Final answer:
The eye contains two muscles to adjust pupil size for optimal light regulation: the sphincter pupillae for constriction and the dilator pupillae for dilation, controlled by the parasympathetic and sympathetic nervous systems respectively.
Step-by-step explanation:
The human eye has two muscles to control the size of the pupil: the sphincter pupillae and the dilator pupillae. These muscles respond to different parts of the autonomic nervous system. The sphincter pupillae is a circular muscle that constricts the pupil under the influence of the parasympathetic nervous system, which is activated by bright light. In this case, signals are sent via the oculomotor nerve to release acetylcholine. Conversely, the dilator pupillae muscle, which is stretched like spokes in a wheel, dilates the pupil under the influence of the sympathetic nervous system. It receives signals through the superior cervical ganglion that release norepinephrine. This duel muscle system allows the pupil to adjust quickly to various light intensities, protecting the retina and helping with vision acuity.