The muscles directly involved in the chewing process, which includes elevation, depression, protrusion, retrusion, and lateral movement of the mandible, are referred to as "masticatory" muscles (from the Latin masticatio – chewing, mastication). Functionally, these muscles work in opposition to the muscles responsible for opening the mouth. Mandibular movement is more complex than it may seem. To observe the muscles of mastication in action, please explore our "Skull, Teeth & TMJ" application.
We have four pairs of masticatory muscles. They all originate from the 1st pharyngeal arch during embryonic development and are therefore innervated by the trigeminal nerve*.
Two other masticatory muscles, the medial and lateral pterygoid, are situated on the inner side of the mandible, and their presence is less evident. Nevertheless, the pterygoid muscles play a crucial role in enabling the full range of lower jaw motion and in coordinating the control of the articular disc's position in the temporomandibular joint.
The masseter is the most prominent masticatory muscle. It consists of two clearly defined parts with differently directed fibers1. Some muscle fibers attach to the temporomandibular joint capsule, so the masseter muscle participates in controlling the position of articular disc. Although some authors reveal three parts2 of masseter muscle or propose more complex classification3, here we will adhere to the latest official Anatomical Terminology.
It is worth to mention that fiber direction is similar between neibourgh deep part of the masseter and the temporal muscle. The deep part of the masseter is a kind of transition between the superficial part of masseter and temporal.
This view also demonstrate the limited connection of the deep masseter to the capsule of the temporomandibular joint.
The temporal muscle, in terms of volume, is the largest among the masticatory muscles1. When it works in conjunction with the masseter muscle, these two muscles are primarily responsible for producing biting force2. Muscle fibers are derived from all the bones of the neurocranium, with the exception of the occipital and ethmoid bones. This arrangement gives rise to the distinctive fan-shaped structure of the temporal muscle.
Note the broad, circular, and complex-shaped connection of a muscle to the coronoid process of the mandible.*
Both pterygoid muscles, the medial and lateral, are indeed attached to the pterygoid process of the sphenoid bone. Due to their deep location and the less obvious spatial relationship between their heads, it can be challenging to correctly identify them.
Note the lower head of the lateral pterygoid passing in between the split heads of the medial pterygoid.
Medial Pterygoid Muscle
The fiber direction of the medial pterygoid muscle is similar to that of the masseter when observed from a lateral perspective. I use the 'M,' which is common for 'masseter' and 'medial,' as a mnemonic to differentiate the medial pterygoid from the lateral one.
The medial pterygoid muscle has two heads: the superficial head is smaller and arises from the maxilla, while the deeper head is larger and originates from the pterygoid fossa.
Lateral Pterygoid Muscle
The lateral pterygoid is the smallest among all the masticatory muscles, yet it is the most complex. Notably, it is the only masticatory muscle with nearly horizontally aligned fibers. This muscle not only attaches to the bones but also to the capsule and articular disc of the TMJ. This is the only masticatory muscle with the almost horizontally aligned fibers. Due to its deep location in the infratemporal fossa and its surrounding tissue, anatomical dissection is very difficult.1, 2.
Functionally and anatomically, the muscle is divided into two heads: upper and lower.
Electromyographic studies show that the heads of the muscle are active in different phases of opening and closing the lower jaw1. This data suggests the primary function of the upper head is to control the velocity at which the discocapsular system is pulled back into place during jaw closing movements, while the lower head pulls the mandible and discocapsular system forward during jaw opening movements2. This situation is quite unique, leading some scientists to consider the two heads of the lateral pterygoid as two functionally distinct muscles3.
The fibers of the lower head of the lateral pterygoid run obliquely. This means that when the muscles work bilaterally, they pull the condyles of the mandible forward, which is necessary for protrusion and depression movements of the mandible. However, in the case of unilateral action, the muscle significantly contributes to the lateral movement of the mandible in the contralateral direction. Simultaneously, the upper head adjusts the position of the articular disc in the temporomandibular joint. Any coordination issues between the heads may result in TMJ disorders, causing pain and limitation of the jaw movement, including partial or complete locking of the lower jaw1, 2, 3.