The nervous system

The nervous system allows you to react to what goes on around you.

Sense organs detect stimuli (receptor). A stimulus is a change in your environment which you may need to react to.

Sense organs: Eyes, ears, nose, tongue, and skin. They all contain different receptors, they change stimulus energy into electrical impulses. A stimulus can be: light, sound, touch, pressure, pain, chemical, change in position or temperature.

For example: Eye: sense organ, it contains light receptors.

Sensory neurones: The nerve cells that carry signals as electrical impulses from the receptors to the sense organs to the CNS.

Relay neurones: The nerve cells that carry signals from sensory neurones to motor neurones.

Motor neurones: The nerve cells that carry signals from the CNS to the effector muscles/glands.

Effectors: Muscles and glands, they respond in different ways. Muscles contract in response to a nervous impulse, glands release hormones.

The central nervous system is where all the information from the sense organs are sent and where reflexes and actions are coordinated. The CNS consists of the brain and spinal cord only. Neurones transmit the information very quickly to and fro from the CNS. Instructions are sent to the effectors which respond accordingly.

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The 5 Sense Organs


  • Contains light receptors, sensitive to light. These cells have a nucleus, cytoplasm, and cell membrance.


  • Contains sound receptors, sensitive to sound, also balance receptors (change in position).


  • Contains smell receptors, sensitive to chemical stimuli.


  • Contains taste receptors, sensitive to bitter, salt, sweet, and sour plus the taste of savoury things like monosodium glutamate, sensitive to chemical stimuli.


  • Sensitive to touch, pressure, pain and temperature change.
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