The impulses from the receptor pass along a sensory neurone to the central nervous system. A motor neurone carries the impulses to the muscles which moves the hand away from the source of heat. This is the RESPONSE
There are two types of responses controlled by the nervous system.
1. Learned responses. These responses are slower and need to be learned (you are not born with them). Example imagine an athlete is waiting to start a 100m race. The starter starts the race with a gun.
The noise of the gun stimulates the sensitive cells in the ears, these cell then send electrical impulses (messages) to the central nervous system, that is the spinal cord and brain, via sensory neurone. Once the impulses get to the brain, it sorts out the message and then co-ordinates a response. The impulses then travel from the brain to the muscles via the motor neurone. When the muscles receive the impulses they contract and move the athlete.
The stimulus is the starter shooting the gun.
The receptors are the cells in the athletes' ears.
The co-ordinator is the athlete's brain.
The effectors are the muscles in the athletes' legs.
The response is to start running.
Stimulus - receptor - sensory neurone - co-ordinator - motor neurone - effector - response
2. Reflex actions. These types of reactions are very fast and are automatic (you are born with them). These actions involve three neurones called sensory, relay and motor neurones. For example: if you touch something hot with your hand.
Pain sensitive receptors in the skin detect pain. The impulses from the receptor pass along a sensory neurone to the central nervous system. At a junction (synapse) between the sensory neurone system and the relay neurone, in the central nervous system, a chemical is released that causes an impulse to be sent along the relay neurone. At a junction (synapse) between the relay and the motor neurone, a chemical is released that causes impulses to be sent along the motor neurone. The motor neurone carries the impulses to the muscles (the EFFECTOR) which moves the hand away from the source of pain. This is the RESPONSE.
A reflex action always follows the path:
Stimulus - receptor - co-ordinator - effector - response
The major difference between learned responses and the reflex action is that the body increases the speed of reflex actions by cutting out a part of the nervous system, that is, the brain.
NOTE: The effector can be either a muscle or a gland. Muscles respond to impulses by contracting, glands respond by secreting.