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The muscles of mastication are a group of muscles responsible for the movement of the jaw, including opening and closing the mouth. There are four main muscles of mastication: the masseter, temporalis, medial pterygoid, and lateral pterygoid. These muscles work together to produce movements such as elevation, depression, and protrusion of the mandible, which is the only bone that moves during activities like talking and chewing. The masseter muscle, in particular, is responsible for the powerful elevation of the mandible, causing the mouth to close.
| Characteristics | Values |
|---|---|
| Muscles Involved | Masseter, Temporalis, Medial Pterygoid, Lateral Pterygoid |
| Innervation | Mandibular branch of the Trigeminal nerve (CN V3) |
| Actions | Elevation of the mandible (closing the mouth), depression, protrusion, retraction, side-to-side movement |
| Blood Supply | Masseteric artery, a branch of the internal maxillary artery |
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What You'll Learn
The masseter muscle
The function of the masseter muscle is to elevate the mandible, causing a powerful closure of the jaw during chewing. This elevation of the mandible occurs during the closing of the jaws. In addition to its role in mastication, the masseter muscle also helps to stabilise the tension of the articular capsule of the temporomandibular joint. The masseter muscle can become enlarged in individuals who habitually clench or grind their teeth, a condition known as bruxism, or even in those who constantly chew gum. This enlargement, or masseteric hypertrophy, is typically asymptomatic and soft, but it can lead to asymmetry of the face due to unequal enlargement of the muscles.
The word "masseter" comes from the Greek word "masasthai," which means "to chew," reflecting its crucial role in the chewing or mastication process. The masseter muscle is a distinguishing feature of hystricognathous creatures, such as mole-rats, and it also provides important insights into the evolution of certain species. For example, in toothed whales, the masseter muscle has become redundant due to a shift from chewing to swallowing, and it now provides the tissue for acoustic fat bodies used for echolocation.
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The temporalis muscle
The unilateral contraction of the temporalis muscle plays an important role in the side-to-side movements of the jaw. The temporalis muscle, working independently on one side, will deviate the jaw outward to the same side. This synchronisation of the temporalis muscle with the masseter and pterygoid muscles produces the side-to-side rocking motion of the mandible associated with chewing and grinding food.
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The medial pterygoid muscle
The function of the medial pterygoid muscle, along with the other muscles of mastication, is to facilitate chewing and grinding food by producing movements of the mandible at the temporomandibular joint.
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The lateral pterygoid muscle
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. This includes forward movement of the mandible (protrusion), depression of the mandible, and movement of the mandibular condyle towards the midline (mediotrusion). Unlike the other three muscles of mastication, the lateral pterygoid is the only muscle that participates in depressing the mandible. It is also the major protractor of the mandible. The unilateral contraction of the lateral pterygoid muscle with the ipsilateral medial pterygoid muscle results in lateral mandibular movement to the contralateral side.
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Innervation and nerve involvement
The muscles responsible for closing the mouth are the masseter, temporalis, medial pterygoid, and lateral pterygoid. These muscles are involved in biting, chewing, and mastication.
The muscles of mastication are innervated by the trigeminal nerve (CN V), specifically by its mandibular branch (V3). This nerve is responsible for providing sensory and motor functions to the muscles involved in mastication. The trigeminal nerve is a three-part nerve that sends signals between the brain and the face, with two such nerves present in the human head. The mandibular branch of the trigeminal nerve provides sensory innervation to the inner lining of the cheeks, the floor of the mouth, the teeth in the lower jaw, the tongue, and the skin below the mouth. It also provides movement information to the muscles involved in chewing, including the masseter and pterygoid muscles.
The trigeminal nerve begins within four nuclei in the brain, with three of these nuclei responsible for sensory information and the fourth controlling motor function. These sensory nuclei merge to form a sensory root near the pons, the central part of the brainstem. This root then becomes the trigeminal ganglion, which splits into three branches: the ophthalmic, maxillary, and mandibular branches.
Damage to the trigeminal nerve can result in a range of symptoms, including facial numbness, pain, vision issues, jaw muscle spasms, and difficulty chewing or swallowing.
In addition to the trigeminal nerve, other nerves are also involved in the complex process of mastication. The mesencephalic trigeminal neurons innervate muscle spindles in the jaw-closer muscles and are active during chewing. These neurons exhibit a resonant property that may facilitate the masticatory rhythm.
The facial nerve (CN VII) is another important nerve involved in facial movements. It controls several facial muscles and provides the sense of taste to parts of the tongue. The parasympathetic nerve fibers of the facial nerve control the salivary glands in the mouth and the lacrimal glands in the corners of the eyes. The cervical branch of the facial nerve allows movement in the chin and the lower corners of the mouth by controlling the platysma muscle in the neck.
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Frequently asked questions
The masseter muscle is one of the four muscles responsible for the action of mastication (chewing) and is the primary muscle that closes the mouth.
The other three muscles of mastication are the temporalis, medial pterygoid, and lateral pterygoid.
The masseter muscle is a powerful elevator of the mandible, causing the mouth to close. It also aids in the protrusion of the mandible, allowing for the anterior motion of the jaw.
The medial pterygoid muscle has a similar function to the masseter muscle, causing elevation and protrusion of the mandible.
The muscles of mastication work together to produce a side-to-side rocking motion of the mandible, which is associated with chewing and grinding food.
