Muscles of mastication: masseter, temporalis and the medial and lateral pterygoids

Mastication is the medical word for chewing. The muscles of mastication are four paired muscles that move the mandible at the temporomandibular joint: masseter, temporalis, medial pterygoid and lateral pterygoid. They develop from the first pharyngeal arch and are therefore supplied by the nerve of that arch, the mandibular nerve, a branch of the trigeminal nerve (cranial nerve V). (TeachMeAnatomy: The Muscles of Mastication.)

OpenStax describes masseter as the main chewing muscle because it elevates the mandible to close the mouth, with temporalis helping to elevate and retract the jaw. The lateral pterygoid depresses and protracts the mandible and moves it from side to side, and the medial pterygoid elevates it and assists side-to-side movement. (OpenStax Anatomy and Physiology 2e, 11.3: Axial Muscles of the Head, Neck, and Back.)

Chewing is the first stage of mechanical digestion. The lips and cheeks keep food between the teeth, and the teeth tear and crush it into smaller particles that can be mixed with saliva and swallowed. The cheek muscle buccinator and the tongue help with this, but they are not muscles of mastication: buccinator is a muscle of facial expression supplied by the facial nerve. (OpenStax Anatomy and Physiology 2e, 23.3: The Mouth, Pharynx, and Esophagus.)

Masseter is the thick, rectangular muscle you can feel at the back of the cheek when you clench your teeth. Its superficial part arises from the maxillary process of the zygomatic bone and the front two-thirds of the lower border of the zygomatic arch, and inserts on the masseteric tuberosity on the lower lateral surface of the ramus, near the angle of the mandible. Its deep part has more vertical fibres that arise from the zygomatic arch and insert higher on the ramus. (Kenhub: Masseter muscle.)

Masseter elevates the mandible to close the jaw with force, and its anterior fibres help to protrude it. It is supplied by the masseteric nerve from the mandibular nerve and by the masseteric artery from the maxillary artery. The parotid duct crosses its outer surface before turning inwards through buccinator to open inside the cheek. (Kenhub: Masseter muscle.)

TeachMeAnatomy calls masseter the most powerful muscle of mastication. Kenhub, on the other hand, describes temporalis as the strongest muscle acting on the TMJ; the comparison depends on how strength is measured, and both muscles are powerful jaw closers. (TeachMeAnatomy: The Muscles of Mastication.)

Temporalis is a broad, fan-shaped muscle on the side of the skull. It arises from most of the temporal fossa below the temporal line and from the temporal fascia that covers it. Its fibres converge into a tendon that passes deep to the zygomatic arch and inserts on the apex and medial surface of the coronoid process of the mandible. (Kenhub: Temporalis muscle.)

The anterior, more vertical fibres of temporalis elevate the mandible, and the posterior, almost horizontal fibres retract it (pull it backwards). It is supplied by the anterior, middle and posterior deep temporal nerves from the mandibular nerve, and by the deep temporal branches of the maxillary artery with help from the superficial temporal artery. (Kenhub: Temporalis muscle.)

The temporal fascia over the muscle can be harvested by surgeons as a graft to repair a perforated eardrum, an operation called myringoplasty. (TeachMeAnatomy: The Muscles of Mastication.)

The medial pterygoid, also called the internal pterygoid or musculus pterygoideus internus in older texts, is a thick muscle on the inner side of the ramus. Its deep head arises from the medial surface of the lateral pterygoid plate of the sphenoid bone. Its small superficial head arises from the maxillary tuberosity and the pyramidal process of the palatine bone. (Kenhub: Medial pterygoid muscle.)

Both heads run downwards, backwards and laterally to insert by a tendinous sheet into a rough triangular area on the medial surface of the ramus and angle of the mandible. Masseter inserts on the opposite, lateral surface of the same region, so the two muscles grip the angle of the mandible from both sides. (Kenhub: Medial pterygoid muscle.)

