of the skull—and the deep part of the temporal fascia. The origin point spans the entire surface of the fossa below the temporal line. The muscle is covered by tough fascia, which can be harvested surgically and used to repair a perforated tympanic membrane.
The temporalis muscle is innervated by the deep temporal branches of the mandibular nerve (V3). It receives its blood supply from the deep temporal branches of the maxillary artery and middle temporal branches of the superficial temporal artery. The anterior and middle fibres of the temporalis muscle function to elevate the mandible (closing the mouth), while the posterior fibres function to retract the mandible. The contraction of the posterior fibres of the temporalis muscle results in the backward movement of the mandible (retrusion).
The temporalis muscle is likely to be involved in jaw pain and headaches. Bruxism, the habitual grinding of teeth, and clenching of the jaw while stressed can lead to overwork of the temporalis muscle and result in pain. The temporalis muscle may be used in reconstructive surgery of the mouth and face.
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Medial pterygoid muscle and its structure
The medial pterygoid muscle is one of the four muscles of mastication, responsible for the chewing movement of the mandible at the temporomandibular joint. It is a thick quadrilateral muscle with two heads: a superficial head and a deep head. The superficial head originates from the maxillary tuberosity and the pyramidal process of the palatine bone, while the deep head originates from the medial aspect of the lateral pterygoid plate of the sphenoid bone.
Both heads of the medial pterygoid muscle attach to the ramus of the mandible near the angle of the mandible, forming an axis for a strong pull of this bone. The 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 insertion of the medial pterygoid forms a tendinous band with the insertion of the masseter muscle, known as the pterygoid-masseteric sling.
The medial pterygoid muscle is innervated by the medial pterygoid nerve, a branch of the mandibular nerve (CN V3), which is one of the three divisions of the trigeminal nerve (CN V). It receives its vascular supply primarily from the pterygoid branches of the maxillary artery, as well as from the facial artery through its muscular branches. The pterygoid branches of the maxillary artery are typically two or three in number, entering the medial aspect of the medial pterygoid muscle from an anterior and inferior direction. Additionally, the medial pterygoid muscle may receive vascular supply from a direct branch of the external carotid artery, which enters the lower part of the muscle posteriorly.
The medial pterygoid muscle has a triple function. Firstly, it contributes to the protrusion of the mandible through bilateral contraction in synergy with the lateral pterygoid muscle. Secondly, unilateral contraction of the medial pterygoid, along with the lateral pterygoid ipsilaterally, results in lateral movement of the mandible towards the opposite side. Lastly, it works together with the masseter and temporalis muscles to elevate the mandible, facilitating the chewing and grinding of food.
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Lateral pterygoid muscle and its function
The lateral pterygoid is a craniomandibular muscle with two heads: the superior and inferior. The inferior head is three times larger than the superior one. The lateral pterygoid 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 is the only muscle of mastication that depresses the mandible.
The lateral pterygoid muscle controls movement at the TMJ, helping to produce forward and lateral motions of the mandible required for chewing and grinding. The sphenoid attachment of the muscle is always fixed, and the direction of pull is oriented towards it. The bilateral contraction of the left and right lateral pterygoid muscles results in translation and rotation within both TMJs. The inferior heads pull the mandibular condyle anteriorly, resulting in the rotation of the condyle against the inferior surface of the articular disc. Simultaneously, the superior heads pull the articular capsule and disc in the same direction to cushion the movements of the condyle. This results in the anterior translation of the disc and condyle, along with the rotation of the condyle, leading to the protrusion and depression of the mandible.
The lateral pterygoid also assists in side-to-side mandibular motion during the grinding of food. The superior head may be active during clenching to prevent the posterior displacement of the mandibular condyle and avoid exertion on the post-condylar structures. The unilateral contraction of the lateral pterygoid with the anterior head of the digastric and mylohyoid muscles results in mandibular depression and mouth opening. Additionally, the unilateral contraction of the lateral pterygoid with the medial pterygoid ipsilaterally leads to lateral excursion to the contralateral side, observed during chewing and clenching.
Dysfunction of the lateral pterygoid muscle can affect mastication and contribute to temporomandibular disorders (TMD). Spasms in this muscle can cause lockjaw. Poor coordination between the superior and inferior heads can disrupt the position of the articular disc of the TMJ, leading to TMD symptoms such as pain in the lateral pterygoid during mandibular movements.
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The four main muscles of mastication
The masseter muscle is rectangular in shape and has two parts: deep and superficial. It originates from the maxillary process of the zygomatic bone and attaches to the ramus of the mandible. Its primary function is to elevate the mandible and approximate the teeth. The intermediate and deep muscle fibres of the masseter retract the mandible, while the superficial fibres protrude it.
The temporalis muscle is fan-shaped and originates from the temporal fossa of the skull, attaching to the coronoid process of the mandible. It is covered by tough fascia, which can be harvested surgically and used to repair a perforated tympanic membrane. The function of the anterior and mid fibres of the temporalis muscle is to elevate the mandible, while the posterior fibres retract it.
The medial pterygoid muscle is thick and rectangular, with a superficial head and a deep head. The superficial head originates from the maxillary tuberosity, while the deep head originates from the medial aspect of the lateral pterygoid plate of the sphenoid bone. Both heads attach to the ramus of the mandible. The medial pterygoid assists with elevation and protrusion of the mandible and contributes to side-to-side mandibular motion for grinding food.
The lateral pterygoid muscle is the sole muscle of mastication that causes depression of the mandible, although this is largely the result of gravity. It also assists with protrusion and side-to-side movement of the mandible. The upper head of the lateral pterygoid originates from the inferior temporal surface of the greater wing of the sphenoid bone, while the lower head originates from the lateral aspect of the lateral pterygoid plate of the sphenoid bone.
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Frequently asked questions
The muscles that protract the mandible are the masseter, medial pterygoid, lateral pterygoid, and temporalis muscles.
The masseter muscle's main function is to elevate the mandible, with a minor contribution to the protrusion of the mandible.
The cardinal mandibular movements of mastication are elevation, depression, protrusion, retraction, and side-to-side movement.
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