Thanks to the function of the autonomic nervous system (ANS) your body is incredibly good at completing important tasks without your conscious input. In this video, the pathways controlling the tone of muscles in the iris (and therefore amount of light that enters the eye) are investigated. There are two types of muscles controlling the size of the iris: 1) pupillary dilator muscle and 2) pupillary sphincter muscle. The orientation of these muscles is paramount to their function; the dilator muscle is orientated radially while the sphincter muscle lies in a circular orientation around the iris. When the dilator muscle contracts, the iris is condensed radially and thus the pupil dilates, allowing more light to enter the eye. Conversely, as the sphincter muscle contracts, the size of the iris enlarges, effectively constricting the pupil and restricting the amount of light entering the eye. In response to ambient light levels, the relative contributions from sympathetic and parasympathetic nerves innervating the iris control the muscle tone and therefore the amount of light entering the eye. When one pathway is activated, there is a reciprocal down regulation of the opposing pathway (not shown here). Some pharmacological agents can mimic these nerves and are often used by optometrists to diagnose conditions of the eye.
This animation is part of a broader ANS learning module for undergraduate students at the University of Melbourne, Australia. A behind the scenes look at the animation process can be found here.