Lesson 10 of 10 / Homeostasis and response
The pupil reflex
How does the eye change the amount of light entering?
I can compare the antagonistic iris-muscle responses in bright and dim light.
I can trace receptors, neurones and effectors in reflexes.Syllabus
Syllabus K325, Topic 8(k). Trace receptors, neurones and effectors in reflexes.
I can explain the pupil reflex in bright and dim light.Syllabus
Syllabus K325, Topic 8(n). Explain the pupil reflex in bright and dim light.
About 5 min
Make a guess. You are not marked.
You walk out of a dark cinema into bright sunlight. Which iris muscles contract?
Show the answer
The circular muscles, which make the pupil smaller.
Circular muscles contract and radial muscles relax, so the pupil constricts and less light enters.
The key idea
Bright light makes circular iris muscles contract and radial muscles relax, narrowing the pupil and reducing light entry.
- Circular muscles
- Contract
- Radial muscles
- Relax
Circular contraction narrows the opening. Radial muscles relax. Less light reaches the retina.
Blue ring: circular muscles. Brown spokes: radial muscles. Dark centre: pupil opening. Line thickness highlights contraction, not measured muscle dimensions.
Reflex route: retina receptors -> sensory pathway -> brain -> motor pathway -> iris muscles. This controls light entry, not lens focus.
Explanation
The pupil is an opening whose diameter is controlled by muscles in the iris. Circular muscles run around the pupil; radial muscles run outwards through the iris. They act antagonistically: their actions oppose one another.
In bright light, circular muscles contract and radial muscles relax. The pupil constricts, reducing the amount of light entering the eye and helping protect the retina from excessive illumination.
In dim light, radial muscles contract and circular muscles relax. The pupil dilates, allowing more light to enter. The pupil itself does not contract as a muscle; its size changes because the surrounding iris muscles act.
Light receptors in the retina detect the stimulus. Sensory signals travel towards the brain, where the reflex is coordinated; motor signals then reach iris muscles. This is an automatic response. It regulates light entry, while accommodation changes lens shape to focus near or distant objects.
Step by step
- 1
State the light condition
Use brightness, not object distance, to choose this response.
- 2
Name both muscle groups
One contracts while the opposing group relaxes.
- 3
Explain the pupil change
Connect pupil diameter to the amount of light reaching the retina.
Worked example
Entering a dim room
Explain the pupil response on moving from bright surroundings into a dim room.
One way to explain it
Radial iris muscles contract and circular muscles relax. The pupil dilates, allowing more light to enter the eye and reach the retina.
Why this answer works
- Dim light calls for more light entry.
- Name the muscles, not just "the pupil contracts".
- Use dilates for the increase in pupil diameter.
Watch out for this
A student says: "A smaller pupil focuses a distant object by flattening the lens." What is wrong with this?
Show the answer
Pupil size controls light entry. Ciliary muscles and suspensory ligaments control lens shape for accommodation.
Check your understanding
Which response follows an increase in light intensity?
Show the answer
Circular iris muscles contract, radial muscles relax and the pupil narrows.
A smaller opening reduces the amount of light entering the eye.
Quick recall
How does the eye change the amount of light entering?