Iris muscle response to light

Steps:

• Bright light enters the eye and stimulates retinal photoreceptors.
• Signals travel via optic nerve to the brain's pretectal nucleus.
• Brain sends parasympathetic signals to the iris sphincter muscle.
• Circular muscles contract, reducing pupil size to limit light entry.

Why B is correct:

• The iris contains circular (sphincter) muscles that constrict the pupil when they contract, as defined in ocular anatomy for the pupillary light reflex.

Why the others are wrong:

• A: Ciliary muscles adjust lens shape for focusing, not pupil size.
• C: Sensory cells (photoreceptors) are in the retina, not the iris.
• D: Sensory neurons transmit signals from retina to brain; ciliary muscles are not involved in pupil constriction.

Final answer: B