Why Our Eyes Have Pupils

Extreme Close-Up Of Human Eye

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The hole or opening that is located in the center of the iris of the eye. The pupil controls the amount of light that enters the eye. Pupil size is controlled by the dilator and sphincter muscles of the iris.

Why We Have Pupils

The pupil controls how much light is let into the eye. It is very similar to a camera aperture which allows more light in for more exposure. At night, our pupils dilate to allow more light in to maximize our vision. In the bright sunlight, our pupil shrinks to a very small diameter to allow us to function normally. Otherwise, we would be very light sensitive. This protects the sensitive photoreceptors in our retina.

Also, when we look at something at a very close distance such as reading a book, our eyes converge and our pupils shrink. When our pupils shrink, it is similar to looking through a pinhole. Looking through a small hole reduces peripheral blur and increases the depth of focus. This improves overall visual acuity. Normal pupil size is between 2.5 to 4.0 mm.

What System Controls the Pupil?

The iris, the colored part of our eye, is made up of pigment and contains two sets of smooth muscles that control the size of the pupil. the sphincter muscle and the dilator muscle. The sphincter muscle is in the shape of a ring at the margin of the pupil. When it contracts, it constricts or decreases the size of the pupil. The dilator muscles are in a radial shape throughout the iris and when it contracts, it dilates or increases the size of the pupil.

Both systems, the parasympathetic and sympathetic systems control the pupil. Our parasympathetic system controls everyday activities such as rest, slowing the heart rate and things like digestion. It controls the pupil size during normal activities during the day and acts to change the pupil size depending on how much light is present. The sympathetic system is a protective system and gives us the typical "fight or flight" responses. In the pupil, if we are scared or feel fear, our pupils dilate very large. This is thought to allow light in so our responses are quicker. 

Significance in Healthcare

Examination of the pupil is very important in medicine because the neurological pathway of the pupil is made up of three different nerves and follows a very long pathway in the body. This three neuron pathway starts in a part of the brain called the hypothalamus and travels down the spinal cord. The second neuron exits the spinal cord travels over the top of the lung and then up the neck where it meets the third neuron.

From here it goes under the subclavian artery and then hitches a ride alongside the carotid artery and back up through the brain and to the eye. Because of its long pathway, doctors use the pupil to diagnose possible disorders that could affect this pathway. For example, lung cancer affecting the top of the lung where the pupil's nerve travels over it could affect the pupillary function. Depending on what loss of function the pupil exhibits, sometimes they can tell where to look for cancer.

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