Biology · Coordination and control. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A sense organ is a group of receptor cells that respond to one kind of stimulus, such as light, sound, touch, temperature or chemicals. The eye is the sense organ for light.
Light is refracted by the cornea and then by the lens, which focuses it on the retina. Receptor cells there send impulses along the optic nerve to the brain.
Two adjustments must be learned. In the pupil reflex, the circular and radial muscles of the iris change the size of the pupil to control how much light enters. In accommodation, the ciliary muscles and suspensory ligaments change the shape of the lens, so that near or distant objects are focused on the retina.
Students mix up the two sets of muscles. Iris muscles (circular and radial) change the pupil for light intensity; ciliary muscles change the lens for distance.
Answer: The pupil widens because the radial muscles contract and the circular muscles relax, so enough light reaches the retina for the person to see in dim light.
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