The Reflex Arc (Cambridge (CIE) IGCSE Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
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The Reflex Arc
Voluntary Responses
A voluntary response is one where you make a conscious decision to carry out a particular action therefore it starts with your brain
An example is reaching out to pick up a cup of coffee
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
Involuntary actions are usually ones which are essential to basic survival and are rapid, whereas voluntary responses often take longer as we consider what the consequences might be before doing it
Reflex Responses
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
A reflex
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)
The reflex pathway
Synapses
Where two neurons meet or join, they do so at a junction called a synapse
Synapses allow junctions between neurones so are important in the nervous system being a connected network of neurones
Nerve impulses can transmit across synapses and be directed along the appropriate route by them eg. to the correct part of the brain
Think about the analogy of railway points that guide the trains onto the appropriate tracks based on that train's destination.
Examiner Tips and Tricks
Reflex actions are:
Automatic
Fast
Protective
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, so they can be easy to miss!
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