The masticatory muscles consist of four paired muscles that are part of the skeletal musculature and are called musculi masticatorii in medical terminology. They move the lower jaw and enable chewing and grinding movements.
What is the masticatory musculature?
The masseter, temporalis, medial pterygoid and lateral pterygoid muscles belong to the masticatory muscles. They are each present on both sides of the skull. Other muscles are involved in the process of mastication, such as various facial muscles and the muscles of the tongue and floor of the mouth, but these are not included in the masticatory muscles. The largest muscle is the temporalis muscle. It originates at the temporal bone and attaches to the lower jaw. It closes the jaw and can pull it backward. The masseter muscle (Musculus masseter) is also involved in the closing movement of the jaw, but additionally allows grinding movements. The internal (Musculus pterygoideus medialis) and external wing muscles (Musculus pterygoideus lateralis) close the jaw, allow grinding movements, and when used unilaterally, they move the jaw in a lateral direction. All muscles of the masseter are innervated by branches of the mandibular nerve, one of the main branches of the 5th cranial nerve (trigeminal nerve).
Anatomy and structure
The masseter muscles are paired, with four on each side of the skull. The largest and strongest is the temporal muscle. It originates at the temporal fascia and temporal fossa and attaches to the coronoid process of the mandible. It is innervated by the deep temporal nerves (Nervi temporales profundi), a branch of the mandibular nerve. The masseter muscle belongs to the pinnated muscles and consists of a deep (pars profunda) and a superficial part (pars superficialis). The deep part originates at the posterior third of the zygomatic arch, while the superficial part comes from the anterior two-thirds. The attachments of the masseter muscle are the outer part of the mandibular angle (angulus mandibulae) and a rough area on the mandible, the tuberositas masseterica. The nervus massetericus, also a branch of the nervus mandibularis provides innervation to this muscle. The internal wing muscle originates at a depression at the base of the skull, the pterygoid fossa, and attaches to the inner surface of the mandible at the pterygoid tuberosity. It is innervated by the medial pterygoid nerve. The external wing muscle is a two-headed skeletal muscle. While the upper muscle head originates at the greater sphenoid wing (Ala major), the lower head originates at a bony process of the sphenoid bone, the pterygoid process. The external wing muscle is innervated by the lateral pterygoid nerve.
Function and Tasks
The very powerful temporal muscle provides nearly 50% of the force required for masticatory movement. It can close the jaw (adduction of the jaw) as well as advance it (protrusion) and retract it (retrusion). For adduction, mainly the vertical muscle fibers are used, while for protrusion and retrusion, mainly the horizontal fibers are active. If the temporal muscle is only used on one side, a lateral displacement of the mandible occurs (laterotrusion). The masseter muscle is also involved in jaw closure. It also raises the lower jaw and can pull it forward. In addition, this muscle helps maintain the tension of the temporomandibular joint capsule. The internal wing muscle assists the masseter muscle in closing the jaw. However, because it is narrower, it can only exert half as much force. If it contracts, the jaw not only closes but also moves forward. With a unilateral contraction, it shifts the lower jaw to the side, which means it makes grinding movements possible. The external wing muscle has a special position among the masseter muscles, because it initiates the opening of the mouth. This movement is taken over and continued by the suprahyoid muscles of the floor of the mouth. This muscle is also involved in advancing the jaw and in grinding movements.
Diseases
Common complaints include pain when chewing or clicking and crunching sounds. They are usually caused by tense chewing muscles. These tensions can occur either due to strong active tension, as in anxiety or anger attacks, or they occur due to malocclusion.When the bite is in the correct position, the temporomandibular joints, bones and muscles work together harmoniously, whereas a malocclusion can lead to uneven loading and thus to excessive tension in the chewing muscles. Nighttime grinding or prolonged dental procedures can also cause painful muscular tension. Often, the pain spreads further and radiates into the teeth or head, mistakenly suggesting the cause is somewhere other than the muscles. Pain in the muscles of mastication is called craniomandibular dysfunction (CMD) or temporomandibular disorders (TMD). Treatment is based on the cause. If there is a malocclusion, it is corrected as far as possible. Against nocturnal grinding, the dentist fits a so-called grinding splint, which is intended to prevent the teeth from grinding against each other. Another disorder of the jaw muscles is lockjaw. In this case, it is no longer possible to open the mouth due to severe muscle spasms. This spasm of the masticatory muscles is also called trismus. Different degrees are distinguished, which are based on the distance between the edges of the front teeth of the upper and lower jaw. In grade I, the restriction of the opening is minimal; in grade II, the distance between the edges of the teeth is about 10 mm; and grade III allows an opening of only 1 mm.
