A simple reflex is a rapid and automatic response to a stimulus. This means that the action is unconscious (although some can be modified). Such a response is important for preventing damage to the body. Examples include:

  • moving your body away from something very hot or sharp
  • knee-jerk to prevent damage to your knee joint
  • pupil reflex in bright light

A simple reflex arc is composed of three neurones which control the reflex actions. It’s a nerve pathway which is innate or built-in.

Five features of a reflex are:

  • it only uses a few neurones
  • it’s very fast as there’s only a few synapses for the message to move across
  • it’s involuntary
  • it’s innate
  • it’s invariable so the response is always the same

If you touch something sharp or hot, for example:

  • the sensation is picked up by a pain receptor in the skin
  • a sensory neurone transports the impulse to an interneurone located in the spinal cord
  • the nerve impulse is then carried by a motor neurone to a muscle which contracts

Secondary responses will also take place as the interneurone. Situated in the spinal cord it tends to also have synapses with other interneurons which relay the impulse to other muscles as well as the brain.