GCSE Biology students often find the reflex arc a difficult topic in the section on human coordination and response.  This is because it is the only type of response they learn about and doesn’t really fit into a sensible flow of ideas on the various types of behaviours organisms can show.  But it is not too complicated, at least if you restrict yourself to ideas that might be tested in the iGCSE exam.

Prior Knowledge (you need to understand these things before you can appreciate a reflex arc)

• basic structure of a neurone/nerve cell
• three different kinds of neurones – sensory, motor and relay – and where they are found in the body
• nerve impulses are electrical events that travel at up to 100ms-1 along nerve cells but cross synapses much more slowly by diffusion of a chemical called a neurotransmitter

Reflex responses

Most human behaviours are complex and involve millions of neurones interacting in the brain.  Our ability to link stimuli (changes in the environment) with an appropriate response can develop over time, can be modified by past experience and can produce different outcomes depending on the circumstances.  For example if you see a fast moving spherical object moving towards your head, you might head it (football), catch it (cricket), hit it (cricket again), duck out of the way (cricket again) or eat it (flying Malteser)

A simple reflex response is much more straightforward:  the same stimulus always produces the same response.  It does not need to be learned but is innate (you are born with it) and in humans, reflex responses tend to be involved in protecting the body from harm or maintaining posture.  The example we look at is called a 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

The reason that you move your arm away before you feel anything is that your brain is not involved in this response.  This produces a rapid, involuntary reaction called a reflex response.  The reason the response is so rapid is that at most three neurones are involved in linking the painful stimulus to the response.  The arrangement of these three neurones is called a reflex arc.

The cell that detects the stimulus is called a sensory neurone.  One end of this cell is a pain receptor in the skin and the other end of this individual cell is found in the spinal cord (see diagram above)  Neurones can be very long cells!  The sensory neurone forms a synapse (junction) with a relay neurone found entirely in the grey matter in the centre of the spinal cord and this in turn synapses with a motor neurone.  The cell body of the motor neurone is on the spinal cord and the other end of this individual cell is a synapse with a skeletal muscle in the arm.

Synapses are the things that slow nerve impulses down and as this whole pathway only includes two synapses (sensory-relay and relay-motor) the response will be as fast as possible.  The response is involuntary as the brain is not involved.

In humans, we can modify most reflex responses using the conscious parts of our brain.  As the sensory neurone synapses with the relay neurone in the diagram, it will also synapse with other neurones carrying nerve impulses up to the brain.  This is why touching a hot plate will hurt (the feeling of pain is in the brain).  There will also be neurones from the brain that can modify the synapse between the relay and motor neurone.  If I told you that I would pay anyone who can touch a hot plate for 2 seconds $10,000 (although of course I don’t have $10,000) many of you would be able to force yourself not to pull your arm away from the hotplate when you touch it.  You could overcome the reflex response with signals from your brain which would know how much fun you could have with $10,000.