MUSCLES OF MASTICATION

The muscles of mastication are masseter, temporalis, medial and lateral pterygoid. These muscles are innervated by the mandibular division of trigeminal nerve.

• Masseter is involved in elevation and protrusion of the mandible.

• Temporalis elevates the mandible, chiefly with its vertical fibers, retracts the protruded mandible with its horizontal posterior fibers. Its unilateral contraction helps in mastication.

• Medial pterygoid elevates the mandible.

• Lateral pterygoid: Its bilateral contraction initiates opening of the mouth by protruding the mandible and moving the articular disk forward. Its unilateral contraction elevates the mandible to the opposite side during mastication.

Branches from the anterior division of the mandibular nerve are the buccal nerve, the nerve to masseter, the deep temporal nerves to the temporalis muscle and the nerve to the lateral pterygoid. The nerve to the medial pterygoid is a branch from the main trunk of the mandibular nerve.

Posterior belly of digastric

• The posterior belly of the digastric (latin: venter posterior musculi digastrici) is the largest of the two bellies of the digastric muscle, and it develops from the second pharyngeal arch.

Origin

• The posterior belly originates from the mastoid notch medial to the mastoid process on the temporal bone.

Insertion

• From the origin site the posterior belly of the digastric stretches forward and becomes or inserts into the intermediate tendon, which attaches to the hyoid bone.

Action

• Upon activation the posterior belly of the digastric elevates the hyoid bone.

Innervation

• The posterior belly of the digastric muscle receives nerve supply from the digastric branch of the facial nerve (CN VII).

Buccinator Muscle 

• Muscle of the cheek.
• The buccinator forms the muscular component of the cheek and is used every time air expanding the cheeks is forcefully expelled.
• It is in the space between the mandible and the maxilla, and is quite deep to the other facial muscles in the area.

 Origin

• The buccinator arises from the posterior part of the maxilla and mandible opposite the molar teeth and the pterygomandibular raphe, which is a tendinous band between the pterygoid hamulus superiorly and the mandible inferiorly and is a point of attachment for the buccinator and superior pharyngeal constrictor muscles.

Insertion

• Motor supply is from the buccal branch of the facial nerve (Cranial Nerve VII).
• Sensory supply is from the buccal branch which is one of the muscular branches of the trigeminal nerve (Cranial Nerve V).

Action

• Aids mastication, tenses cheeks in blowing and whistling, aids closure of mouth

• Temporalis muscle: The temporalis muscle elevates the mandible.
• Masseter muscle: It elevates the mandible. It works functionally with the temporalis and pterygoid muscles to move the mandible at the TMJ, it is a muscle of mastication.
• Lateral pterygoid muscle: Contraction causes the mandibular condyle and the articular disc to move anteriorly, resulting in both protraction and depression of the mandible. It works synergistically with the medial pterygoid muscle to move the mandible from side to side.
• Medial pterygoid muscle: It elevates the mandible and moves it from side to side.

The reflex arc of masseteric reflex involves two nuclei of the trigeminal nerve, namely, the mesencephalic nucleus and the motor nucleus of trigeminal nerve .

Lateral Pterygoid

• The primary function of the lateral pterygoid muscle is to pull the head of the condyle out of the mandibular fossa along the articular eminence to protrude the mandible.
• A concerted effort of the lateral pterygoid muscles acts in helping lower the mandible and open the jaw whereas unilateral action of a lateral pterygoid produces contralateral excursion, usually performed in concert with the medial pterygoids
• Unlike the other three muscles of mastication, the lateral pterygoid is the only muscle of mastication that assists in depressing the mandible (opening the jaw).
• At the beginning of this action it is assisted by the digastric,mylohyoid and geniohyoid muscles.

Depression of the Mandible

* Depression of the mandible is brought about by contraction of the digastrics, the geniohyoids, and the mylohyoids; the lateral pterygoids play an important role by pulling the mandible forward.

Elevation of the Mandible

* Elevation of the mandible is brought about by contraction of the temporalis, the masseter, and the medial pterygoids. The head of the mandible is pulled backward by the posterior fibers of the temporalis.

Protrusion of the Mandible

* In protrusion, the lower teeth are drawn forward over the upper teeth, which is brought about by contraction of the lateral pterygoid muscles of both sides, assisted by both medial pterygoids.

Refraction of the Mandible

* The articular disc and the head of the mandible are pulled back into the mandibular fossa. Retraction is brought about by contraction of the posterior fibers of the temporalis.

Lateral Chewing Movements

* These are accomplished by alternately protruding and retracting the mandible on each side. For this to take place, a certain amount of rotation occurs, and the muscles responsible on both sides work alternately like turning the chin to the left side produced by left lateral pterygoid and right medial pterygoid and vice versa.

Lateral Pterygoid

• The primary function of the lateral pterygoid muscle is to pull the head of the condyle out of the mandibular fossa along the articular eminence to protrude the mandible.

• Temporalis muscle: The temporalis muscle elevates the mandible.
• Masseter muscle: It elevates the mandible. It works functionally with the temporalis and pterygoid muscles to move the mandible at the TMJ, it is a muscle of mastication.
• Lateral pterygoid muscle: Contraction causes the mandibular condyle and the articular disc to move anteriorly, resulting in both protraction and depression of the mandible. 
• Medial pterygoid muscle: It elevates the mandible and moves it from side to side