The eyelid closure reflex is a so-called polysynaptic foreign reflex that serves to protect the eyes from foreign body exposure and dehydration. The reflex can be triggered by tactile, visual, or auditory stimuli; startle can also activate the reflex. It always affects both eyes, even in the case of tactile or optical stimuli that occur in only one eye.
What is the eyelid closure reflex?
The eyelid closure reflex is a so-called polysynaptic foreign reflex that serves to protect the eyes from foreign body exposure and dehydration. The eyelid closure reflex, which serves to directly protect the eyes against foreign body intrusion (e.g., insects or particles displaced by wind), is triggered by tactile stimuli on the cornea or the immediate eye environment. However, the reflex can also be activated by bright light stimuli. In this case, it serves mainly to protect the retina and the photoreceptors (rods and cones) within it against damage caused by excessive light incidence. If the reflex is triggered by acoustic stimuli, e.g. by a loud bang, or by a fright situation, it is a kind of prophylactic protection of the eyes. The involuntary, regularly recurring eyelid closure, which occurs without external stimulus and protects the eyes from drying out, is also part of the reflex. The eyelid closure reflex is a so-called foreign reflex because it does not originate from the organ that is affected, but because the organ acting is another. There must be a nervous synaptic circuit to establish the connection between the affected organ and the acting part of the body. Therefore, extraneous reflexes are also called polysynaptic reflexes. The disadvantage is that this makes the reflex much more sluggish compared to an immediate reflex, which does not require synaptic circuitry with the central nervous system. The time from tactile stimulus to eyelid closure is about 250 milliseconds.
Function and task
The eyelid closure reflex performs important functions to mechanically protect the eyes from exposure to foreign objects and from sudden strong light incidence that could damage photoreceptors in the retina. In addition, the reflex, in the form of the regularly recurring involuntary closure of the eyelids, provides the necessary tear film on the cornea, which seals the eye to the outside. The wetting of the cornea with tear fluid ensures that the eyeball can move smoothly in the truest sense of the word and that the incident light passes through clearly, thus ensuring an unclouded and undistorted projection onto the retina. The mechanical protective effect of eyelid closure is further supported by the so-called Bell phenomenon. Simultaneously with the closing of the eyelids, the eyes roll upwards-outwards completely unconsciously and unnoticed. This rotates the immediately functional area of the eye, lens and pupil, out of the “danger zone” and thus achieves a further prophylactic protective effect. The eyes rotate into a position that they also assume during sleep. In addition, the Westphal-Piltz phenomenon occurs during the reflexively triggered eyelid closure. Together with the eyelid closure and the Bell phenomenon, both pupils constrict. This phenomenon also most likely serves a prophylactic protective purpose. The constricted pupils protect the photoreceptors from any strong flashes of light that may occur. Since the eyelid closure reflex is a foreign reflex, the reflex can be conditioned to a certain degree or it can be attenuated by prepulse inhibition. This is the only reason why it is possible to use contact lenses, for example. Contact lens wearers have to “train away” the eyelid closure reflex a little in order to be able to use contact lenses at all without triggering the reflex. Weakening the reflex by prepulse inhibition means that touching the cornea several times weakens the reflex overall because the brain adjusts to further, stronger stimuli.
Diseases and complaints
Impairment or failure of the eyelid closure reflex, including the repetitive involuntary closure of the eyelid to wet the cornea, leads to problems with dry cornea even in the short term, resulting in itchy or burning eyes and conjunctival irritation and even conjunctivitis. The risk of injury to the eyes increases because the protective reflex is absent or greatly reduced.Causes for the failure of the eyelid closure reflex can be the afferent sensory or the efferent motor nerve fibers or the processing centers of the sensory messages. Similarly, muscle paralysis (facial paresis) of the facial muscles and especially of the eyelid muscles can lead to a failure of the eyelid closure reflex. Sensory tactile messages that can trigger the reflex travel via an afferent lateral branch of the 5th cranial nerve, the trigeminal nerve, to various nuclei before being conducted to the reflex center of the brainstem. In the case of a strong light stimulus, the stimuli are transmitted via the optic nerve. The “instructions” to close the eyelid travel via the efferent branches of the 7th cranial nerve, the facial nerve, to the eyelid muscles. This means that if only a single part of the reflex arc has disturbances, as in an electrical series circuit, the overall reflex is disturbed. Nerve diseases such as facial paralysis or paralytic lagophthalmos with paralysis of the eyelid muscles cause partial or total failure of the eyelid closure reflex. Suppression of the reflex can also be caused by anesthetics. The effectiveness of local anesthesia in the eye area can be checked by provoking the reflex. If the reflex fails to appear, the local anesthesia is absolutely effective. In blind persons, permanent eyelid closure usually occurs. Similarly, permanent eyelid closure can be provoked by certain irritants such as capsaicin. Capsaicin is the substance notorious for its pungency and is found in peppers. The same or a similar active ingredient is also the main component of the notorious pepper sprays. Contact of the active ingredient with the eyes causes a spasmodic eyelid closure that lasts at least 30 to 40 minutes.
