Which statement about mydriasis is correct?

Which statement about mydriasis is correct?

• A. It results from parasympathetic activation.
• B. It results from sympathetic nervous system activation releasing norepinephrine.
• C. It results from somatic motor activation.
• D. It is a central phenomenon independent of autonomic input.

Answer: (B)

Mydriasis is produced by the sympathetic branch of the autonomic nervous system. When this system activates, norepinephrine is released onto the dilator pupillae muscle (the radial muscle) of the iris, causing the pupil to dilate. In other words, sympathetic drive pulls the pupil open, which is why dilation occurs in low light or during stress. Parasympathetic activation does the opposite: it releases acetylcholine to stimulate the sphincter pupillae, causing constriction (miosis). Somatic motor input does not control pupil size; it’s managed by autonomic pathways. And the pupil’s size is not independent of autonomic input—the central nervous system modulates these autonomic pathways, with the primary mechanism for dilation being sympathetic output and norepinephrine release.

Mydriasis is produced by the sympathetic branch of the autonomic nervous system. When this system activates, norepinephrine is released onto the dilator pupillae muscle (the radial muscle) of the iris, causing the pupil to dilate. In other words, sympathetic drive pulls the pupil open, which is why dilation occurs in low light or during stress.

Parasympathetic activation does the opposite: it releases acetylcholine to stimulate the sphincter pupillae, causing constriction (miosis). Somatic motor input does not control pupil size; it’s managed by autonomic pathways. And the pupil’s size is not independent of autonomic input—the central nervous system modulates these autonomic pathways, with the primary mechanism for dilation being sympathetic output and norepinephrine release.