Acting together, the two medial pterygoids elevate the mandible and, with the lateral pterygoids, protrude it. One medial pterygoid acting alone rotates the mandible slightly medially, helping to swing the chin towards the other side. The muscle is supplied by the nerve to medial pterygoid from the mandibular nerve. The lingual and inferior alveolar nerves pass between it and the ramus, which is relevant to dental nerve blocks. (Kenhub: Medial pterygoid muscle.)

The lateral pterygoid is a short, thick muscle with two heads that runs almost horizontally backwards to the jaw joint. Its superior head arises from the infratemporal crest and surface of the greater wing of the sphenoid. Its much larger inferior head arises from the lateral surface of the lateral pterygoid plate. (Kenhub: Lateral pterygoid muscle.)

The superior head inserts into the front of the capsule and articular disc of the TMJ, and the inferior head inserts into the pterygoid fovea, a small pit on the front of the neck of the mandible's condylar process. Because its fibres are horizontal, the lateral pterygoid is the main muscle that pulls the mandible forwards (protrusion). (TeachMeAnatomy: The Muscles of Mastication.)

When both lateral pterygoids contract, the inferior heads pull the condyles forwards and the superior heads pull the discs forwards with them, so the jaw protrudes and the mouth opens. When one side contracts, the condyle on that side moves forwards and inwards and the chin swings to the opposite side; this works with the medial pterygoid of the same side when we grind food. (Kenhub: Lateral pterygoid muscle.)

The lateral pterygoid is supplied by the nerve to lateral pterygoid from the mandibular nerve and by pterygoid branches of the maxillary artery. The maxillary artery and the buccal nerve pass between its two heads, and the lingual and inferior alveolar nerves appear at its lower border. (Kenhub: Lateral pterygoid muscle.)

The infratemporal fossa is the space below the skull base, deep to the ramus of the mandible. Its lateral wall is the ramus and condylar process of the mandible; its medial wall is the lateral pterygoid plate; in front is the back wall of the maxillary sinus; behind is the carotid sheath; its roof is the greater wing of the sphenoid; and the medial pterygoid forms its floor. (TeachMeAnatomy: The Infratemporal Fossa.)

The medial and lateral pterygoids lie inside the fossa, while masseter and temporalis attach to its borders. The lateral pterygoid divides the contents: branches of the mandibular nerve lie deep to it and the maxillary artery runs superficial to it. The mandibular nerve enters through the foramen ovale and gives the auriculotemporal, buccal, lingual and inferior alveolar nerves; the chorda tympani joins the lingual nerve here. (TeachMeAnatomy: The Infratemporal Fossa.)

The fossa also contains the pterygoid venous plexus, which connects with the cavernous sinus inside the skull. This connection is a route by which infection from the face can spread into the cranial cavity. Superiorly the fossa opens into the temporal fossa, and it communicates with the pterygopalatine fossa through the pterygomaxillary fissure. (TeachMeAnatomy: The Infratemporal Fossa.)

The temporomandibular joint is an atypical synovial joint between the condylar process (head) of the mandible and the mandibular fossa and articular eminence of the temporal bone. The articular eminence, also called the articular tubercle, is the rounded ridge of bone just in front of the mandibular fossa. (Radiopaedia: Temporomandibular joint.)

Unlike most synovial joints, the TMJ's articular surfaces are covered by fibrocartilage instead of hyaline cartilage. A fibrocartilage articular disc separates the bones completely and divides the joint into two compartments, each with its own synovial membrane. (Kenhub: Temporomandibular joint.)

In side view the disc has a bow-tie shape, with thick anterior and posterior bands joined by a thin intermediate zone. Behind the posterior band is the loose, richly innervated retrodiscal tissue (bilaminar zone). The capsule is loose above the disc and tight below it, anchoring the disc to the condylar neck, and the lateral pterygoid inserts into the capsule, the disc and the condylar neck. (Radiopaedia: Temporomandibular joint.)

