Agonist / antagonist muscle pairing in the form of the sphincter and dilator muscles in the iris allow for quick and powerful movements of both the constriction and the dilatory response.
Like the (striated) skeletal muscles, the smooth iris muscles come in pairs. Contraction is per definition an active process, and hence it is fast. Take the familiar example of the arm; when the agonist (biceps) contracts, the arm bends. Its antagonist (triceps) relaxes at the same time to give minimal resistance to the motion. The process reverses when the arm is stretched again (Fig. 1).
A similar mechanism operates in the iris to regulate the pupil response (Fig. 2). In this case it is smooth musculature: the sphincter muscle forms a ring around the pupil and constricts the pupil. The dilator muscles radiate from the pupil such that contraction dilates the pupil. Also here, the action of the dilator is antagonistic to that of the sphincter and the dilator must relax to allow the sphincter to decrease pupil size. Their actions regulate the amount of light that enters the eye as well as the depth of field. The iris sphincter is controlled by the parasympathetic system, whereas the iris dilator is controlled by the sympathetic system (source: University of Texas).
A constriction and relaxation alone cannot do the job, unless some basic tension would exist that would keep the arm continuously bent; in that case a constant muscle tone would be needed to keep the arm fixated somewhere in the middle of its dynamic range. Increased tension would bend it, relaxation would stretch it. However, a decrease in tonus would never allow it to be stretched completely without the assistive force of gravity or another source of power. In addition, it would be a really sluggish process. E.g., using your arms to get up from the ground would be impossible.
In terms of the eye, the pupil response would be fast in the active process, say the constriction, but very sluggish at best in the opposite direction. Viscosity and friction of the tissue might even prevent the antagonistic (dilation in this example) movement altogether, leaving your pupils stuck in a constricted state (in our example), hampering quick adaptation to dark environments.
- Tudor & Deaconescu, MATEC Web of Conferences (2018); 178(3): 07005
Fig. 1. Arm control. source: (Tudor & Deaconescu, 2018)
Fig. 2 Pupil response. source: Mammoth memory