During eyelid closure, the upper and lower eyelids meet until the palpebral fissure is fully closed and the eye is no longer visible. The seventh cranial nerve of the mimic muscles is primarily involved in eyelid closure, thus protecting the eye from drying out and from dangerous stimuli with the help of the eyelid closure reflex. When the nerve is paralyzed, eyelid closure is incomplete.
What is eyelid closure?
During eyelid closure, the upper and lower eyelids meet until the palpebral fissure is fully closed and the eye is no longer visible. In addition to an upper eyelid, the human eye is equipped with a lower eyelid. Between the eyelids lies the so-called palpebral fissure, through which the eye is visible. When the upper and lower eyelids meet, the palpebral fissure is completely closed and the eye is fully covered. The active joining of the upper and lower eyelids is also called eyelid closure. The human eye is protected and moistened by the closure of the eyelid. As part of the so-called eyelid closure reflex, eyelid closure occurs automatically in response to certain stimuli, in the form of a foreign reflex. The eyelid is closed by the mimic musculature and can also take place consciously in addition to the reflex-like unconscious variant, as long as the control of the mimic musculature is given. Especially the orbicularis oculi muscle and with it the seventh cranial nerve are involved in the conscious and unconscious eyelid closure. The muscle is thus irreplaceable for the moistening and general protection of the cornea. In the form of wetting with tear fluid, the muscle prevents the eye from drying out during eyelid closure. The term eyelid closure is associated with conscious, nonautomatic eyelid closure rather than the associated reflex.
Function and task
As the visual perception system of humans, the eyes are one of the most important perceptual instances of all. In the course of evolution, they have sometimes ensured human survival. For this reason, the eyes are equipped with many different protective functions. One of these is the closure of the palpebral fissure. By closing the eyelids, the eyes do not dry out. The eyelid closure reflex additionally keeps environmental hazards away from the eye and takes place in response to stimuli moving toward the eye. The orbicularis oculi muscle is the most important muscle for the eyelid closure function of the eye. It is located in the area of the orbital opening and is also called the ocular ring muscle. It surrounds the eye in a circle and thus encloses the palpebral fissure. The muscle belongs to the mimic musculature and consists of three different parts. The pars orbitalis originates at the processus frontalis at the maxima and the pars nasalis at the os frontale. This part surrounds the palpebral fissure. The pars palpebralis takes its origin at the ligamentum palpebrale mediale and the pars lacrimalis originates at the crista lacrimalis posterior, where it surrounds the lacrimal sacs. Innervation of the orbicularis oculi muscle is through the rami temporales and rami zygomatici of the seventh cranial nerve. Because the muscle is fused to the corium, the skin follows its movements. The reflex movement of the eyelid is called the eyelid closure reflex and corresponds to an extraneous reflex that does not carry afferents and efferents in the same organ. The afferent limb of the reflex is the ophthalmic nerve if tactile stimuli trigger eyelid closure. If, on the other hand, optical stimuli, such as bright light, are involved in the protective reflex, the optic nerves form the afferent limb. After switching the stimuli to the trigeminal complex, they are conducted via the superior colliculus or the nucleus ruber to the formatio reticularis, from where they travel to the reflex center of the brainstem and thus reach the facial nucleus. The efferent limb of the reflex is the seventh cranial nerve, which causes contraction of the orbicularis oculi muscle in response to the stimuli. The eyelid closure reflex always occurs in both eyes. This is true even when only one of the eyes is threatened by a stimulus.
Diseases and complaints
Failure of the orbicularis oculi muscle is one of the most obvious complaints that can occur in connection with eyelid closure. Such partial or complete failure is the result of paralysis of the mimic muscles and, accordingly, is caused primarily by failure of the facial nerve. A paralysis of this nerve is a peripheral paralysis and can occur, for example, in the context of a polyneuropathy or a nerve injury.A polyneuropathy may in turn be due to vitamin deficiency, a past infection or poisoning as the primary cause. In the case of seventh cranial nerve palsy, the symptomatic picture corresponds to incomplete eyelid closure, known as lagophthalmos. Due to the incomplete eyelid closure, in many cases the cornea dries out and allows a so-called xerophthalmia to develop. Patients with incomplete eyelid closure therefore usually perceive the discomfort as a burning or foreign body sensation in the eye. Sometimes keratitis e lagophthalmo develops in the context of incomplete eyelid closure. This is an inflammation of the cornea, which in some cases causes ulcers. These ulcers are also known as corneal ulcers. When the patient attempts to close the eyelid despite ulceration, Bell’s phenomenon occurs. The eyeball rotates temporally upward. In addition to peripheral paralysis of the seventh cranial nerve, scars, for example, can cause incomplete eyelid closure. In case of scar tissue in this area, the eyelids shorten and for this reason do not meet anymore, because they do not reach each other in length. Other causes of incomplete eyelid closure are exophthalmos, coma or an ectropion. The latter condition is a malposition of the eyelid that causes the insufficient eyelid closure.
