Logan Steele
Müller's muscle, also known as the superior tarsal muscle, is a smooth muscle that originates from the undersurface of the levator palpebrae superioris muscle and inserts onto the superior tarsal plate of the upper eyelid. The muscle is innervated by the sympathetic nervous system, specifically by postganglionic sympathetic fibers that originate in the superior cervical ganglion. These fibers travel through the internal carotid plexus and communicate with the oculomotor nerve before innervating the superior tarsal muscle. While the exact function of Müller's muscle is not yet fully understood, it is believed to play a role in elevating the upper eyelid and adjusting its position in response to changes in light intensity and pupil size.
| Characteristics | Values |
|---|---|
| Muscle Type | Smooth muscle |
| Muscle Width | 15 mm |
| Muscle Length | 10 mm |
| Muscle Thickness | 0.1–0.5 mm |
| Muscle Function | Allows for the retraction and elevation of the upper eyelid |
| Innervation Type | Sympathetic innervation |
| Innervation Source | Superior cervical ganglion |
| Vascular Supply | Superior branches of the ophthalmic artery |
| Nerve Fibers | Postganglionic sympathetic fibers |
| Nerve Pathway | Via the internal carotid plexus |
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What You'll Learn
- The superior tarsal muscle is innervated by the sympathetic nervous system
- The role of the muscle is to elevate the upper eyelid
- The muscle is supplied blood by the ophthalmic artery
- Injury to the innervation of the superior tarsal muscle can cause ptosis
- The precise course of the postganglionic fibres is unknown
The superior tarsal muscle is innervated by the sympathetic nervous system
The superior tarsal muscle, also known as Müller's muscle, is an accessory smooth muscle that allows for the retraction and elevation of the upper eyelid. This muscle is innervated by the sympathetic nervous system.
Müller first described the superior tarsal muscle in 1858. It is a smooth muscle measuring about 10 mm long, 15 mm wide, and 0.1–0.5 mm thick. The superior tarsal muscle originates from the undersurface of the levator palpebrae superioris muscle (LPSM) and inserts onto the superior tarsal plate of the upper eyelid. The superior tarsal plate is made of dense connective tissue and supports the upper eyelid, helping to maintain its shape.
The superior tarsal muscle has a unique function in assisting the levator palpebrae superioris by maintaining upper eyelid elevation after being raised by the levator palpebrae superioris. It also lifts the upper eyelid by an additional 2 mm beyond the range of the levator palpebrae superioris, following a sympathetic nervous system response. This muscle receives a sympathetic innervation derived from the cervical sympathetic chain via third neuron fibres that pass from the carotid plexus.
The precise course of these postganglionic fibres remains unclear. However, it has been suggested that they follow the motor nerve fibres of the levator palpebrae superioris muscle and the internal carotid and ophthalmic arteries or join the ophthalmic division of the trigeminal nerve. The superior tarsal muscle is often affected in thyroid eye disease-associated ophthalmopathy as the disease-mediated inflammation and fibrosis occurring within the muscle cause further upper lid retraction.
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The role of the muscle is to elevate the upper eyelid
The superior tarsal muscle, also known as Müller's muscle, is a smooth muscle that originates from the undersurface of the levator palpebrae superioris muscle (LPSM) and inserts onto the superior tarsal plate of the upper eyelid. The superior tarsal plate is made of dense connective tissue and functions to support the upper eyelid and maintain its shape.
Müller's muscle is innervated by the sympathetic nervous system, specifically by postganglionic sympathetic fibres that originate in the superior cervical ganglion. These fibres travel via the internal carotid plexus, where they communicate with the oculomotor nerve as it passes through the cavernous sinus. The exact course of these postganglionic fibres is not yet fully understood.
Müller's muscle has a well-established role in elevating the upper eyelid. It assists the levator palpebrae superioris muscle by maintaining upper eyelid elevation after it has been raised. The superior tarsal muscle also lifts the upper eyelid an additional 2 mm beyond the range of the levator palpebrae superioris, in response to the sympathetic nervous system. This additional elevation is observed when an individual is alert or startled, as opposed to when they are fatigued or sleepy.
The precise function and purpose of Müller's muscle have been a subject of debate and are not yet clearly defined. One hypothesis suggests that the muscle adjusts the upper eyelid's position in response to changes in pupil size caused by varying light intensity. This hypothesis is supported by observations of slight changes in eyelid height in different light conditions, particularly noticeable with high magnification and an infrared camera.
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The muscle is supplied blood by the ophthalmic artery
The superior tarsal muscle, also known as Müller's muscle, is a smooth muscle that allows for the retraction and elevation of the upper eyelid. It is innervated by the sympathetic nervous system, specifically by postganglionic sympathetic fibres that have synapses in the superior sympathetic cervical ganglia. These fibres wrap around the internal carotid artery, forming the internal carotid plexus, and enter the skull. From here, the plexus fibres reach the orbit via the ophthalmic artery and subsequently innervate the superior tarsal muscle.
The ophthalmic artery is a key vessel that supplies blood to the structures of the eye and surrounding region. It arises from the internal carotid artery, which is one of the main blood vessels supplying the brain and head region. The ophthalmic artery then branches off into several smaller arteries, each supplying different parts of the eye and associated structures.
