Describe to me the pupillary light reflex, and how the differences in light intensity cause this change.

Pupils can either dilate or constrict in response to changes in light intensity.

In very bright light - the eyes constrict - as a protective mechanism
In very dull light/dark conditions - the eye dilates - to maximise the amount of light entering the eye
The changes in pupil size are mediated by two sets of muscles in the eye.

The Circular muscles and the radial muscles - they are antagonistic (they work in opposite directions)When the circular muscles contract the radial relaxwhen the radial muscles contract the circular muscles relax
So in low light levels - to dilate our pupils, two changes occurcircular muscles in the iris relaxradial muscles contract
And in high intensity light - to constrict our pupils...radial muscles relaxcircular muscles contract