The orbicularis oculi muscle is a circular facial muscle positioned around the eye. It influences eye protection and communication. Its actions are integral to daily ocular activities.
Anatomy and Primary Actions
The orbicularis oculi is a flat, broad muscle encircling the eye, extending across the eyelids and orbital rim. It has three parts: the orbital, palpebral, and lacrimal portions. The orbital part forms the outer ring around the eye socket, originating from the nasal part of the frontal bone, the frontal process of the maxilla, and the medial palpebral ligament. This section is responsible for forceful eye closure, such as squinting, protecting against bright light or debris.
The palpebral part lies within the eyelids themselves, originating from the medial palpebral ligament. This portion controls the more subtle, involuntary movements, most notably blinking. Blinking is a reflexive action occurring approximately 15 to 20 times per minute, which continuously spreads the tear film across the corneal surface, lubricating the eye and washing away debris.
The smallest and deepest part is the lacrimal portion, situated behind the medial palpebral ligament and lacrimal sac. This part contributes to tear drainage by compressing the lacrimal sac and pulling the eyelids medially, directing tears into the lacrimal canals. All parts of the orbicularis oculi are innervated by branches of the facial nerve (cranial nerve VII). The orbital segment also integrates with neighboring muscles, like the frontalis, for broader facial expressions involving the forehead and eyebrows.
Common Conditions and Aesthetic Relevance
Dysfunction of the orbicularis oculi muscle can lead to several medical conditions. One such condition is benign essential blepharospasm, a focal dystonia characterized by involuntary and often forceful contractions of the orbicularis oculi muscles, leading to repetitive blinking and eye closure. This can severely impact daily activities and, in advanced cases, cause functional blindness due to the inability to keep the eyes open. While the exact cause is unknown, it is thought to involve both genetic and environmental factors.
Another condition affecting this muscle is weakness or paralysis due to facial nerve damage, such as in facial nerve palsy. Since the facial nerve innervates the orbicularis oculi, its damage can result in incomplete eyelid closure, a condition known as lagophthalmos. This incomplete closure can expose the cornea to damage and lead to dry eye conditions.
Beyond medical conditions, the orbicularis oculi muscle is relevant to the formation of “crow’s feet” wrinkles. These lines appear at the outer corners of the eyes from repetitive contractions of the orbital part of the muscle during expressions like smiling, laughing, or squinting. Over time, as skin elasticity diminishes, these dynamic wrinkles can become static lines visible even at rest.
Botulinum toxin injections are a common aesthetic treatment targeting these wrinkles. When injected into the orbicularis oculi, this neurotoxin temporarily relaxes the muscle by blocking nerve signals, thereby smoothing out the crow’s feet lines. This treatment can also help reduce further wrinkle formation. Doses for crow’s feet typically range from 10 to 24 units per side, depending on individual muscle activity and wrinkle severity.