The lateral pterygoid muscle has upper and lower bellies that are separated by the epimysial tissue. The upper head originates from the infratemporal surface of the greater wing of the spheroid bone. The lower head originates from the lateral surface of the lateral pterygoid plate. Insertion points for the upper head are both into the disc through an orifice of the external capsule as well as into the upper part of the fovea of the anterior part of the condyle. The inferior belly inserts entirely into the anterior neck of the condyle. The upper and lower bellies have distinctly different electromyographic activity. On mandibular opening, the inferior head contracts and the superior head maintains resting activity. On closure of the mandible, the superior head contracts while the inferior is at rest. Functionally, then, it is the superior head that may influence discal position. Hence, because the superior head inserts both into the condyle and into the disc, it is felt to function by making rapid adjustments in the relative positioning of the disc and condyle during closure. Thus, the relative alignment of the disc over the condyle becomes more than merely mechanically dictated by the shape of the anterior and posterior bands of the disc, and it may, in fact, be under more precise neurologic control by the central nervous system.
Muscles function by moving the insertion point closer to the origin. Therefore, in the TMJ, the condyle and disc will move anteriorly and medially toward the infratemporal fossa and lateral pterygoid plate when the lateral pterygoid muscle is in a foreshortened state. The most common form of muscle foreshortening is through contraction. In fact, all muscles have underlying electrical activity that allows an average number of myofibrils to be contractile at any point in time. This defines a "resting length" of the muscle.
However, the resting length of a muscle is also defined by the laxity of its fascial tissues. The upper and lower bellies of the lateral pterygoid muscle each have an outer-connective tissue layer of epimysium and perimysium, and collagenous septa divide the muscle fascicles. The reticular layer around the individual muscle fibers makes up the endomysium. All muscles must on occasion be stretched in order to maintain the laxity of fibrous connective tissue layers.
Before leaving the muscle, it is important to consider the vascular network that feeds the lateral pterygoid muscle as well as that blood supply that courses through the muscle. The maxillary artery arises from the external carotid artery and immediately gives rise to feeder vessels into the inferior half of the lateral pterygoid muscle. More anteriorly, the maxillary artery initially lies superficial to the inferior belly; toward the origin of the muscle, the artery dives between the two bellies. Therefore, anteriorly, the artery will eventually supply branches to the pterygopalatine fossa. More posteriorly, there are two important dependent areas of maxillary artery blood flow. Crossing first inferiorly and then medially to the lateral pterygoid is the middle meningeal artery in its extracranial course just before it enters the foramen spinosum at the medial part of the articular eminence of the TMJ. Hence, patency of this vessel may be influenced by the posture of the TMJ structures. Second, the posterior blood supply from the maxillary artery must also eventually perfuse at the mandibular condyle. Those branches that perfuse the head of the condyle course through the superior belly of the lateral pterygoid muscle and perfuse the cortical bone to supply the underlying marrow. The neck of the condyle receives its blood supply from vessels that must pass through the upper and lower bellies of the lateral pterygoid muscles.