Pupillary light reflex

The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict (miosis/myosis; thereby allowing less light in), whereas a lower intensity of light causes the pupil to dilate (mydriasis, expansion; thereby allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye. Light shone into one eye will cause both pupils to constrict.

Pupil is the dark circular opening in the center of the iris and is where light enters the eye. Based on analogy with a camera, pupil is equivalent to aperture, whereas iris is equivalent to shutter. Pupillary reflex should have been named iris reflex, because iris is the actual muscular structure that responds to light and pupil is merely the passive opening formed by the active iris. Pupillary reflex is synonymous with pupillary response, which may be pupillary constriction or dilation. Pupillary reflex is conceptually linked to the side (left or right) of the reacting pupil, and not to the side from which light stimulation originates. Left pupillary reflex refers to the response of the left pupil to light, regardless of which eye is exposed to a light source. Right pupillary reflex means reaction of the right pupil, whether light is shone into the left eye, right eye, or both eyes. In contrast, the terms direct and consensual refers to the side where the light source comes from, relative to the side of the reacting pupil. A direct pupillary reflex is pupillary response to light that enters the ipsilateral (same) eye. A consensual pupillary reflex is response of a pupil to light that enters the contralateral (opposite) eye. Thus there are four types of pupillary light reflexes, based on this terminology of absolute (left versus right) and relative (same side versus opposite side) laterality:

It is a misconception to think that, for example, left consensual pupillary reflex is the left eye pupillary reflex when light is shone to the right eye. Likewise, this applies to the terminology "right consensual pupillary reflex".

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