2.90 describe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object

Reflex: fast, involuntary action that protects the body from damage
Reflex arc:


A simple reflex response is one that does not require thinking, and takes place before you feel it. For example, withdrawal reflex to a painful stimulus e.g. touching a hot plate on a cooker.
The response to this is that you contract muscles in your arm to move your hand away from the hot plate.  The key idea is that you will do this before you feel the heat or burn the skin.  The sequence of events is

• touch the hot plate (pain receptors stimulated in the skin)
• move your arm away (reflex arc)
• feel the pain (brain receives the nerve impulses and a conscious sensation of pain is felt)

Detailed Process:

1. heat (Stimulus) is detected by the finger (receptor) and will stimulate sensory neurones to carry electrical impulses to CNS 
2. electrical impulses will be carried from CNS to relay neurons which will carry it through the CNS (where a response is decided) and carried to motor neurons
3. motor neuron will carry electrical impulses to muscles in the arm (effector)
4. muscles (effector) will contract and will withdraw finger away from the hot object (response)