Because this is the first of three chapters dealing with muscles and their functions, it seems appropriate to mention briefly some terms relevant to this subject. In general, a person reading about muscles constantly sees five terms: origin, insertion, action, nerve supply, and blood supply. Because nerve supply and blood supply are self-explanatory, we will be primarily concerned with the first three. The origin of a muscle is generally considered to be the end of the muscle that is attached to the least movable structure. The insertion of a muscle is the other end of the muscle, which is attached to the more movable structure. In some muscles, there could be movement at either end and so the terminology is a bit confusing. The action is the work that is accomplished when the muscle fibers contract. If you are familiar with the action of a muscle but are not sure which end is the origin or the insertion, keep in mind that in general, the insertion moves toward the origin when the muscle is contracted. Likewise, if you know the direction of the muscle fibers, you can usually deduce the action by imagining the insertion moving toward the origin and picturing what happens. Keep in mind that there may at times be more than one function to a particular muscle because either end of a muscle can function as an insertion if the other end is fixed or because different fiber directions within the muscles and different groups of fibers may be called into function, thereby creating different actions.
The muscles of mastication are four pairs of muscles attached to the mandible and primarily responsible for elevating, protruding, retruding, or causing the mandible to move laterally. They develop from the first (mandibular) pharyngeal arch, which is also responsible for the development of some of the bony facial structures. Because they develop from this arch, they are innervated by the nerve of the first arch, the fifth cranial nerve (trigeminal nerve). More specifically, the muscles are innervated by the third part of the fifth nerve, which is called the mandibular division or V3. The blood supply to these muscles comes from the maxillary artery, which is a branch of the external carotid artery. Blood vessels and nerves are further discussed in Chapters 32 and 34.
The masseter muscle is probably the most powerful of the muscles of mastication. It takes its origin from two areas on the zygomatic arch. The superficial head originates from the inferior border of the anterior two thirds of the zygomatic arch. The deep head arises from the inferior border of the posterior third of the zygomatic arch and the entire medial side of the zygomatic arch. The fibers of the superficial head run down and slightly back to be inserted into the angle of the mandible on the lateral side. The deep head has vertically oriented fibers. When the masseter muscle contracts, it elevates the mandible, closing the mouth (Fig. 28-1).
The temporal muscle, frequently called the temporalis muscle, has a very wide origin from the entire temporal fossa and the fascia covering the muscle. The anterior fibers run almost vertically, but the posterior fibers run in a more horizontal direction over the ear. All these fibers insert into the coronoid process of the mandible and sometimes run down the anterior border of the ramus of the mandible as far as the third molar (Fig. 28-2). If the entire muscle contracts, the overall action pulls up on the coronoid process and elevates the mandible, closing the mouth. If only the posterior fibers are contracted, the result is a horizontal pulling of the coronoid process in a posterior direction. This pulls the mandible backward, which is referred to as retruding the mandible.
In studying the origin of the medial pterygoid muscle, it is probably best to examine a model of the skull while reading the description. The muscle has two origins. The larger and major origin is from the medial side of the lateral pterygoid plate and the pterygoid fossa as well as a tiny area of the palatine bone at the lower end of the medial and lateral pterygoid plates. This area is called the pyramidal process of the palatine bone. The smaller origin is just anterior to that area, coming from the maxillary tuberosity just behind the third molar. All the fibers run down and slightly posteriorly and laterally to be inserted into the angle of the mandible on the medial side. This is just opposite the masseter insertion on the lateral side. When the muscle contracts, the resulting action is elevation of the mandible and closing of the mouth (Fig. 28-3).
The lateral pterygoid muscle also has two separate origins. The smaller, superior origin or head arises from the area called the infratemporal crest of the greater wing of the sphenoid bone. The larger, inferior origin or head arises from the lateral side of the lateral pterygoid plate. This is just opposite the origin of the medial pterygoid muscle. The fibers from both origins of the lateral pterygoid muscle run horizontally in a posterior direction. Some fibers from the superior head penetrate the capsule of the temporomandibular joint (TMJ) and insert into the anterior border of the disc of the joint. The remainder of the fibers from that origin and the fibers of the inferior head insert into the neck of the condyle on the anterior and medial side (see Fig. 28-3).
The lateral pterygoid muscle has several actions. The inferior head pulls the condyle forward and helps protrude and depress the mandible. The disc is also brought forward because of its attachment to the condyle. When both left and right inferior heads function, the mandible is protruded and depressed. If only one lateral pterygoid is contracted, there will be lateral excursion to the opposite side of the contracted muscle. The superior head of the lateral pterygoid functions primarily in the action of biting or what is sometimes called the power stroke. It functions to guide the posterior movement of the disc and condyle as it goes back to a centric position. In other words, as other muscles are pulling posteriorly, the upper head of the lateral pterygoid is relaxing and controlling that movement (see Fig. 28-3).