Final answer:
A sudden flash of bright light would cause contraction of the pupillary constrictor muscles, triggering the pupillary light reflex. This autonomic response is designed to reduce the amount of light entering the eye by making the pupil smaller, and it occurs bilaterally, even if the light is presented to only one eye.
Step-by-step explanation:
A sudden flash of bright light would primarily cause contraction of the pupillary constrictor muscles. This physiological response is known as the pupillary light reflex. When light hits the retina, the optic nerve sends a signal to the brain, which then triggers the oculomotor nerve to initiate a parasympathetic response. This leads to the release of acetylcholine on the circular fibers of the iris, resulting in the pupils contracting to reduce the amount of light entering the eye.
The pupillary light reflex is an example of an autonomic reflex, which is different from somatic reflexes that govern voluntary muscle movements. In the context of this reflex, if a sudden flash of light is presented to one eye, it will cause a bilateral response - both pupils will constrict, not just the one exposed to the stimulus. This helps to maintain consistent vision and light perception.
The size of the iris does not increase; it is the contraction or relaxation of the muscles within the iris that leads to changes in pupil size - constriction or dilation. No relaxation occurs in the ciliary body or ciliary ligaments in direct response to a flash of bright light since these structures are involved in lens curvature, affecting focus rather than light intake.