Reflexes (WJEC GCSE Biology)

• A reflex action is an involuntary response that 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
• Reflex actions are:
  • Automatic
  • Rapid
  • Protective
• Some examples include:
  • The withdrawal reflex:
    • Contractions in limb muscles to cause sudden withdrawal of the limb from a potentially damaging stimulus, such as high heat or a sharp object; this protects the limb from harm
  • Blinking
    • Contraction of muscles in the eyelid to close the eyes against a potentially harmful stimulus, e.g. an allergen or a bright light; this protects the eyes against damage
  • The pupil reflex
    • Changes in pupil size occur due to contraction of muscles in the eye
      • The pupil becoming smaller protects the back of the eye from damage caused by bright light
      • The pupil becoming larger allows more light to enter the eye in dark conditions

Higher Tier Only

• A reflex arc is the pathway taken by electrical impulses as they travel along neurones during a reflex response
• Several components make up a reflex arc; these include:
  • Stimulus - (plural stimuli) a change in the internal or external environment that is detected by receptor cells
  • Receptor - cells within the sense organs that detect a stimulus
  • Coordinator - neurones in the CNS, e.g. in the spinal cord
    • Note that for reflex arcs this will not involve neurones in the conscious parts of the brain
  • Effector - a muscle or gland to bring about the response
• 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 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 coordinator in the spinal cord
  3. An electrical impulse is passed to a relay neurone in the spinal cord
  4. A relay neurone passes the impulse to a motor neurone via a synapse
  5. A motor neurone carries an impulse to a muscle in the leg, which acts as the effector
  6. When stimulated by the motor neurone the muscle contracts and pulls the foot up and away from the sharp object in response
• This all occurs within a fraction of a second
• The direction of travel in a reflex arc is always from the stimulus and receptor cells towards the spinal cord and then to the effector
  • The sensory neurone travels from the sensory receptor to the CNS
  • The motor neurone travels from the CNS to the effector

Reflex arc diagram

The reflex arc pathway for a pain-withdrawal reflex; reflex actions are automatic and rapid