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Science Revision notes

The nervous system

The nervous system allows the body to respond to changes in the environment. This is a
process usually coordinated by the brain. Reflex actions are extra-rapid responses to
stimuli, and this process also involves the nervous system, but bypasses the brain.

Receptors and effectors…

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chemicals in food


chemicals in the air




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sound and position of the head


The central nervous system - CNS - in humans consists of the brain and spinal cord. When a
receptor is stimulated, it sends a signal along the nerve cells - neurones - to the brain. The
brain then co-ordinates the response.


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A motor neurone

Where two neurones meet, there is a tiny gap called a synapse. Signals cross this gap
using chemicals. One neurone releases the chemical into the gap. The chemical diffuses
across the gap and makes the next neurone transmit an electrical signal.

Reflex actions
When a receptor…

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In dim light:
Radial muscles of the iris contract.
Circular muscles of the iris relax.
More light enters the eye through the dilated pupil.


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