2.85 reflex arc

http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/human/thenervoussystemrev3.shtml

This is what happens:

1. receptor detects a stimulus - change in the environment
2. sensory neurone sends signal to relay neurone
3. motor neurone sends signal to effector
4. effector produces a response