The Orbicularis Oculi Muscle is a sphincter (ring-shaped) muscle that surrounds the eyelids and enables them to be closed. Its contracting fibers originate from the nasal portion of the frontal bone, the frontal process of the maxilla, and the anterior surface and borders of the medial palpebral ligament. These fibers are widely distributed around the eye, forming a thin, broad muscle layer extending toward the temples and cheeks.
Anatomy
Morphology and Connection Points
The Orbicularis Oculi Muscle is a circular muscle that surrounds the eye and plays a vital role in facial expression and eye protection. It consists of three main parts: the orbital, palpebral, and lacrimal parts. The palpebral part is further divided into preseptal and pretarsal parts. While the lacrimal part is a distinct anatomical structure, it is functionally distinct. The orbital part, with its thick structure, encompasses the outer periphery of the eye and is surrounded by fibers that form a complete ellipse at the lateral palpebral commissure. The upper fibers of this region, combined with the frontalis and corrugator supercilii muscles, contribute to facial expressions. The palpebral part, composed of thinner fibers, is responsible for the gentle closure of the eyelids, originating at the bifurcation of the medial palpebral ligament. The pretarsal part controls the involuntary blink reflex, while the preseptal part is involved in larger movements. The lacrimal part lies behind the medial palpebral ligament. It originates from the orbital surface of the occipital bone and, by coiling around the lacrimal sac, is involved in the drainage of tears. It contains the muscles of Horner and Riolan; the latter, in particular, prevents the outflow of tears by drawing the eyelid margins closer.
Embryological Origin and Classification
The Orbicularis Oculi Muscle originates embryologically from the second branchial arch (hyoid arch), a feature that places it in the group of muscles of facial expression. The second branchial arch provides the formation of various muscle structures in the head and neck region during embryonic development; these muscles are typically innervated by the facial nerve (nervus facialis, cranial nerve VII). The fact that the orbicularis oculi muscle is also innervated by the facial nerve is consistent with this embryological origin and confirms its place in the classification of facial muscles. The muscle develops by the migration of neuroectodermal muscle progenitor cells derived from the facial ectoderm to the second branchial arch. This developmental process mediates the muscle's morphological characteristics that contribute to facial expressions. Consequently, the orbicularis oculi muscle is classified as a typical muscle of facial expression in terms of both embryological origin and neural innervation.
Nerve Innervation and Vascular Supply
The Orbicularis Oculi Muscle is innervated by the temporal and zygomatic branches of the facial nerve (cranial nerve VII). This motor innervation provides innervation to both the palpebral (eyelid) and orbital (around the eye socket) portions of the muscle, enabling voluntary or reflex closure of the eyelids. Arterial blood supply to the muscle is primarily provided by branches of the facial artery, superficial temporal artery, ophthalmic artery, and infraorbital artery. Venous drainage occurs via the facial and ophthalmic veins. The orderly organization of these neural and vascular structures allows the orbicularis oculi muscle to function quickly and harmoniously.
Function and Clinical Relationships
The Orbicularis Oculi Muscle is the only muscle capable of closing the eyelids. The palpebral region handles involuntary blinking and closing during sleep, while the orbital region consciously handles the task of tightly closing the eyes. Contraction of this muscle stretches the medial palpebral ligament, creating negative pressure on the lacrimal sac, which allows tears to drain through the lacrimal ducts. This mechanism is critical for tear circulation. The muscle's antagonist, the levator palpebrae superioris, lifts the upper eyelid, allowing the eye to open. Clinically, loss of orbicularis oculi muscle function in facial nerve palsies can lead to inability to close the eyelids and dry and inflamed cornea. This is frequently observed in pathologies such as Bell's palsy. Furthermore, the muscle's excitability can be tested with the corneal reflex, which allows facial nerve function to be assessed even in unconscious patients. The accessory muscles, the corrugator supercilii, procerus, and frontalis muscles, work together or antagonistically with the orbicularis oculi to contribute to eyebrow movements and facial expressions.
