Uploaded by Jo Bracken

the nervous system revision

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What are reflexes?
Reflexes are fast, automatic, protective
biological control systems that link a stimulus to
a response. A stimulus is a change in the
internal or external environment of an organism.
Reflex reactions happen without you having to
think about them – they are involuntary.
This is because the central nervous
system (CNS) sends electrical signals to
the muscles before the brain can pick up
and process the message.
Many reflexes, such as sneezing and focusing
your eyes, occur naturally, but other reflexes
can be learned, i.e. conditioned responses.
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Reflexes and animals
Reflex reactions govern much of how the simplest
animals behave and help to ensure their survival.
Reflex actions in simple
animals help them to:
 shelter from predators
 find food.
Humans also demonstrate simple
reflexes. These include newborn
reflexes, such as grasping or
sucking, pupil reflexes and
knee-jerk reflexes.
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Why are reflexes so fast?
Reflexes need to be fast in order to protect the body. If you
pick up a hot plate, the reflex action will make you drop it
almost immediately. This is to protect your hand from burning.
The quicker you drop
the plate, the less
damage will be
done to your hand.
However, we can consciously override reflexes. If the hot
plate had your dinner on it, you might try to resist the reflex
to drop it so that you could put it down safely.
Reflexes happen so quickly because they often only involve
three neurones – sensory, relay and motor neurones.
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Neurones are specialized cells that convey information in
the form of nerve impulses. The nervous system uses
electrical impulses to ensure a swift response to any
internal or external stimuli.
Hormones (chemicals secreted by
glands) travel in the blood and
produce slower and longer-lasting
responses; examples include
testosterone and insulin.
In single-cell organisms, hormones and a nervous system
are not necessary. These systems of communication have
developed with the evolution of multi-cellular organisms.
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What are receptors and effectors?
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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
The chemical diffuses across the gap and
makes the next neurone transmit an
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What is a synapse?
A synapse is a junction between two neurones across
which electrical signals pass. The human body contains up
to 500 trillion synapses.
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The release of neurotransmitters
When a nerve impulse arrives at the end of one neurone it
triggers the release of neurotransmitter molecules from
synaptic vesicles.
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Continuing the impulse
The neurotransmitters diffuse across the synaptic cleft and
bind with receptors on the next neurone, triggering another
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Stimulus to response
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Key terms
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Multiple-choice quiz
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