Mydriasis is best described as pupil dilation due to which system?

• A. Pupillary constriction due to the parasympathetic CN III releasing acetylcholine.
• B. Pupillary dilation due to the sympathetic nervous system releasing norepinephrine.
• C. Equal pupil size in bright light due to optic nerve function.
• D. Pupil dilation unrelated to autonomic activity.

Answer: (B)

Pupillary dilation is driven by the sympathetic division of the autonomic nervous system. When sympathetic activity increases, the radial dilator pupillae muscle contracts, caused by norepinephrine acting on alpha-1 adrenergic receptors, which widens the pupil (mydriasis). The pathway starts in the brain, travels down to the spinal cord, ascends to the superior cervical ganglion, and reaches the eye via the long ciliary nerves. In contrast, the parasympathetic system (via the oculomotor nerve and acetylcholine) constricts the pupil by stimulating the sphincter pupillae. Dilation unrelated to autonomic activity or equal pupil size in bright light do not reflect the actual autonomic control of the iris.