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Keeping in touch

We perceive what happens on the outside world through
our senses
Receptors in the sense organs convert the stimulus into
electrical signals that travel in the brain

Sense organs in humans

Eyes which contain cells sensitive to light
Ears which contain cells sensitive to sounds
Which contain…

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Tongue which contains cells sensitive to tastes
Nose which contains cells sensitive to smells
Skin which contains cells sensitive to touch and

All these

All these cells send messages to the Central Nervous
System ( CNS )
These are made up of the brain and the spinal cord


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When the central nervous system receives a signal it
makes a decision whether to take action or not to take
It then responds by causing muscles or glands to do
something, this is called co ­ ordination
The cells that carry the messages are called nerves
Nerves are bundles…

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There are several kinds of neurones, the most important being :
Sensory neurones that carry messages to the brain
Motor neurones that carry messages from the brain

Sensory neurones

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Motor Neurones

The branches of the neurones ( dendrites ) are connected
to other dendrites from other cells
There is a tiny gap called a synapse

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Synapses 2

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The signal causes special chemicals, neurone transmitters,
to cross the gap from one nerve ending to another which
transfers the signal
The neurone transmitters are then destroyed by special

Co ­ ordination by the Central Nervous System

The Central Nervous System has to decide how the body
should respond,…

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These type of responses is very rapid and does not involve
conscious thought
It is often used to protect the body from damage

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The reflex arc

Receptor Sensory Relay Motor Effector
Neurone Nerone Neurone

The eye

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The iris and the pupil

The coloured part of the eye us called the iris
The iris has two sets if muscles
These control the amount of light going in the eye

In dim light

Large pupil lets more light into the eye

In bright light


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