Superior tarsal muscle
Superior tarsal (Muller's muscle) is a small muscle found within the superior eyelid. It is a smooth muscle, but it is considered as a structural component of the larger skeletal muscle of the eyelid; levator palpebrae superioris. It extends from the deep aspect of levator palpebrae superioris to the upper margin of the superior tarsal plate of the eyelid.
Being a smooth muscle, the superior tarsal is controlled by the autonomic nervous system, precisely its sympathetic division. The main function of this muscle is to elevate the upper eyelid in the state of sympathetic predominance, such as excitement, fear or surprise. Moreover, the static tone of the muscle also contributes to maintaining the eyes open and keeping the size of palpebral fissure constant in the state of awakeness.
|Origin||Deep surface of levator palpebrae superioris muscle|
|Insertion||Superior tarsal plate of the eyelid|
|Action||Elevates and retracts the upper eyelid|
|Innervation||Sympathetic nervous system (via internal carotid plexus)|
|Blood supply||Ophthalmic artery|
In this article, we will discuss the anatomy and function of the superior tarsal muscle.
Superior tarsal is a smooth muscle of the upper eyelid. It originates from the deep surface of levator palpebrae superioris and inserts inferiorly, to the superior tarsal plate of the upper eyelid.
The superior tarsal muscle receives its innervation from the sympathetic nervous system. More specifically, the postganglionic sympathetic fibers stem from the superior cervical ganglion and form the internal carotid plexus around the cervical segment of internal carotid artery. Following the petrous and cavernous segments of internal carotid artery, these nerve fibers enter the skull and traverse the cavernous sinus. Finally, the nerve fibers for the superior tarsal muscle access the orbit in a form of a tight nervous plexus wrapped around the ophthalmic artery, which is a branch of internal carotid artery. Note that the ophthalmic artery provides the blood supply for the superior tarsal muscle.
The main action of the superior tarsal muscle is to elevate the upper eyelid in the state of sympathetic predominance. The function of this action is to enable a wider visual field in states of acute life-threatening situations (fight or flight response). In addition, the muscle has a static tone, which adds up to the tone of the levator palpebrae superioris muscle. The tones of these two muscles are in balance with the opposing tone of orbicularis oculi, thereby maintaining the eyes open and defining the size of palpebral fissure.
Learn more about the structures of the upper eyelid with our articles, video tutorials, quizzes and atlas images.
Horner's syndrome occurs when the sympathetic nerve supply to the face and eye is damaged. This condition usually occurs due to trauma of the neck and shoulder regions and consequential injury of the cervical sympathetic ganglia and/or nerves. Injuries of this kind will result with a loss of effective innervation of the smooth muscles and glands in the head. Horner's syndrome is characterized by the triad of symptoms; partial ptosis (drop of the upper eyelid), miosis (constricted pupil) and loss of hemifacial sweating (anhidrosis). Sometimes, it can also be followed by the posterior displacement of the eyeball within the orbit (enophthalmos).