Blinking, or blink reflex, is the involuntary movement (opening and closing) of either one or both eyelids simultaneously. It is essential in the function of spreading tears and removing irritants from the surface of the cornea (front part of the eye) and the conjunctiva (the mucous membrane that covers the cornea and the lining of the eyelids). The rate or frequency of a blink varies between species. Each mammalian species expresses a characteristic blink rate that is constant under unchanging conditions. However, the blink rate can be affected by factors such as fatigue, eye injury, disease, or medication. In humans, the blink reflex is the first and most reliable form of the startle reflex, which is an avoidance response induced by a threatening stimulus. Further, the blink reflex can be increased by emotional stimuli.
Function and Purpose of the Blink Reflex
The blink reflex seen in animals and humans is made for the local protection of the cornea and the conjunctiva. This is because animals are highly visual and need to protect their ability to see. It enables self-preservation of the eyes, either from a threat of real danger to the animal’s life (like being attacked) or from undefined fears of the surrounding conditions.
Blinking also provides moisture to the eye by irrigation. Specifically, the eyelid provides suction across the eye from the tear duct (located in the lower eyelid near the nose) to the rest of the eyeball; this prevents the eye from drying out. Thus, the tears and lubricant that the eyes secrete are essential for eye health.
Lastly, blinking protects the eye from irritants in the environment. A second line of defense against dust and other elements that could irritate the eye is eyelashes, which are hairs attached to the upper and lower eyelids. Eyelashes function to catch these irritants during a blink reflex before they reach the eyeball.
Anatomy of the Blink Reflex
Several muscles control the blink response. The main muscles in the upper eyelid are the obicularis oculi and the levator superioris muscles. These control the opening and closing of the eye. The obicularis oculi muscle closes the eye, while the levator superioris muscle contracts and opens the eye. The contraction of the obicularis oculi muscle protects important structures, like the retina (the photosensitive lining of the back of the eye), from external trauma. The smooth muscle of the upper lid, the Muller’s muscle (also known as the superior palpebral muscle), also helps in widening the lid aperture, which enables the widening of the eyes.
The nerves that supply the blink reflex are cranial nerves V and VII (or trigeminal and facial nerves). Specifically, the ophthalmic branch (V1) of the trigeminal nerve senses the stimulus on the cornea, lid, or conjunctiva. The temporal and zygomatic branches of the facial nerve initiate the motor response, or blinking action.
See also: Facial Nerve; Reflex; Trigeminal Nerve; Visual System
Hall, Arthur. (1945). The origin and purposes of blinking. British Journal of Ophthalmology, 29(9), 445—467.
Pearce, J. M. S. (2008). Observations on the blink reflex. European Neurology, 59(3—4), 221—223.
Shahani, Bhagwan. (1970). The human blink reflex. Journal of Neurology, Neurosurgery, and Psychiatry, 33(6), 792—800.