The act of chewing is termed as mastication. The muscles of mastication move the temporomandibular joint and are largely responsible for elevation, depression, protrusion, retraction and lateral motion of the mandible. This process of mechanical breakdown of food is controlled by the brainstem. Some may think that this is already a primitive skill that we have, but in reality this skill must be learned. By learning this skill in the infancy stage, the muscles that are needed for this process are developed, giving us the ability to break down the food that we eat.
The muscles of mastication function bilaterally to effect movements of the single bone or the mandible at two joints. The action of the elevator muscle produces the chewing motion. This action by the elevator muscle is coupled with the contraction of the lateral pterygoid muscle.
The Temporalis and Masseter Muscles
These two muscles are often contracted unconsciously during stress, as evidenced by the clenching of our teeth, giving rise to potentially severe bitemporal and preauricular headaches. The muscles can easily be palpated when contracted. The masseter muscle is probably one of the strongest muscle n the body, if we take its size into consideration. It originates from the zygomatic arch, the lateral part. Each side of the jaw has a masseter muscle and an alteration in the tightness or looseness of this muscle can lead to temporomandibular joint disorder. This powerful muscle can raise the jaw or clench the teeth and figuratively is associated to aggressive or angry states. The temporalis muscle inserts on the internal surface of the coronoid process and can be best palpated at the side of the head.
The medial and lateral pterygoids are in the infrtemporal fossa and cannot be palpated. It is responsible for the elevation or protrusion of the mandible. Their individual function is to move the mandible from side to side. The lateral pterygoid is also partly responsible in our action of opening the lower jaw. The mandibular nerve separates the lateral pterygoid into two separate bellies. Usually these two bellies work together in harmony but in certain conditions, these bellies resonate differently and can contribute to the development of the temporomandibular joint disorder. The clicking sound that patients with TMJ hear comes from the inability of the lateral pterygoid to hold the articular discs in place.
The medial pterygoid muscle attaches itself behind the upper tooth, the last upper tooth to be exact. The medial pterygoid muscle is the counterpart of the masseter muscle. While the masseter works on the outside, the medial pterygoid muscle works on the inside.
The muscles of mastication are all innervated by branches of the fifth cranial nerve or otherwise known as the trigeminal nerve. The muscles of facial expression on the other hand are all supplied by the seventh cranial nerve.