The medial pterygoid is a thick, quadrilateral muscle of mastication

Mastication

Mastication or chewing is the process by which food is crushed and ground by teeth. It is the first step of digestion and it increases the surface area of foods to allow more efficient break down by enzymes. During the mastication process, the food is positioned between the teeth for grinding by...

.

The mandibular branch of the fifth cranial nerve

Cranial nerves

Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...

, the trigeminal nerve

Trigeminal nerve

The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...

, innervates the medial pterygoid muscle.

Origin and insertion

It consists of two heads.

• The bulk of the muscle arises as a deep head from just above the medial surface of the lateral pterygoid plate
  Lateral pterygoid plate
  The lateral pterygoid plate of the sphenoid is broad, thin, and everted; its lateral surface forms part of the medial wall of the infratemporal fossa, and gives attachment to the lateral pterygoid muscle; its medial surface forms part of the pterygoid fossa, and gives attachment to the medial...
  
  .
• The smaller, superficial head originates from the maxillary tuberosity
  Maxillary tuberosity
  At the lower part of the infratemporal surface of the maxilla is a rounded eminence, the maxillary tuberosity, especially prominent after the growth of the wisdom tooth; it is rough on its lateral side for articulation with the pyramidal process of the palatine bone and in some cases articulates...
  
   and the pyramidal process of the palatine bone
  Palatine bone
  The palatine bone is a bone in many species of the animal kingdom, commonly termed the palatum .-Human anatomy:...
  
  .

Its fibers pass downward, lateral, and posterior, and are inserted, by a strong tendinous lamina, into the lower and back part of the medial surface of the ramus

Ramus

Ramus can refer to:* A branch* A portion of a bone , as in the Ramus of the mandible or Superior pubic ramus* A nerve ramus such as the Dorsal ramus of spinal nerve* Petrus Ramus...

 and angle of the mandible, as high as the mandibular foramen

Mandibular foramen

The Mandibular foramen is an opening on the internal surface of the ramus for divisions of the mandibular vessels and nerve to pass.-Contents:...

. The insertion joins the masseter muscle

Masseter muscle

In human anatomy, the masseter is one of the muscles of mastication.In the animal kingdom, it is particularly powerful in herbivores to facilitate chewing of plant matter.-Origin and insertion of the two heads:...

 to form a common tendinous sling which allows the medial pterygoid and masseter to be powerful elevators of the jaw.

Innervation

Like the lateral pterygoid

Lateral pterygoid muscle

The lateral pterygoid is a muscle of mastication with two heads. It lies superiorly to the medial pterygoid.-Origin and insertion:...

, and all other muscles of mastication (apart from buccinator

Buccinator

The buccinator muscle is a muscle at the side of the face.Buccinator may also refer to:* Buccinator artery * Buccinator lymph node* Buccinator nerve * An ancient Roman buccina player...

 which is innervated by the facial nerve (VII)) the medial pterygoid is innervated by the anterior root (motor root) of the mandibular branch of the trigeminal nerve (V). Note: the buccinator muscle is not a muscle of mastication; instead it is classified as a facial muscle.

Actions

Given that the origin is on the medial side of the lateral pterygoid plate and the insertion is from the internal surface of the ramus of the mandible down to the angle of the mandible

Angle of the mandible

At the junction of the lower border of the ramus of the mandible with the posterior border is the angle of the mandible, which may be either inverted or everted and is marked by rough, oblique ridges on each side, for the attachment of the Masseter laterally, and the Pterygoideus internus medially;...

, its functions include:

• Elevation of the mandible (closes the jaw)
• Minor contribution to protrusion of the mandible
• Assistance in mastication
• Excursion of the mandible; contralateral excursion occurs with unilateral contraction.