Describe the 'reflex arc'

The relfex arc is an innate response designed to keep us safe (this means we do not consiously think about doing it). One of the most common examples for this question is a person touching a hot flame and immediately withdrawing their hand to prevent damage (burning). The main stages in the reflex arc are as follows:

1.   The sensory receptors detect the stimulus (Nerves in your fingers will detect the heat of the flame)

2.   The sensory neurone passes this 'message' on via the relay neurone to the spinal cord in the central nervous system

3.   The 'message' is then passed from the spinal cord to a motor neurone

4.   This motor neurone sends the 'message' to an effector (in this example the effector may be a muscle which contracts to remove the finger from the flame)

Every time the 'message' is passed between neurones or the spinal cord, it is passes on via a synapse. A synapse works like this:

1.   A 'message' comes to the end of a neurone

2.   It causes a chemical called a neurone transmitter (usually acetylcholine) to be released into the gap between the end of the first neurone and the start of the next.

3.   Thhis chemical diffuses accross the gap to the next neurone. It binds to receptors on the surface and continues the 'message'.