The muscles of mastication are four paired muscles that control chewing by moving your lower jaw. They are the masseter, temporalis, medial pterygoid, and lateral pterygoid. All four are powered by the same nerve (the mandibular branch of the trigeminal nerve) and work together to produce five basic jaw movements: closing, opening, pushing forward, pulling back, and shifting side to side.
The Masseter
The masseter is the most prominent chewing muscle and the one you can feel bulging at the angle of your jaw when you clench your teeth. It runs from the cheekbone (zygomatic arch) down to the corner and outer surface of the lower jawbone. Its primary job is elevation, meaning it pulls the jaw upward to close the mouth and press the teeth together. The superficial fibers also help push the jaw forward slightly, while the deeper fibers assist with pulling it back.
Despite its relatively compact size, the masseter generates remarkable force. In healthy adults with natural teeth, maximum bite force typically falls between 530 and 1,000 newtons in males and 490 and 880 newtons in females, with median values around 810 N and 670 N respectively. Values above 1,000 N are considered excessive, while values below 650 N may indicate a deficit. That force comes primarily from the masseter working in concert with the temporalis and medial pterygoid.
The Temporalis
The temporalis is a large, fan-shaped muscle that fills the shallow depression on the side of your skull above the ear. You can feel it tighten in the temple area when you clench. It originates along the temporal line of the skull and converges into two strong tendons that attach to the coronoid process, a bony projection on the upper part of the jawbone.
Different sections of the temporalis perform different tasks depending on the direction of their fibers. The vertical fibers at the front pull the jaw straight up, making them powerful closers. The oblique middle fibers assist with rotation. The horizontal fibers running toward the back of the skull are responsible for retracting the jaw, pulling it backward after you’ve thrust it forward. This makes the temporalis especially important for controlled, precise biting movements, not just raw closing force.
The Medial Pterygoid
The medial pterygoid sits on the inner surface of the jaw, roughly mirroring the masseter on the outer surface. Together they form a muscular sling around the angle of the jawbone. The medial pterygoid helps close the mouth and push the jaw forward when both sides contract simultaneously.
Its more distinctive role emerges during one-sided contraction. When the medial pterygoid on one side fires along with the lateral pterygoid on the same side, the jaw swings toward the opposite side. This lateral grinding motion is essential for breaking down tough food between the back teeth.
The Lateral Pterygoid
The lateral pterygoid is unique among the four muscles because it is the only one that opens the jaw. It sits deep in the face, running roughly horizontally from the base of the skull to the neck of the jawbone’s condyle (the rounded knob that fits into the jaw joint). Opening your mouth is largely a passive event driven by gravity, but the lateral pterygoid initiates and controls the movement, especially when you open against resistance or need to open quickly.
This muscle has two heads: an upper head and a lower head. The upper head has fibers that connect, at least partially, to the disc inside the temporomandibular joint (TMJ). Research shows that roughly 29 to 31 percent of the upper head’s fibers integrate into that disc, which is why the lateral pterygoid plays a central role in TMJ disorders. When it contracts bilaterally, it protrudes the jaw. When it contracts on one side, it shifts the jaw to the opposite side, working with the medial pterygoid to produce the grinding pattern of chewing.
How They Work Together
Chewing is not simply opening and closing. A single chewing cycle involves a looping motion: the jaw drops, shifts to one side, rises to crush the food, then returns to center. The lateral pterygoid opens and protrudes the jaw to start the cycle. The masseter, temporalis, and medial pterygoid then fire to close the jaw with force. Meanwhile, the pterygoids on one side create the lateral shift that grinds food against the molars. The posterior fibers of the temporalis pull the jaw back into position to reset for the next cycle.
All four muscles receive motor signals from the mandibular nerve, which is the third and lowest branch of the trigeminal nerve (cranial nerve V). This is the only branch of the trigeminal nerve that carries motor fibers. The blood supply comes from branches of the maxillary artery: the masseteric artery feeds the masseter, the deep temporal arteries supply the temporalis, and small pterygoid branches supply both pterygoid muscles.
Accessory Muscles in Jaw Movement
While these four are formally classified as the muscles of mastication, several other muscles assist with jaw movement. The suprahyoid muscles (a group of small muscles connecting the jaw to the hyoid bone in the throat) help pull the jaw open, especially during wide opening or forceful depression. The buccinator muscle in the cheek and the tongue work together to position food between the teeth during chewing. These are not classified as muscles of mastication in the strict anatomical sense, but chewing would be ineffective without them.
Common Problems With These Muscles
Disorders affecting the chewing muscles fall under the umbrella of temporomandibular disorders, or TMDs, which include more than 30 conditions affecting the jaw joint and surrounding muscles. About 11 to 12 million adults in the United States experience pain in the TMJ region. The most common symptom is pain in the chewing muscles or the jaw joint itself.
Muscle-specific TMDs often involve the masseter and temporalis. Chronic clenching or grinding (bruxism) overworks these muscles, leading to soreness, fatigue, and tension headaches that radiate from the temples. The lateral pterygoid is frequently involved in disc displacement within the TMJ because of its direct attachment to the joint disc. When this muscle goes into spasm or develops trigger points, it can cause clicking, locking, or pain with jaw opening. Myofascial pain in any of the four muscles can refer discomfort to the ear, teeth, or side of the face, sometimes mimicking a toothache or earache.