The Müller muscle, also known as the superior tarsal muscle, is a smooth muscle located in the upper eyelid that assists in elevating the eyelid.
Here's a more detailed breakdown:
-
Location: The Müller muscle is positioned above the levator palpebrae superioris muscle (the main muscle responsible for lifting the eyelid) and is closely associated with the conjunctiva.
-
Function: Its primary function is to provide additional lift to the upper eyelid. It contributes approximately 1-2mm of eyelid elevation. This is crucial for maintaining an open and alert appearance.
-
Innervation: The Müller muscle is innervated by the sympathetic nervous system. This means that its contraction is involuntary and influenced by the body's "fight or flight" response. Stimulation of the sympathetic nervous system can cause the muscle to contract, widening the eyes.
-
Clinical Significance:
-
Ptosis: Damage or dysfunction of the sympathetic nervous system can lead to drooping of the upper eyelid (ptosis). Horner's syndrome, for instance, which involves disruption of the sympathetic pathway, often presents with ptosis due to impaired function of the Müller muscle.
-
Thyroid Eye Disease (Graves' Orbitopathy): In some cases, inflammation and fibrosis can affect the Müller muscle, contributing to eyelid retraction (the eyelid being pulled upwards too much).
-
Surgical Procedures: The Müller muscle is sometimes surgically targeted in procedures to correct ptosis or eyelid retraction. Müller muscle-conjunctival resection (MMCR) is a common surgical technique to correct ptosis by shortening the muscle.
-
In summary, the Müller muscle is a vital accessory muscle in the upper eyelid that plays a significant role in eyelid elevation and is influenced by the sympathetic nervous system. Its dysfunction can result in noticeable clinical symptoms such as ptosis.
