A receptor is able to pass on information about the surroundings to other neurones as an electrical impulse and this is achieved via a synapse.

A neurone secretes a chemical messenger, known as the neurotransmitter, which passes from one cell to the other across the synapse. The receiving neurone is then able to pass on this electrical impulse and a chain is created. The nervous system is then able to respond accordingly.

Reflex actions

Reflex actions are actions carried out automatically by the body, reactions that you don’t even have to think about. For example, if you put your hand on a hot stove your body’s reflex action is to move it away quickly before it gets badly burnt.

Usually, a reflex action involves three types of neurone:

• motor neurones connect directly to the CNS and co-ordinate the correct response
• relay neurones carry electrical impulses from the CNS to the effector
• sensory neurones transport an electrical impulse from the receptors to the CNS

If you take the stove example above, the process of a simple reflex action is as follows:

You put your hand on a hot stove.

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Receptors in your skin register this.

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The electrical impulse is passed to a sensory neurone.

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The sensory neurone passes on the information to a relay neurone in the CNS as neurotransmitter through a synapse.

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Neurotransmitter is then released between the relay neurone and a motor neurone.

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The motor neurone then sends an electrical impulse to the effector or the specific muscle or gland which needs to carry out the response which, in this example, is your hand.

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The muscles in your hand contract and move away from the hot stove.

(A gland, on the other hand, would respond by secreting chemical substances.)