The Nervous System

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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
Receptors are groups of specialised cells. They can detect changes in the environment,
which are called stimuli, and turn them into electrical impulses. Receptors are often
located in the sense organs, such as the ear, eye and skin. Each organ has receptors
sensitive to particular kinds of stimulus.
sense organs receptors sensitive to
touch, pressure, pain and

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chemicals in the air
Eyes…read more

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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.
An effector is any part of the body that produces the response.…read more

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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 is stimulated, it sends a signal to the central nervous system, where the
brain co-ordinates the response. But sometimes a very quick response is needed, one that
does not need the involvement of the brain.…read more

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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.…read more


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