The muscles of mastication are four paired muscles on each side of your skull that control your jaw. They open and close your mouth, slide your jaw forward and backward, and shift it side to side for chewing. The four primary muscles are the masseter, temporalis, medial pterygoid, and lateral pterygoid. All four are powered by the same nerve: the mandibular branch of the trigeminal nerve, the fifth cranial nerve.
Masseter
The masseter is the thick, rectangular muscle you can feel bulging at the back of your jaw when you clench your teeth. It runs from your cheekbone down to the lower edge of your jawbone and is the most powerful of the four muscles. Its primary job is closing the jaw, and it generates most of the force when you bite down on food. In the molar region, average maximum bite force reaches roughly 350 to 520 newtons in men and 220 to 440 newtons in women, depending on the study. That force drops significantly at the front teeth, where averages fall closer to 100 to 190 newtons.
The masseter has two layers. The superficial layer, closer to the skin, originates from the zygomatic process (the bony arch of your cheekbone). The deeper layer originates from the underside of that same arch. Both layers insert onto the outer surface of the jawbone’s vertical branch, called the ramus. Because of its position and leverage, the masseter is the muscle most associated with jaw clenching and teeth grinding.
Temporalis
The temporalis is a broad, fan-shaped muscle that fills the shallow depression on the side of your skull above your ear. You can feel it contract by placing your fingertips on your temples and biting down. It originates from the parietal bone and the surrounding temporal fossa, then narrows into a tendon that passes under the cheekbone and attaches to the coronoid process, a bony projection on the upper part of the jawbone.
Its front fibers run nearly vertically, making them effective at closing the jaw alongside the masseter. Its rear fibers run more horizontally. When those posterior fibers contract, they pull the jaw backward, a movement called retraction. This is what happens when you slide your lower jaw back after jutting it forward. The temporalis also contributes to side-to-side grinding motions during chewing.
Medial Pterygoid
The medial pterygoid sits on the inner surface of the jawbone, roughly mirroring the masseter on the outer surface. Together, these two muscles form a sling around the jaw’s ramus. The medial pterygoid originates from the pterygoid plate of the sphenoid bone (a wing-shaped bone deep in the skull) and inserts near the angle of the mandible on its inner side.
Like the masseter, the medial pterygoid’s main role is elevating the jaw to close the mouth. It also assists with protrusion (pushing the jaw forward) and contributes to the side-to-side grinding motion essential for breaking down tough food. When the medial pterygoid on one side contracts alone, it shifts the chin toward the opposite side.
Lateral Pterygoid
The lateral pterygoid is the only one of the four that primarily opens the jaw rather than closing it. It’s a short, triangular muscle with two heads. The superior head originates from the greater wing of the sphenoid bone, and the inferior head originates from the outer surface of the pterygoid plate. Both heads converge into a tendon that attaches to the neck of the mandibular condyle, the rounded knob where the jawbone meets the skull at the temporomandibular joint (TMJ).
When both lateral pterygoids fire together, they pull the jaw forward (protrusion). When only one side contracts, it swings the chin to the opposite side. The lateral pterygoid also plays a role in opening the mouth by pulling the condyle forward and downward along the joint surface. This forward sliding of the condyle is what allows you to open your mouth wide rather than simply hinging it open a small amount.
How These Muscles Work Together
Chewing isn’t a simple up-and-down motion. It’s a complex cycle that combines several jaw movements in quick succession, and each movement recruits a different combination of muscles.
- Closing the jaw (elevation): The masseter, temporalis, and medial pterygoid all contract together. This is the most powerful movement the jaw produces.
- Opening the jaw (depression): The lateral pterygoid works with muscles below the jaw (the digastric, mylohyoid, and geniohyoid), assisted by gravity.
- Pushing the jaw forward (protrusion): Mainly driven by the lateral pterygoid, with some help from the medial pterygoid.
- Pulling the jaw back (retraction): The posterior fibers of the temporalis lead this movement, with assistance from the deep fibers of the masseter.
- Side-to-side grinding (lateral excursion): When the lateral and medial pterygoids on one side contract, the chin swings to the opposite side. Alternating this movement between sides creates the circular grinding pattern you use to crush food between your molars.
Accessory Muscles That Help With Chewing
The four primary muscles don’t work alone. Several secondary muscles assist during eating. The buccinator, the thin muscle in your cheek, keeps food pushed back between your teeth so you don’t constantly bite the inside of your mouth. The suprahyoid muscles (the digastric, mylohyoid, and geniohyoid) sit beneath the chin and pull the jaw downward when you open your mouth against resistance, like biting into an apple. The infrahyoid muscles, which run from the hyoid bone down to the collarbone and breastbone, stabilize the hyoid so the suprahyoids can effectively pull the jaw open.
Common Problems With the Masticatory Muscles
The most common disorder affecting these muscles is myofascial pain, a condition where the muscles develop tender, painful spots that ache during chewing or even at rest. It falls under the broader category of temporomandibular disorders (TMD). Symptoms typically include dull, aching jaw pain, difficulty opening the mouth fully, and soreness in the temples or along the cheeks. In some cases, the pain spreads beyond the muscle itself or refers to other areas of the face and head.
Anxiety appears to increase the risk of myofascial jaw pain. One large study found a 1.8-fold increase in myofascial pain among people with anxiety disorders. Habits like teeth clenching, grinding (bruxism), and prolonged gum chewing can overwork these muscles and trigger symptoms. Masseter hypertrophy, where the masseter visibly enlarges from chronic clenching, is another recognized condition that can change the shape of the lower face and sometimes cause discomfort.
Diagnosis typically involves a clinician pressing on the masseter and temporalis to reproduce familiar pain. If pressing on one spot produces pain that stays local, it’s classified as local myalgia. If the pain spreads outward or refers to a distant area, it’s categorized as myofascial pain with spreading or referral, which can mimic headaches or earaches.