Medial rectus muscle

Origin and insertion

Medial rectus is the shortest but strongest of the four recti. It originates from the medial part of the common tendinous ring, between the superior and inferior recti, and the adjacent surface of the dura mater (meninges) that wraps the optic nerve (CN II).

The muscle courses anteriorly, sliding over the medial part of the eye to cross its equator and reach the anterior half of the eyeball. It then inserts onto the medial side of the eyeball, around 5 millimeters posterior to corneoscleral junction. The inserting muscle fibers pierce the fascial sheath of the eyeball (Tenon’s capsule), which in turn reflects back and creates a thin fascial sleeve around the muscle’s tendon. This sleeve gives off an expansion called the medial check ligament that connects the medial rectus muscle with the medial wall of orbit.

Relations

Medial rectus muscle runs cushioned in the periorbital fat tissue, superior to the floor of the orbit and inferior to the superior oblique muscle, ophthalmic artery and nasociliary nerve. The lateral surface of this muscle encloses a triangular space with the posteromedial surface of the eye and optic nerve. The inferior branch of the oculomotor nerve traverses this space in order to reach and penetrate the lateral surface of the muscle. 

Innervation

This muscle is innervated by the inferior branch of oculomotor nerve (CN III). These fibers come from the nucleus of oculomotor nerve which is a somatic motor nucleus, thus imposing voluntary control over this muscle.

Blood supply

Blood supply to medial rectus comes from the ophthalmic artery, a branch of the internal carotid artery.

Function

When contracting, each medial rectus muscle adducts the eyeball, i.e. pulls the eye medially. This action is important in two types of ocular movements; conjugate and disconjugate. 

Conjugate movements are when both eyeballs move in the same direction. Medial rectus takes part in conjugate movements of the eyes in a horizontal plane (i.e. left and right), working together with lateral rectus. For directioning the gaze to one side, medial and lateral recti have to function synchronously;

• Ipsilateral medial rectus relaxes, ipsilateral lateral rectus contracts. This sequence pulls the ipsilateral eye toward the desired direction.
• Contralateral medial rectus contracts, contralateral lateral rectus relaxes. This sequence pulls the contralateral eye toward the desired direction.

As lateral rectus is supplied by the abducens nerve (CN VI), the synchrony between these muscles is established by the medial longitudinal fasciculus that connects the oculomotor and abducens nuclei.

Disconjugate movements are when the eyeballs converge or diverge from the midline. When both medial recti contract, they converge the eyeballs toward the midline. Convergence is a part of the accommodation reflex, when the eyes adjust to observe a close object.