The position of the upper eyelid plays an important role and varies with the degree of care.
It is therefore of interest to understand the mechanisms that cause eyelid movement and, in particular, the relationship of local automatic reflexes to this function.
Consciousness generally also invokes a lot of eye work of both movement and visual function.
However, it has been found that the muscles that control the position of the eye do not appear to possess simple stretch reflexes, nor does proprioceptive information appear to reach a conscious level.
The blink reflex is a defense movement that arises when the lids of the eyes are closed if a strong light or noise suddenly appears.
The blink reflex has two components: R1 and R2. Direct stimulation of the facial nerve produces a compound muscle action potential (PAMC) of the facial nerve.
- A1 : It is the short loop reflex, which occurs only on the stimulation side.
- R2 : This is a longer loop reflex that occurs bilaterally.
This response corresponds to the clinically observable blink.
How the palpebral reflex occurs
It has been established that both components of the blink reflex are cutaneous reflexes that represent a highly organized and useful mechanism in man.
Afferent fibers for the blink reflex have been identified in the human supraorbital nerve and their conduction velocity has been estimated for the first time in man.
Both components of the blink reflex have been shown to be mediated by the same group of afferent fibers.
The blink reflex is stimulated by momentarily shining a bright light directly into the infant’s eyes, causing the infant to blink. This reflex should not be inhibited.
A study of the vestibulo-palpebral reflex could establish a basis for its use in diagnosing vestibular disorders at an early age.
To determine how premature a birth has been and possibly to determine the individual characteristics of the cortical activity that is manifested at such an early stage by reflex suppression.
The finding of abnormal blink reflex responses in individuals raises the idea that they may be useful in detecting early changes and in the follow-up of patients with disorders.
Studies of the blink reflex, with a particular emphasis on late responses, may be helpful in revealing subclinical cranial nerve abnormalities in disease.
Additional studies could be designed to find out whether the blink reflex abnormalities caused are reversible or irreversible.
Studies could be done to find out if the different etiologies of hypothyroidism (autoimmune, iodine deficiency, etc.) have any effect on the blink reflex.
Imaging studies may be done to see if any structural pontine lesions can be visualized in the region of the circuit.
Functional imaging studies (nuclear imaging, functional magnetic resonance imaging) could be performed to visualize the functional aspects of the brain stem circuits.
Eyelid movements during blinking
Disjunctive oblique eye movements accompany spontaneous, voluntary, and reflex blinking.
Depending on the gaze position before blinking, the amplitude of the horizontal and vertical components of eye movement during blinking varies systematically.
With adduction and downward gaze, the range is minimal. With abduction, the horizontal breadth increases, while with looking up, the vertical breadth increases.
Unilateral supraorbital electrical stimulation at low currents elicits eye movements with a bilateral late component.
At stimulus intensities approximately two to three times the threshold, the ipsilateral blink reflex response can be observed in the orbicularis oculi muscle along with an early ipsilateral eye movement component at a latency of approximately 15 ms.
In addition, during the electrical blink reflex, early ipsilateral and late bilateral components can also be identified in upper eyelid movement.
In contrast to the late bilateral component of upper eyelid movement, the early ipsilateral component of upper eyelid movement appears to open the eye to a greater degree.
This early ipsilateral component of upper eyelid movement occurs more or less simultaneously with the early eye movement component.
It is suggested that both early ipsilateral movements after electrical stimulation do not have a central neuronal origin. The late components of eye movement slightly precede the late components of the eyelid movement.
Synchrony between late components of eyelid movements and eye movements.
As well as the similarity of the components of the oblique eye movement in different types of blinks, it suggests the existence of a premotor neural structure that acts as a generator that coordinates the impulses to different sub-nuclei of the oculomotor nucleus.
As well as the nucleus of the facial nerve during blinking independent of the ocular saccadic system and / or the vergence system.
The profile and direction of eye movement rotation during blinking support the idea that it may be secondary to the retraction of the eyeball; an additional co-contraction of the inferior and superior rectus muscles would be sufficient to explain both the ocular retraction and the rotation in the horizontal vertical and torsional planes.
Relationship of eyelid movement to the blink reflex
Although the blink reflex is standard neurophysiological investigation, its relationship to eyelid movement has not been clearly established.
Closing of the eyelid does not necessarily occur in a single movement.
After glabellar bypass, the first component of a two-stage movement began with relaxation of the levator palpebre, whereas with stimulation of the supraorbital nerve, contraction of the orbicularis oculi produced the first movement.
Compound muscle action potential after direct stimulation of the facial nerve produced only minimal eyelid movement, and greater closure was associated with a higher latency ocular orbicularis reflex.
Corneal stimulation causes single-component eyelid movement. Therefore, the eyelid movement pattern differs for each stimulus, reflecting variations in the contraction of the orbicularis oculi and the inhibition of the levator palpebre.