## Step4

Pupillary reflex

When you look at a human eye, you will see a white part, a coloured part and a dark spot. This dark spot is called the pupil. Actually the pupil is a hole in your eye where the light falls through. The pupil can become bigger and smaller to control the amount of light going in.

Watch the following videos:

Pupillary reflex in a human

Papillary reflex in the W-shaped pupil of the cuttlefish

Your pupil will automatically become smaller when there is a lot of light and become bigger when there is very little light. This way the pupillary reflex makes sure there is a more or less constant amount of light entering our eyes. This is called a negative feedback loop, these negative feedback loops makes sure something stays at the same level without us having to think about it. Make sure to study the picture below to understand the negative feedback loop in our pupillary reflex.

The negative feedback loop of the pupillary reflex.

Now with a partner, try to answer the next questions:

1. What happens with the pupil when there is a lot of light?

2. What happens with the pupil when there is not much light?

3. What does this sentence mean?: 'The pupillary reflex is an example of negative feedback loops.'