The levator palpebrae superioris is a skeletal muscle in the upper eyelid that is primarily responsible for lifting the upper eyelid. It is a thin, flat muscle that originates from the lesser wing of the sphenoid bone, located in the orbit of the skull. The muscle extends forward and narrows as it approaches the eyelid, forming the levator aponeurosis, a tendinous structure that inserts into the upper tarsal plate of the eyelid. This muscle plays a crucial role in the mechanics of eyelid movement and is under the control of the oculomotor nerve (cranial nerve III).
Location
The levator palpebrae superioris muscle is located in the orbit, the bony cavity of the skull that houses the eyeball. It originates at the lesser wing of the sphenoid bone, near the apex of the orbit. From there, it runs forward above the superior rectus muscle and attaches to the upper eyelid’s tarsal plate and skin. Its tendon spreads out within the eyelid to ensure the entire upper lid is elevated during contraction.
Structure and Anatomy
The levator palpebrae superioris is a complex muscle that plays a crucial role in the elevation of the upper eyelid. Its anatomy involves intricate connections between bony structures, tendons, and other muscles in the orbital region. Below is a detailed description of its anatomical structure.
Origin
The levator palpebrae superioris originates from the lesser wing of the sphenoid bone, a part of the cranial skeleton located near the apex of the orbit. Specifically, its point of origin is located just above the optic foramen, through which the optic nerve and ophthalmic artery pass. This bony origin provides the necessary anchor for the muscle’s action in elevating the upper eyelid.
Course and Path
After originating from the sphenoid bone, the levator palpebrae superioris travels forward within the superior part of the orbit. As it moves anteriorly, it runs above the superior rectus muscle, one of the extraocular muscles responsible for moving the eye. The muscle maintains close proximity to the superior rectus, but their functions and innervation are distinct.
Levator Aponeurosis
At the front of the orbit, the muscle narrows into a tendon known as the levator aponeurosis. This tendinous structure spreads out and attaches to multiple parts of the upper eyelid. The aponeurosis inserts into the tarsal plate, which is a dense connective tissue structure that provides shape and support to the upper eyelid. Additionally, fibers of the levator aponeurosis extend into the skin of the eyelid, contributing to the lifting action that elevates the entire upper eyelid when the muscle contracts.
Insertion
The levator palpebrae superioris inserts into the following structures:
- Tarsal Plate: The tarsal plate is a thick, supportive structure that gives rigidity to the upper eyelid. The levator muscle’s insertion into this plate allows for the coordinated movement of the eyelid.
- Eyelid Skin: Some fibers of the levator aponeurosis extend beyond the tarsal plate and attach to the skin of the upper eyelid, contributing to the elevation of the skin during the opening of the eye.
Müller’s Muscle (Superior Tarsal Muscle)
In addition to its connection with the tarsal plate and skin, the levator palpebrae superioris muscle is closely associated with Müller’s muscle, also known as the superior tarsal muscle. Müller’s muscle is a small, smooth muscle that lies underneath the levator palpebrae superioris. It assists in maintaining the elevation of the upper eyelid and is controlled by the sympathetic nervous system. The interaction between these two muscles ensures both voluntary and involuntary control of eyelid position.
Blood Supply
The levator palpebrae superioris muscle receives its blood supply from branches of the ophthalmic artery, which is a branch of the internal carotid artery. Specifically, the lacrimal artery and supraorbital artery provide blood flow to the muscle. These vessels run within the orbit, providing oxygenated blood to the levator muscle and its associated structures.
Nerve Supply
The levator palpebrae superioris is innervated by the superior division of the oculomotor nerve (cranial nerve III). This nerve provides motor control to the levator muscle, allowing for the voluntary elevation of the upper eyelid. The oculomotor nerve enters the orbit through the superior orbital fissure and splits into two divisions: the superior and inferior branches. The superior branch innervates both the levator palpebrae superioris and the superior rectus muscle.
Relation to Extraocular Muscles
The levator palpebrae superioris is anatomically related to several extraocular muscles within the orbit, most notably the superior rectus muscle. The levator palpebrae superioris runs directly above the superior rectus, and their fibers are separated by connective tissue sheaths. Despite their proximity, these muscles serve different functions: the levator palpebrae superioris elevates the upper eyelid, while the superior rectus moves the eyeball upward.
