Reflex arc

Neurones work together to make a reflex action

Different types of neurones work together in a reflex action.

This creates an automatic, and rapid response to a stimulus, which protects the body from potentially harmful conditions, such as touching something hot.

A reflex action follows this general sequence and does not involve the conscious part of the brain, which makes it much quicker.

The nerve pathway followed by a reflex action is called a reflex arc. For example, a simple reflex arc happens if we accidentally touch something hot.

The nerve pathway in the arm
  1. Receptor in the skin detects a stimulus (the change in temperature).
  2. Sensory neurones send electrical impulses to relay neurones, which are located in the spinal cord. They connect sensory neurones to motor neurones.
  3. Motor neurone sends electrical impulses to an effector.
  4. Effector produces a response (muscle contracts to move hand away).

The signal is sent to the brain in additional relay neurones shortly afterwards or you wouldn't remember burning your hand. The time saved by sending this signal later than normal reduces damage to your body.

Where two neurones meet, there is a small gap, a synapse.

  1. An electrical impulse travels along the first axon.
  2. This triggers the nerve-ending of the neurone to release chemical messengers called neurotransmitters.
  3. These chemicals diffuse across the synapse (the gap) and bind with receptor molecules on the membrane of the second neurone.
  4. The receptor molecules on the second neurone bind only to the specific neurotransmitters released from the first neurone. This stimulates the second neurone to transmit the electrical impulse to continue its journey.

Synapses also allow one neurone to start signals in more than one neurone at the same time.


Scientists have studied how fast a neurone can transmit an electrical impulse in frogs.

They use the equation:

\text{Speed}= \frac{\text{Distance}}{\text{Time}}

They measured the length of the neurone as 50 mm (0.05 m) and measured the time the impulse took to travel the length of the impulse as 1.5 ms (0.0015 s) Calculate the speed of the impulse to two decimal places in m/s.

Speed = \frac{0.05}{0.0015} = 33.33 \text{m/s} =