The Sense of Sight / All About the Eyes

submitted by Dr. Gary Farr Last Updated June, 14, 2002

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Page: 10

The visual process

The work of the auxiliary structures

The protective mechanisms

Blink reflexes

Reflex blinking may be caused by practically any peripheral stimulus, but the two functionally significant reflexes are (1) that resulting from stimulation of the endings of the fifth cranial nerve in the cornea, lid, or conjunctiva—the sensory blink reflex, or corneal reflex—and (2) that caused by bright light—the optical blink reflex. The corneal reflex is rapid (0.1 second reflex time) and is the last to disappear in deepening anesthesia, impulses being relayed from the nucleus of the fifth nerve to the seventh cranial nerve, which transmits the motor impulses. The reflex is said to be under the control of a medullary centre. The optical reflex is slower; in man, the nervous pathway includes the visual cortex (the outer substance of the brain; the visual centre is located in the occipital—rear—lobe). The reflex is absent in children of less than nine months.

Normal rhythm

In the waking hours the eyes blink fairly regularly at intervals of two to 10 seconds, the actual rate being a characteristic of the individual. The function of this is to spread the lacrimal secretions over the cornea. It might be thought that each blink would be reflexly determined by a corneal stimulus—drying and irritation—but extensive studies indicate that this view is wrong; the normal blinking rate is apparently determined by the activity of a “blinking centre” in the globus pallidus of the caudate nucleus, a mass of nerve cells between the base and the outer substance of the brain. This is not to deny that the blink rate is modified by external stimuli.

There is a strong association between blinking and the action of the extraocular muscles. Eye movement is generally accompanied by a blink, and it is thought that this aids the eyes in changing their fixation point.

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Additional information regarding conditions of the eyes can be found here.