- An involuntary (or reflex) response does not involve the brain as the coordinator of the reaction and you are not aware you have completed it until after you have carried it out
- This is an automatic and rapid response to a stimulus such as touching something sharp or hot
- As it does not involve the brain, a reflex response is quicker than any other type of nervous response
- This helps to minimise the damage to the body
- The pin (the stimulus) is detected by a pain/pressure/touch receptor in the skin
- Sensory neurone sends electrical impulses to the spinal cord (the coordinator)
- Electrical impulse is passed on to relay neurone in the spinal cord
- Relay neurone connects to motor neurone and passes the impulse on
- Motor neurone carries impulse to a muscle in the leg (the effector)
- The muscle will contract and pull the foot up and away from the sharp object (the response)
Reflex actions are:
A common exam question is to be asked to draw arrows on the neurones in the reflex arc diagram to show the direction of movement of the impulse.
Make sure you read questions carefully – not all questions have a line underneath them to write an answer!
- Neurones never touch each other
- The junctions (gaps) in between them are called synapses
- The electrical impulse travels along the first axon
- This triggers the nerve-ending of the presynaptic neurone to release chemical messengers called neurotransmitters from vesicles which fuse with the presynaptic membrane
- The neurotransmitters diffuse across the synaptic gap and bind with receptor molecules on the membrane of the second neurone (known as the post synaptic membrane)
- This stimulates the second neurone to generate an electrical impulse that travels down the second axon
- The neurotransmitters are then destroyed to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent
- Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions
- As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system – eg this is where heroin works
For maximum marks you will need to be able to understand the structure and functioning of a synapse and explain what happens at each step.