Simple Reflex Arc (Edexcel IGCSE Biology)

• A reflex response (also known as an involuntary response) does not involve the conscious part of the brain as the coordinator of the reaction
• Awareness of a response having happened occurs after the response has been carried out
• Responses are therefore automatic and rapid – this helps to minimise damage to the body and aids survival
  • Pain-withdrawal, blinking, and coughing are all examples of reflex responses that help us to avoid serious injury, such as damage to the eye or choking

The reflex arc

• A reflex arc is the pathway of a reflex response (specifically, the pathway taken by electrical impulses as they travel along neurones)
• An example of a reflex response is the pain-withdrawal reflex that occurs when someone steps on a pin
• The reflex arc for this response is outlined below:
  1. The pin (the stimulus) is detected by a (pain/pressure/touch) receptor in the skin on the person's foot
  2. A sensory neurone sends electrical impulses to the spinal cord (the coordinator)
  3. An electrical impulse is passed to a relay neurone in the spinal cord (part of the CNS)
  4. A relay neurone synapses with a motor neurone
  5. A motor neurone carries an impulse to a muscle in the leg (the effector)
  6. When stimulated by the motor neurone, the muscle will contract and pull the foot up and away from the sharp object (the response)
• This all occurs within a fraction of a second

The reflex arc pathway (in this case for a pain-withdrawal reflex). Reflex actions are automatic and rapid; they do not involve the conscious part of the brain.