Location and Topographic Relations
The orbicularis oculi muscle belongs to the facial muscles derived from the second pharyngeal arch. It is located superficially and has the following topographic features:
- In the structure of the eyelids: The muscle lies between the skin and the tarsal plate (eyelid cartilage). Circular muscle bundles run near the margins of both the upper and lower eyelids.
- Relation to other muscles: The inferior portion of the orbicularis oculi lies most superficially. Deep to it are the zygomaticus minor and major muscles (mm. zygomatici minor et major), which overlie the levator labii superioris muscle (m. levator labii superioris).
Anatomical Parts
Anatomically and functionally, the muscle is divided into three portions:
- Palpebral part (pars palpebralis) — involved in blinking.
- Orbital part (pars orbitalis) — involved in forceful eye closure (squinting).
- Lacrimal part (pars lacrimalis) — dilates the lacrimal sac upon contraction.
Blood Supply and Innervation
- Innervation: Supplied by the facial nerve (n. facialis, CN VII). The zygomatic branch (r. zygomaticus) approaches the muscle, running within the subcutaneous tissue over the anterior third of the zygomatic arch.
- Blood supply: The muscle is supplied by the supratrochlearis (a. supratrochlearis), supraorbital (a. supraorbitalis), and angular (a. angularis) arteries. The frontal branch of the superficial temporal artery (r. frontalis a. temporalis superficialis) also contributes to its blood supply.
Role in Reflex Arcs
The orbicularis oculi muscle serves as the efferent effector component of protective reflexes:
- Corneal reflex: Normally, touching the cornea causes both eyes to close. The afferent signal travels from the ocular mucosa via the ophthalmic nerve (CN V1) to the pontine sensory nucleus. From there, the impulse relays to the facial nerve motor nucleus, which triggers bilateral contraction of the orbicularis oculi muscle.
- Blink reflex: Rapid eye closure in response to intense visual stimuli. The signal descends from the superior colliculi via the tectospinal/tectobulbar pathways to the motor nucleus of the facial nerve, causing the muscle to contract.
Clinical Significance
- Facial nerve palsy: Leads to facial asymmetry and lagophthalmos—the inability to completely close the eyelids due to impaired function of the orbicularis oculi muscle. This also results in the loss of the corneal reflex.
- Trigeminal neuralgia: Severe attacks of facial pain can trigger a painful tic (tic douloureux), which is a reflex contraction of the facial expression muscles on the affected side. The orbicularis oculi muscle is frequently involved in this process.