Pupil

• The pupil is located in the middle of the iris, the colored disc in eyes made up of connective tissue and smooth muscle fibers. According to Dr. Ted Montgomery, the pupil is an opening that "is circular and is comparable to the aperture of a camera." It allows for the regulation of light passing through to the retina at the back of the eye.
  
  

Pupillary Light Response

• When light enters one eye and not the other, all that happens is the one pupil becomes constricted (smaller) and the other stays the same. The pupillary light response, or the pupillary reflex, occurs when light enters the eye and the pupil constricts.
  
  

Nerve Impulses

• According to Exploratorium, the light that your eye detects is converted to nerve impulses that travel along the optic nerve. A portion of these impulses travel from the optic nerve to the muscles that control the pupil size. More light that is shined in the eyes will create more impulses, thus causing the muscles to close the pupil. A part of the optic nerve on one eye crosses over and connects to the muscles that control the pupil size of the other eye. This is the reason the pupil of one eye can change in size while the other one stays as it was.
  
  

Other Information

• Just as the pupil constricts when more light enters the eye, the opposite holds true when there is not enough light. When a room is dark or dim, the iris dilator muscle tugs away from its center, making the iris dilate and allowing more light to reach the retina.