Select the correct answers to the questions from the given options. (Do not copy the questions, write the correct answer only.)
Karan was standing on a high stool and cleaning the ceiling fan. He suddenly loses balance and sustains a head injury. An examination reveals that his pupils have lost the capacity to constrict in bright light. Which structure has been damaged?
Topic: Brain — control of the pupillary reflex
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Pick an option and check your answer. The full worked answer is below either way.
Answer
(b) Medulla oblongata.
The narrowing of the pupil in bright light is the PUPILLARY REFLEX — an involuntary, automatic response. Involuntary actions of this kind are controlled by the MEDULLA OBLONGATA, so damage there abolishes the reflex.
Why the others fail — all three are parts of the EYE, not control centres: (A) SUSPENSORY LIGAMENTS hold the lens in place and alter its curvature for focusing. They have nothing to do with pupil size. (C) and (D) The lens and the eyelid are parts of the eye, not control centres. The pupil is narrowed by the muscles of the IRIS, and those muscles are being told what to do by the brain. Since the eye itself is working, the fault must lie in the structure issuing the instruction.
THE TRAP: the words "loses balance" in the question point straight at the cerebellum, and many candidates choose it for that reason. Read what is actually being asked — the SYMPTOM is a failed pupillary reflex, not a failure of balance.
Fix the main control centres: cerebrum = thinking, memory, voluntary action; cerebellum = balance and coordination; medulla oblongata = involuntary actions such as heartbeat, breathing, swallowing and reflexes of this kind.
Medulla oblongata.