Gliding (translation) for protrusion, retraction and side-to-side movement happens in the upper (discotemporal) compartment, while hinge-like rotation for opening and closing happens in the lower (discomandibular) compartment. When the mouth opens wide, the condyle and disc translate forwards so that the condyle comes to lie beneath the articular eminence. (Radiopaedia: Temporomandibular joint.)

The joint is reinforced by the lateral (temporomandibular) ligament, a thickening of the capsule from the articular tubercle to the neck of the mandible that prevents posterior dislocation. The sphenomandibular and stylomandibular ligaments lie outside the capsule. The joint is supplied mainly by the auriculotemporal and masseteric branches of the mandibular nerve and by branches of the external carotid artery, especially the superficial temporal artery. (TeachMeAnatomy: The Temporomandibular Joint.)

Elevation, the powerful closing movement, is produced by temporalis, masseter and the medial pterygoid. Depression, opening the mouth, is mainly due to gravity, and the digastric, geniohyoid and mylohyoid muscles help when there is resistance. Protrusion is produced by the lateral pterygoid with help from the medial pterygoid, and retraction by the posterior fibres of temporalis. (TeachMeAnatomy: The Temporomandibular Joint.)

Grinding food from side to side is not a separate movement at one joint. It comes from alternating protrusion and retraction on each side, so the mandible swings around one condyle and then the other. (TeachMeAnatomy: The Temporomandibular Joint.)

In practice the two kinds of movement are combined: Kenhub's TMJ article notes that rotation and translation usually happen together during normal opening and closing. (Kenhub: Temporomandibular joint.)

The lateral pterygoid also works while the mouth closes. Its inferior heads contract eccentrically (lengthening while active) to control the backward glide of the condyles into the fossae, and the retrodiscal tissue behind the disc limits how far forward the condyle can travel. (Kenhub: Lateral pterygoid muscle.)

The pterygomandibular raphe is a tendinous band that runs from the hamulus of the medial pterygoid plate to the mylohyoid ridge on the inner surface of the mandible. The buccinator attaches to its front edge and the superior pharyngeal constrictor to its back edge. (Radiopaedia: Pterygomandibular raphe.)

Inside the mouth the raphe is covered by a fold of mucosa, the pterygomandibular fold, between the anterior pillar of the tonsil and the retromolar area behind the last molar. Dentists use this fold as a landmark when giving an inferior alveolar nerve block to numb the lower teeth. (Radiopaedia: Pterygomandibular raphe.)

The most common TMJ injury is anterior dislocation, which can happen during a wide yawn, a large bite or a blow to the side of the face. The head of the mandible slides forwards out of the mandibular fossa, in front of the articular eminence, and the patient cannot close the mouth. Posterior dislocation is very rare because the postglenoid tubercle and the strong lateral ligament resist it. (TeachMeAnatomy: The Temporomandibular Joint.)

Trismus is a persistent spasm of the masseter that makes it hard to open the mouth. It is a feature of tetanus and of abscesses in the space beneath masseter, which usually spread from an infected lower third molar. The masseter (jaw jerk) reflex is also tested in clinical examination; an abnormal response may point to a lesion of the trigeminal nerve or brainstem. (Kenhub: Masseter muscle.)

Tension in temporalis is a common cause of pain over the temple, for example with tooth grinding (bruxism) or after long dental treatment with the mouth open. Kenhub notes that temple pain also needs inflammation of the superficial temporal artery (giant cell arteritis) to be ruled out. (Kenhub: Temporalis muscle.)

Mandible

The lower jaw bone.

Maxilla

The upper jaw bone, paired.

Pterygomandibular raphe

A tendinous band between the pterygoid hamulus and the mandible, not a true muscle.

Sphenoid bone

A butterfly-shaped bone at the base of the skull.

Temporal bone

A paired bone forming part of the side and base of the skull.

Zygomatic bone

The cheekbone.