Medial pterygoid muscle

Origin and insertion

Medial pterygoid muscle consists of two heads; superficial and deep, that are separated by the inferior head of lateral pterygoid muscle at their origin. 

• The deep part forms the bulk of the muscle and arises from the medial surface of the lateral pterygoid plate of sphenoid bone and from the pyramidal process of the palatine bone.
• The smaller, superficial part arises from the maxillary tuberosity and the grooved surface of the pyramidal process of palatine bone.

As these two sets of fibers descend posterolaterally, they insert to the triangular impression found on the medial surface of the ramus and angle of mandible. The fibers attach via a strong tendinous lamina that extends from the inferior alveolar (a.k.a. mandibular) foramen superiorly, to the mylohyoid groove anteroinferiorly.

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Relations

Medial pterygoid muscle is located in the infratemporal fossa lying deep to masseter and temporalis muscles and medial to lateral pterygoid muscle. 

The outer surface of the muscle lies against the inner surface of mandible, from which it is separated by the lateral pterygoid muscle, sphenomandibular ligament, maxillary artery, mandibular nerve and its lingual and inferior alveolar branches. Near its insertion, the outer surface of the medial pterygoid muscle is related to the process of the parotid gland. 

The inner surface of the muscle is in relation to tensor veli palatini, styloglossus and salpingopharyngeus muscles. The latter two muscles separate the medial pterygoid muscle from the superior pharyngeal constrictor. Near its insertion, the medial pterygoid muscle is medially related to the lateral surface of the submandibular gland, and the facial artery, which descends between these two structures. 

Innervation

Medial pterygoid muscle is innervated by the medial pterygoid branches of mandibular nerve (CN V3), one of the three divisions of trigeminal nerve (CN V).

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Blood supply

Medial pterygoid muscle receives blood supply from pterygoid and buccal branches of maxillary artery. To a lesser extent, ascending palatine artery and muscular branches of facial artery also contribute to the blood supply of this muscle.

Function

Due to its posteroinferior direction and the fact that its mandibular attachment is always mobile, medial pterygoid muscle can exhibit several actions:

• Unilateral contraction slightly medially rotates the mandible. When this action occurs simultaneously with the contraction of the ipsilateral lateral pterygoid, it results with a noticeable movement; same sided part of mandible swings anteriorly and medially. In addition, alternating contraction of the medial pterygoid and lateral pterygoids causes side-to-side movements of the mandible.
• Bilateral contraction elevates the mandible. This action, when combined with bilateral contraction of the lateral pterygoids protrudes the mandible.

Medial pterygoid usually acts together with other masticatory muscles. Therefore, its actions combine with those of other masticators to facilitate chewing and food grinding between the maxillary and mandibular teeth.