Müller's muscle, in particular, receives its vascular supply from the superior branches of the ophthalmic artery. These superior branches provide blood flow not only to Müller's muscle but also to several other important structures in the eye, including the levator muscle, superior rectus, superior oblique, and lateral rectus muscles.
The ophthalmic artery also has a medial muscular branch that supplies blood to the extraocular muscles not supplied by the superior branches. This medial branch is important for the function of these muscles, which play a crucial role in eye movement and coordination.
The blood supply provided by the ophthalmic artery is essential for the health and function of Müller's muscle. The artery delivers oxygenated blood, providing the necessary oxygen and nutrients for the muscle to function properly. Any disruption to the blood supply, such as through injury or disease, can have significant impacts on the muscle's ability to contract and relax, ultimately affecting the movement of the upper eyelid.
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Injury to the innervation of the superior tarsal muscle can cause ptosis
The superior tarsal muscle, also known as Müller's muscle, is a smooth muscle that originates from the undersurface of the levator palpebrae superioris muscle (LPSM) and inserts onto the superior tarsal plate of the upper eyelid. The superior tarsal muscle is innervated by the sympathetic nervous system, specifically by postganglionic sympathetic fibres that originate in the superior cervical ganglion. These fibres travel via the internal carotid plexus and communicate with the oculomotor nerve before entering the superior tarsal muscle on its inferior aspect.
Müller's muscle serves as an accessory upper eyelid retractor, allowing for the retraction and elevation of the upper eyelid. It assists the levator palpebrae superioris muscle in maintaining upper eyelid elevation and can lift the eyelid an additional 2 mm beyond the range of the levator palpebrae superioris. This function is particularly important in response to changes in light intensity, as it helps to adjust the upper eyelid's position to variations in pupil size.
Injury to the innervation of the superior tarsal muscle can indeed cause ptosis, or a drooping eyelid. This condition is often observed in individuals with Horner's syndrome, a disorder characterised by oculosympathetic paresis. The ptosis seen in Horner's syndrome is typically mild, ranging from 2 to 3 mm, and is a result of sympathetic denervation.
The superior tarsal muscle is routinely addressed in oculoplastic surgery, particularly in procedures aimed at correcting ptosis. One common approach is the Müller muscle-conjunctival resection, which involves removing a portion of the superior tarsal muscle to remedy the ptosis. Other surgical techniques, such as external levator advancement, are also employed to repair ptosis and improve eyelid function.
While the primary function of Müller's muscle relates to eyelid elevation, its precise physiologic purpose remains a subject of debate. Some studies suggest that it plays a secondary role in maintaining the tone of the eyelids, while others propose that it may have a more significant contribution to levator action than previously thought. Further research is needed to fully elucidate the normal physiologic function and mechanisms of Müller's muscle.
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The precise course of the postganglionic fibres is unknown
The superior tarsal muscle, also known as Müller's muscle, is an accessory smooth muscle that allows for the retraction and elevation of the upper eyelid. It receives its innervation from the sympathetic nervous system. Postganglionic sympathetic fibres originate in the superior cervical ganglion and travel via the internal carotid plexus, where they communicate with the oculomotor nerve. These fibres then continue to the superior division of the oculomotor nerve, where they enter the superior tarsal muscle.
Müller's muscle is believed to play a role in adjusting the upper eyelid's position in response to changes in pupil size and light intensity. However, its precise physiologic function and purpose remain unclear and are a subject of debate. While it is known that the muscle receives sympathetic innervation, the exact course of these postganglionic fibres is not fully understood.
It has been suggested that the postganglionic fibres may follow the motor nerve fibres of the levator palpebrae superioris muscle and the internal carotid and ophthalmic arteries. Alternatively, they may join the ophthalmic division of the trigeminal nerve. However, further research is needed to confirm the precise pathway of these fibres.
The role of Müller's muscle in eyelid movement is complex and may involve multiple mechanisms. It is proposed that the muscle assists in maintaining the tone of the eyelids, working in conjunction with the levator aponeurosis to transmit movement from the LPS muscle to the tarsus. This results in a slight elevation of the eyelid, typically observed when an individual is fatigued or sleepy.
In summary, while the general pathway of the postganglionic fibres innervating Müller's muscle is understood, the precise course and specific mechanisms involved remain a subject of ongoing investigation. The function and purpose of Müller's muscle are not yet clearly defined, and further research is necessary to fully comprehend the role of this muscle in eyelid movement and adjustment to changing light conditions.
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Frequently asked questions
Müller's muscle, also known as the superior tarsal muscle, is a smooth muscle that allows for the retraction and elevation of the upper eyelid.
The superior tarsal muscle receives innervation from the sympathetic nervous system. More specifically, it is innervated by postganglionic sympathetic fibres that originate in the superior cervical ganglion.
The function of Müller's muscle is not entirely clear, but it is believed to be an accessory muscle that helps to raise the upper eyelid. It may also serve to adjust the upper eyelid's position in response to changes in pupil size or light intensity.
Müller's muscle is approximately 10 mm long, 15 mm wide, and 0.1-0.5 mm thick.
Damage to Müller's muscle or its innervation can cause a drooping eyelid, a condition known as ptosis. This is often seen in Horner's syndrome, where the ptosis is typically milder than in oculomotor nerve palsy.