Fascial Attachments
The levator palpebrae superioris muscle is surrounded by a fascial sheath that is interconnected with the fascial covering of the superior rectus muscle. This fascial structure contributes to the coordinated movement of the eye and eyelid. The fascial attachments also ensure that the muscle remains in place within the orbital cavity as it contracts and relaxes.
Function
The levator palpebrae superioris is a specialized muscle with the primary function of lifting the upper eyelid. This action is essential for normal vision, allowing the eye to be fully exposed to light and enabling visual focus. Below is a detailed description of its functions.
Elevation of the Upper Eyelid
The primary function of the levator palpebrae superioris is to raise the upper eyelid, a movement that is necessary for the eye to open fully. The muscle works by contracting and pulling the upper eyelid upwards, exposing the cornea and the pupil. This movement is crucial for both visual acuity and field of vision, as it allows the eye to take in light and form a clear image. The muscle’s contraction is voluntary, meaning it is consciously controlled, enabling a person to open their eyes when awake and alert.
Maintaining an Open Eye
In addition to elevating the eyelid, the levator palpebrae superioris helps maintain the eyelid in an open position. Once the upper eyelid is raised, the muscle continues to exert force to keep it elevated, allowing the eye to remain open for extended periods. This function is important for activities that require sustained attention, such as reading or looking at a distant object. The muscle’s tone is continuously adjusted to keep the eyelid in the proper position.
Coordination with Müller’s Muscle
While the levator palpebrae superioris is primarily responsible for the voluntary elevation of the eyelid, it works in conjunction with Müller’s muscle (superior tarsal muscle) to assist with involuntary movements. Müller’s muscle provides additional elevation of the eyelid, particularly in maintaining eyelid height during alert states or sympathetic activation (such as during stress or excitement). While Müller’s muscle is controlled by the sympathetic nervous system, the levator palpebrae superioris is controlled by the somatic nervous system, allowing for both voluntary and involuntary control of eyelid position.
Coordination with Superior Rectus Muscle
The levator palpebrae superioris is functionally linked to the superior rectus muscle, which elevates the eyeball. This coordination allows the upper eyelid to rise simultaneously with the eye when looking upward. As the superior rectus moves the eyeball upwards, the levator palpebrae superioris ensures that the upper eyelid moves in unison, maintaining an open field of vision. This synchronized action prevents the eyelid from obstructing the view as the eye moves, which is essential for clear upward vision.
Assisting in Blinking
While the levator palpebrae superioris is primarily responsible for raising the eyelid, it plays a role in the mechanics of blinking. During a blink, the orbicularis oculi muscle contracts to lower the eyelid, and when the blink is completed, the levator palpebrae superioris contracts to raise the eyelid again. Although blinking is largely controlled by the orbicularis oculi, the reopening of the eye is the responsibility of the levator palpebrae superioris, allowing for rapid cycles of closing and opening during a blink.
Control of Light Exposure
By elevating the upper eyelid, the levator palpebrae superioris regulates how much light enters the eye. When the eyelid is fully raised, it maximizes the amount of light reaching the retina, which is important in low-light conditions or when focusing on detailed objects. Conversely, partial or reduced contraction of the levator muscle results in a narrowed palpebral fissure (the opening between the eyelids), decreasing the amount of light entering the eye. This function is especially useful in bright light conditions, where the eyelids naturally lower to protect the retina from excessive exposure.
Maintaining Visual Field
The full elevation of the upper eyelid by the levator palpebrae superioris ensures an unobstructed visual field. If the muscle fails to function properly, such as in conditions like ptosis (drooping of the eyelid), the visual field can become blocked, impairing the upper portion of the field of view. The muscle’s proper function is essential for maintaining full access to the visual environment, whether looking straight ahead or in upward gaze.
Clinical Significance
The levator palpebrae superioris is critically important for maintaining normal eyelid function and vision. Its dysfunction can result in ptosis, a condition where the upper eyelid droops, partially or completely covering the eye. Ptosis can be congenital or acquired and may lead to obstructed vision, especially in severe cases. Causes of levator muscle dysfunction include trauma, aging, neurological disorders (such as third cranial nerve palsy), and muscle diseases like myasthenia gravis.
Surgical repair, such as levator resection or levator advancement, may be required in severe cases of ptosis to restore proper eyelid function. Early detection and treatment of levator dysfunction are essential for preventing long-term vision problems and improving quality of life.
