Final answer:
The opening at the center of the iris that grows larger or smaller as the iris contracts or relaxes is called the pupil. It adjusts in size based on the amount of light, contracting in bright light and dilating in dim light due to the release of neurotransmitters like acetylcholine and norepinephrine from the parasympathetic and sympathetic nervous systems, respectively.
Step-by-step explanation:
As the iris contracts and relaxes, an opening at its center, the pupil grows larger or smaller. The control of pupillary size is a response to the amount of light that the retina receives. When light is bright, the parasympathetic postsynaptic neuron releases acetylcholine, which stimulates the circular muscles of the iris to contract, making the pupil smaller. Conversely, in dim light, the sympathetic nervous system releases norepinephrine, causing the radial fibers of the iris to contract, which dilates the pupil, allowing more light to enter.