The nervous system
When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. This is a reflex action.
Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.
This is what happens:
The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action.
In bright light:
In dim light:
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