Drugs that dilate the pupil without cycloplegia operate by stimulating which iris muscle?

Drugs that dilate the pupil without inducing cycloplegia typically work by stimulating the radial dilator muscle of the iris. The radial dilator muscle, when contracted, pulls the iris peripherally and causes the pupil to enlarge. This mechanism allows for widening of the pupil while preserving the ability to accommodate, or focus on near objects, as there is no paralysis of the ciliary muscle involved in the process.

In contrast, the constrictor muscle, also known as the sphincter muscle, is responsible for constriction of the pupil. The orbicularis muscle is involved in eyelid closure and not in the dilation of the pupil. Therefore, the radial dilator is specifically the muscle that facilitates dilation without affecting accommodation. This understanding is crucial for clinical practice, particularly when considering medications that may influence pupil size without causing significant changes to vision clarity at near distances.