Simple Reflex Arc (Edexcel GCSE Biology: Combined Science)

• 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)
  7. 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

The neurones of the reflex arc

• There are three main types of neurone in a reflex arc: sensory, relay and motor
  • Sensory neurones carry impulses from sense organs to the CNS (brain or spinal cord)
  • Relay neurones are found inside the CNS and connect sensory and motor neurones
  • Motor neurones carry impulses from the CNS to effectors (muscles or glands)

You need to be able to recognise the three types of neurone in a reflex arc