Reflex Arc

Types of Neurones

There are three main types of neurones:

  • Sensory neurones – Carry electrical impulses from the receptors to the central nervous system (brain and spinal cord)
  • Relay neurones – Connect the sensory and motor neurones
  • Motor neurones – Carry information from the central nervous system to the effectors

Reflex Actions

Different neurones work together to produce a rapid and automatic response to a specific stimulus. This response is called a reflex action. Reflexes are very effective at preventing injury because they don’t involve the conscious part of the brain. If the conscious part of the brain was involved, this would waste valuable time in preventing injury.

The route that information follows during a reflex is called a reflex arc.

The passage of the reflex arc goes through the CNS:

1. Receptors detect a stimulus and send the information to the sensory neurone.

2. An electrical impulse passes from the receptor, along a sensory neuron, to the central nervous system (the spinal cord, not the brain).

3. The central nervous system (CNS) processes the information and determines the appropriate response.

4. Electrical impulses are then sent along a motor neurone to an effector, which is usually a muscle.

5. The muscle contracts.


When a bird spots a large cat:

2. The bird’s eyes have receptors which become stimulated.

  • The cat is the stimulus

2. The information is carried along sensory neurones from the receptors to the CNS.

3. The CNS then determines the necessary response.

4. The motor neurones send the information to the bird’s wings, which are the effectors.

5. The bird’s wing muscles contract to help it fly away.


Synapses are gaps where two neurones meet. They are relatively small in size and electrical impulses are unable to jump across them.

1. An electrical impulse travels along the long axon of the first neuron, in the direction of the arrow.

2. The impulse reaches the end of the neurone. This triggers the synaptic vesicle to release chemicals called neurotransmitters into the synaptic cleft.

3. Chemicals called neurotransmitters diffuse across the synapse, down a concentration gradient.

4. The neurotransmitters then bind with the receptor molecules on the second neurone, which are specific to the neurotransmitters released by the first neurone.

5. This stimulates the second neurone to transmit the electrical impulse, which travels down the second axon.

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