Blinking is the rapid closing and opening of the eyelid

Eyelid

An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...

. It is an essential function of the eye

Eye

Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

 that helps spread tears

Tears

Tears are secretions that clean and lubricate the eyes. Lacrimation or lachrymation is the production or shedding of tears....

 across and remove irritants from the surface of the cornea

Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 and conjunctiva

Conjunctiva

The conjunctiva covers the sclera and lines the inside of the eyelids. It is composed of rare stratified columnar epithelium.-Function:...

. Blink speed can be affected by elements such as fatigue, eye injury, medication, and disease. The blinking rate is determined by the "blinking center", but it can also be affected by external stimulus. When an animal (usually human

Human

Humans are the only living species in the Homo genus...

) chooses to blink only one eye as a signal to another in a social setting (a form of body language

Body language

Body language is a form of non-verbal communication, which consists of body posture, gestures, facial expressions, and eye movements. Humans send and interpret such signals almost entirely subconsciously....

), it is known as wink

Wink

A wink is a facial expression made by briefly closing one eye. A wink is an informal mode of non-verbal communication usually signalling shared hidden knowledge or intent, which may also include, in some contexts, sexual attraction....

ing. Some animals (for example, tortoise

Tortoise

Tortoises are a family of land-dwelling reptiles of the order of turtles . Like their marine cousins, the sea turtles, tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The tortoise...

s and hamster

Hamster

Hamsters are rodents belonging to the subfamily Cricetinae. The subfamily contains about 25 species, classified in six or seven genera....

s) blink their eyes independently of each other.

Function and anatomy of blinking

Blinking provides moisture to the eye by irrigation using tears and a lubricant the eyes secrete. The eyelid provides suction across the eye from the tear duct to the entire eyeball to keep it from drying out.

Blinking also protects the eye from irritants. Eyelash

Eyelash

An eyelash or simply lash is one of the hairs that grow at the edge of the eyelid. Eyelashes protect the eye from debris and perform some of the same function as whiskers do on a cat or a mouse in the sense that they are sensitive to being touched, thus providing a warning that an object is near...

es are hairs attached to the upper and lower eyelids that create a line of defense against dust and other elements to the eye. The eyelashes catch most of these irritants before they reach the eyeball.

There are multiple muscles that control reflexes of blinking. The main muscles, in the upper eyelid, that control the opening and closing are the orbicularis oculi and levator palpebrae superioris muscle

Levator palpebrae superioris muscle

The levator palpebrae superioris is the muscle in the orbit that elevates the superior eyelid.-Structure:...

. The orbicularis oculi closes the eye, while the contraction of the levator palpebrae muscle opens the eye. The Müller’s muscle, or the superior tarsal muscle

Superior tarsal muscle

The superior tarsal muscle is a smooth muscle adjoining the levator palpebrae superioris muscle that helps to raise the upper eyelid.-Structure:...

, in the upper eyelid and the inferior palpebral muscle in the lower eyelid are responsible for widening the eyes. These muscles are not only imperative in blinking, but they are also important in many other functions such as squinting and winking. The inferior palpebral muscle is coordinated with the inferior rectus to pull down the lower lid when one looks down. Also, when the eyes move, there is often a blink; the blink is thought to help the eye shift its target point.

Nervous coordination

Though one may think that the stimulus triggering blinking is dry or irritated eyes, it is most likely that it is controlled by a "blinking center" of the globus pallidus

Globus pallidus

The globus pallidus also known as paleostriatum, is a sub-cortical structure of the brain. Topographically, it is part of the telencephalon, but retains close functional ties with the subthalamus - both of which are part of the extrapyramidal motor system...

 of the lenticular nucleus, a body of nerve cells between the base and outer surface of the brain. Nevertheless, external stimuli can contribute. The average length of a blink is 300-400 milliseconds.

Greater activation of dopaminergic pathways dopamine production in the striatum

Striatum

The striatum, also known as the neostriatum or striate nucleus, is a subcortical part of the forebrain. It is the major input station of the basal ganglia system. The striatum, in turn, gets input from the cerebral cortex...

 is associated with a higher rate of spontaneous eye blinking. Conditions in which there is reduced dopamine availability such as Parkinson's disease

Parkinson's disease

Parkinson's disease is a degenerative disorder of the central nervous system...

 have reduced eye blink rate, while conditions in which it is raised such as schizophrenia

Schizophrenia

Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...

 have an increased rate.

Children

Infants do not blink at the same rate of adults; in fact infants only blink at an average rate of one or two times in a minute. The reason for this difference is unknown, but it is suggested that infants do not require the same amount of eye lubrication that adults do because their eyelid opening is smaller in relation to adults. Additionally, infants do not produce tears during their first month of life. Infants also get a significant amount more sleep than adults do, and, as discussed earlier, fatigued eyes blink more. However, throughout childhood the blink rate increases, and by adolescence, it is usually equivalent to adults.

Adults

It is a myth that women blink nearly twice as much as men. Generally, between each blink is an interval of 2–10 seconds; actual rates vary by individual averaging around 10 blinks per minute in a laboratory setting. However, when the eyes are focused on an object for an extended period of time, such as when reading, the rate of blinking decreases to about 3-4 times per minute. This is the major reason that eyes dry out and become fatigued when reading.

Eye blinking can be a criterion for diagnosing medical conditions. For example, excessive blinking may help to indicate the onset of Tourette syndrome

Tourette syndrome

Tourette syndrome is an inherited neuropsychiatric disorder with onset in childhood, characterized by multiple physical tics and at least one vocal tic; these tics characteristically wax and wane...

, stroke

Stroke

A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

s or disorders of the nervous system

Nervous system

The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

. A reduced rate of blinking is associated with Parkinson's disease

Parkinson's disease

Parkinson's disease is a degenerative disorder of the central nervous system...

. Parkinson's patients have a distinct serpentine stare that is very recognizable.