Comprehensive Osteopathic Medical Licensing Examination (COMLEX USA) Practice

What is the PNS effect on the eye?

1. Increased pupil dilation

2. Decreased accommodation

3. Miosis

4. Myopia

The correct answer is: Miosis

The effect of the parasympathetic nervous system (PNS) on the eye primarily involves the modulation of pupil size and the ability to focus on close objects. When the PNS is activated, one of its hallmark responses is miosis, which is the constriction of the pupil. This occurs due to the contraction of the circular muscle of the iris, which is innervated by parasympathetic fibers. Miosis allows more precise control over the amount of light entering the eye, particularly in bright conditions, and enhances near vision by increasing the depth of focus. The other options reflect effects that do not occur with PNS activation. For instance, increased pupil dilation is primarily mediated by the sympathetic nervous system, which causes the radial muscles of the iris to contract. Decreased accommodation is typically linked to reduced PNS activity; the PNS is responsible for stimulating accommodation (the ability of the lens to focus on near objects). Myopia, or nearsightedness, is a refractive error related to the shape of the eye or lens function and is not an effect caused by the PNS. In summary, the primary action of the PNS on the eye is to induce miosis, aligning with its role in promoting rest and