What Is the Pupil and How Does It Change Size? How Your Eyes and Brain Control Light Intensity on Your Retina

Nerve responses and specialized structures control the aperture that regulates the amount of light that enters the eye and falls on the photoreceptors.

The eye is the brain’s window on the world, the first portal to the processes associated with vision. And similar to cameras and taking pictures, either too much or too little light can lead to images that lack for detail, display distorted coloration, and generate lower resolution images than desired. The eye has a specialized structure at its anterior (front) end, the pupil, which plays a critical role in maximizing visual inputs.

The Eye and Refraction of Light

The front part of the eye is most involved in the process of refraction of light. That is, the structures of the anterior segment of the eye are most involved in the process of bending (refracting) light so that the images will focus on the retina, the light-sensitive and sensing structure in the posterior (rear) of the eye. The front part of the eyeball, or globe, contains the cornea, the anterior and posterior chambers, the iris (the visibly colored circle in the front of the eye), the ciliary bodies, the lens, and the pupil.

What Is the Pupil?

Most simply, the pupil is an aperture, an opening. In this case, it is the opening through the iris that lets light pass to and through the ocular lens and onward towards the retina. The pupil is the dark spot in the center of the iris. It appears dark because light that enters the eye is absorbed by the internal structures of the eye and doesn’t reflect back through the pupil.

What Controls the Pupil?

The size of the opening through the iris, known as the pupil, is controlled in response to contraction and relaxation of the different muscles that are part of the iris. The dilation (widening) or constriction (narrowing) depends on numerous signals in a circuit that involves the pupillary muscles, the retinal ganglion cells, the optic nerve (the second cranial nerve, CN II), regions of the brain and brainstem and the oculomotor nerve (the third cranial nerve, CN III).

How Does Pupil Size Change?

When light shines into the eye, the signals from the retina travel to the higher centers of the brain for the processing of visual input. But when it comes to constricting the pupil, neural signals exit the retina and then pass through a different circuit that goes to deeper, older structures of the brain and a signal goes back out through a different nerve back to specific muscles of the iris, the sphincter pupillae, causing them to constrict, narrowing the pupil. This is known as the pupillary reflex. It can not be consciously controlled and serves as a useful indicator of neural function through specific parts of the brainstem. More importantly, this reflex is “consensual”; that is if light is shone specifically into one eye, the pupil of the other eye will constrict right along with the pupil that is being illuminated.

There are another set of muscles in the iris that are known as the dilator pupillae. When these muscles are activated, via a different neural pathway than the sphincter pupillae muscles, the pupil will dilate. Both the constricting and dilating pathways are themselves countered by other neural signals that can inhibit their action. Ultimately, the size of the pupil is determined by the intensity of contraction of the different muscles, regulated by both their positive and negative signals.

Drugs That Change Pupil Size

When someone has a dilated exam at the eye doctor, specific drugs are used that block the action of the muscles controlling pupillary constriction. This aids in the examination of the eye and its internal structures. So drugs like tropicamide, phenylephrine and atropine can cause dilation of the pupil, but so can cocaine, amphetamines and other illicit drugs. Constriction of the pupil can be caused by drugs such as pilocarpine or neostigmine, but can also be caused by drugs of abuse such as heroin or morphine.

In everyday use, the pupil changes size in response to ambient illumination. A lot of light and the pupil constricts to help with visual acuity and keep the light sensing cells of the retina from getting “overexposed”. When it is dark out or a person is in a dimly lit room, the pupil dilates to allow as much light as possible to enter the eye. A remarkably involved system for something that seems so simple.

To learn more about the eye, visit the interactive eye diagram at the US National Eye Institute