THE MUSCLES OF MASTICATION
Embryologically, the muscles of mastication develop from
the first pharyngeal arch. Consequently they are innervated by a branch of the trigeminal nerve (CN V), the mandibular nerve.
The muscles of mastication
are a group of muscles associated with movements of the jaw (temporomandibular joint). They are one of the major muscle groups in the head - the other being the muscles of facial expression. There are four muscles of mastication - the masseter, temporalis, medial pterygoid and lateral pterygoid.
Lateral Pterygoid
has a triangular shape, with two heads; superior and inferior. It has horizontally orientated muscle fibres, and thus is the major protractor of the mandible. Attachments: The superior head originates from the greater wing of the sphenoid. The inferior head originates from the lateral pterygoid plate of the sphenoid. The two heads converge into a tendon, which attaches to the neck of the mandible. Actions: Acting bilaterally, the lateral pterygoids protract the mandible, pushing the jaw forwards. Unilateral action produces the 'side to side' movement of the jaw. Innervation: Mandibular nerve (V3).
Masseter
is the most powerful muscle of mastication. It is quadrangular in shape, and can be split into two parts; deep and superficial. The entirety of the muscle lies superficially to the pterygoids and temporalis, covering them. Attachments: The superficial part originates from maxillary process of the zygomatic bone. The deep part originates from the zygomatic arch of the temporal bone. Both parts attach to the ramus of the mandible. Actions: Elevates the mandible, closing the mouth. Innervation: Mandibular nerve (V3).
Medial Pterygoid
muscle has a quadrangular shape, with two heads; deep and superficial. It is located inferiorly to the lateral pterygoid. Attachments: The superficial head originates from the maxilla. The deep head originates from the lateral pterygoid plate of the sphenoid bone. Both parts attach to the ramus of the mandible, near the angle of mandible. Actions: Elevates the mandible, closing the mouth. Innervation: Mandibular nerve (V3).
Temporalis
originates from the temporal fossa - a shallow depression on the lateral aspect of the skull. The muscle is covered by tough fascia which can harvested surgically and used to repair a perforated tympanic membrane (an operation known as a myringoplasty). Attachments: Originates from the temporal fossa. It condenses into a tendon, which inserts onto the coronoid process of the mandible. Actions: Elevates the mandible, closing the mouth. Also retracts the mandible, pulling the jaw posteriorly. Innervation: Mandibular nerve (V3).