Levator palpebrae superioris muscle

Origin and insertion

Levator palpebrae superioris is a triangular muscle that extends along the roof of the orbit, from the apex of orbit to the superior eyelid. It originates with a short and narrow tendon from the inferior aspect of the lesser wing of sphenoid bone, superior and anterior to the common tendinous ring. The muscle belly gradually widens as it courses anteriorly toward the eyelid. The muscle fibers penetrate the upper eyelid, inserting to its parts via two aponeurotic fascicles;

• Deep fibers attach to the anterior surface of the superior tarsus 
• Superficial fibers radiate through the eyelid and orbicularis oculi to finally attach to the skin of the superior eyelid. 

The most lateral fibers of the muscle’s aponeurosis attach to the orbital tubercle of zygomatic bone, whereas the most medial fibers attach to the medial palpebral ligament.

Some authors recognize another component of the levator palpebrae superioris called the superior tarsal muscle. It is a small slip of smooth muscle that attaches to the underside of the levator palpebrae superioris near its insertion point.

Relations

Levator palpebrae superioris lies superior to the superior rectus muscle, which separates it from the eyeball. Both muscles are enclosed by connective tissue sheaths which are fused along their related surfaces. The sheaths are separated only in their anterior parts at the level in which the tendons of these muscles separate. The interval between them is filled by a thick connective tissue plate onto which the superior conjunctival fornix attaches. In addition, the levator palpebrae superioris is attached to the check ligament of the superior rectus. All of these connections contribute to the coordinated simultaneous contraction of these two muscles.
 
As the levator palpebrae superioris fans out as a wide aponeurosis, the lateral fibers pass between the orbital and palpebral parts of the lacrimal gland to attach to the orbital tubercle. The medial fibers run adjacent over the reflected tendon of superior oblique muscle.

Innervation

Levator palpebrae superioris receives somatic motor innervation from the superior division of oculomotor nerve (CN III). Superior tarsal muscle receives sympathetic innervation from the carotid plexus whose fibers join the oculomotor nerve while it passes through the cavernous sinus.

Blood supply

Levator palpebrae superioris receives arterial blood supply from the internal carotid artery, via ophthalmic artery and its supraorbital branch.

Function

The main actions of the levator palpebrae muscle are the retraction and elevation of the superior eyelid and widening of the palpebral fissure. These actions are limited by several anatomical features of the muscle; 

• Lateral and medial attachments of the muscle’s aponeurosis which prevent further elevation of the eyelid once they are stretched maximally
• Orbital septum, which is a fibrous part of eyelid that when compressed maximally, prevents further retraction of the eyelid.
• Antagonistic actions of the orbicularis oculi, that counteracts the levator palpebrae superioris since it pulls the eyelid in the opposite direction. 

Levator palpebrae superioris aids the superior rectus and elevates the eyelid when the gaze is directed superiorly. This happens automatically because the fascial sheaths of the levator palpebrae superioris and superior rectus are fused via check ligament; this is why when the superior rectus contracts, levator palpebrae superioris follows. Levator palpebrae superioris has a static tone that is in balance of the opposing tone of orbicularis oculi, thereby maintaining the eyes open and defining the size of palpebral fissure.

Being innervated by the sympathetic nervous system, superior tarsal muscle elevates the eyelid in states of a “flight or fight” response. Hence, this muscle causes additional widening of the palpebral fissure during moments of excitement, fear, surprise and others.