• Reflexes have to be quick and they are so because they are automatic. The message of a reflex does not go up the brain which is why they are involuntary.
  • Many reflexes occur naturally such as accommodation and sneezing but some can be learned e.g. conditional reflex.
  • These prevent us from being injured, or at least reduce the potential injury.
  • The passage of information in a reflex is called a reflex arc.

Reflex arc


  1. The neurones in the reflex arc go through the spinal cord or through an unconscious part of the brain (you don’t have to think).
  2. When a stimulus (e.g. pin) is detected by the receptors, impulses are sent along a sensory neurone to the CNS.
  3. In the CNS the sensory neurone passes on the message to a relay neurone.
  4. Relay neurones relay the impulses to a motor neurone.
  5. The impulses then travel along the motor neurone to the effector (e.g. muscle)
  6. The muscle then contracts and moves the hand away from the pin.
  7. As you don’t have to think about this response, it’s quicker than a normal response.

What occurs at a synapse

  1. Electrical impulse travels along the first neurone (pre-synaptic)
  2. If the impulse is strong enough it causes neurotransmitters to be released across the synapse (gap between two neurones).
  3. The neurotransmitter is detected by receptors on the next neurone (post-synaptic)

Once enough neurotransmitter is detected the post-synaptic neurone transmits the electrical signal again.