An important group of head and neck muscles are the muscles of mastication—that is “chewing”. These four pairs of muscles operate the only diarthrotic synovial joints in the skull, the paired temporomandibular joints. These are joints between the head, or condyle, of the mandible and the mandibular fossa on the temporal bone.
Joints in the Jaw
By shape, they are combination hinge and gliding joints—hinge for the opening-and-closing action and gliding when talking about side-to-side and front-to-back actions.
A cartilaginous disc between the bones aids in keeping the mandible in place by deepening and cushioning the mandibular fossa, while the articular tubercle on the inferior aspect of the zygomatic arch helps keep the mandible from anterior displacement.
Effect of Gravity
Now, when we think about it, gravity can open the jaw. That is, any one can fall asleep in his chair with his mouth hanging open. And regarding cadavers and others in eternal rest, most people don’t realize that it’s often part of standard embalming procedure to wire the upper and lower jaw together.
The cadavers we use in anatomy are utilized twice for teaching and learning, so we see evidence of many standard funeral practices in the lab. First, the cadavers are used for teaching mortuary science students to embalm, and then the bodies are transferred to labs where they are used to teach anatomy.
When we cover head and neck anatomy, students are often surprised to see a small screw in both the upper and lower jaw and a loop of wire holding those two screws together. This is an important funeral practice, since at an open-casket visitation, no one wants a deceased friend or relative’s mouth to gape open.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Muscles of the Jaw
Coming back to muscles of the jaw, there are two paired muscles that help us close our jaw and clench our teeth—the temporalis and masseter muscles. The temporalis sits on either side of the head at the temple—hence the name—and if we clench our teeth, we can feel it tensing.
The masseter is more associated with the mandible itself, and if we put our fingers on the sides of our jaw while clenching our teeth, we can feel it work.
The temporalis has a broad origin along the temporal lines on the sides of the skull and inserts on the coronoid process of the mandible. The masseter originates along the zygomatic arch and has a large insertion along the lateral ramus of the mandible.
These are both powerful muscles, and in some primates—like the gorilla—the temporalis muscles are so well developed that there’s a crest on the top of the skull, the sagittal crest, where the temporalis muscles anchor superiorly. The sagittal crest is not only found in gorillas, which are primarily herbivores, but also in some carnivores, like some dinosaurs, large cats, sea lions, some dogs, and even in a few omnivores like badgers and orangutans.
The two other pairs of muscles that act on the temporomandibular joint are the medial pterygoid and lateral pterygoid. Both are located deep to the mandible and originate on the pterygoid plates of the sphenoid bone. They insert on the deep surface of the mandible, so neither pterygoid muscle can be easily seen without removing part of the ramus of the mandible to expose an area known as the infratemporal fossa.
Both the medial and lateral pterygoids can aid in side-to-side movements of the jaw, as when performing grinding actions between molar teeth. This is also sometimes called lateral excursions of the mandible, which makes these movements sound awfully fun.
Not to be Confused with Facial Expression Muscles
Both pterygoids are also involved with protraction of the mandible—which means pushing the jaw out anteriorly—while retraction of the mandible is accomplished by the posterior fibers of the temporalis muscle pulling the jaw back into place.
Despite their location in and around the face, muscles of the jaw are not facial expression muscles, and are not supplied by the facial nerve. All muscles of chewing are supplied by cranial nerve V, the trigeminal nerve. Its third branch, the mandibular nerve, exits the cranial cavity to reach the infratemporal fossa—and this control of mandibular movements is one of the reasons for its name.
Injuries to the Jaw
One of the most common injuries caused to the jaw is temporomandibular joint disorder—commonly just called TMJ, but TMD is actually the preferred acronym. The causes are elusive and varied; it can occur after an injury to the jaw or the joint, from arthritis in the TMJ, or from long-term bruxism—that’s grinding of the teeth.
TMD can also arise from muscular causes—like stress that causes some people to clench their teeth too much. Sometimes an audible clicking or scraping sound occurs when they chew.
Some individuals even have dislocation of the mandible or a locking of the joint that can prevent jaw movements. Often, physical therapy helps many people, as the PT students practice on each other—using gloved fingers in mouths and so forth.
Common Questions about the Muscles of the Jaw
There are two paired muscles that help us close our jaw and clench our teeth—the temporalis and masseter muscles.
The medial and lateral pterygoids can aid in side-to-side movements of the jaw, as when performing grinding actions between molar teeth.
The causes of temporomandibular joint disorder are elusive and varied; it can occur after an injury to the jaw or the joint, from arthritis in the TMJ, or from long-term bruxism. It can also arise from muscular causes, like stress that causes some people to clench their teeth too much.