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THE SYNAPSE
Where nerve
impulses convert toneurotransmitters
The Sanger Institute
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The synapse is where the nerve impulsepasses from one cell to the next
The electrical signal (the action potential)stops and a chemical signal takes over tocross the gap between the cells
The chemical messenger is called a
neurotransmitter The neurotransmitter crosses the gap by
diffusion, which creates a small delay
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'esigner signals
The advantage of using neurotransmitter is
that the nerve impulse can be given some
more specificity eurotransmitters can also control the
operation of the nervous system by
inhibition orexcitation
any drugs that try to cure problems in the
nervous system operate at synapses
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eurotransmitters and
hormones
In many ways neurotransmitters are
hormonesworking over a very short
distance (about !"nm) Some of them work both at synapses and
in the circulatory system
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*+ ction potential arrives at
terminal button
embrane receptor
for neurotransmitter
-a!.
channel
/esicle storing
neurotransmitter
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'ept of %iology, Saint 0ouis 1niversity
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Ca2+ Ca2+Ca2+
Ca2+
2.'epolarisationopens -a!.channels
-a!.enters terminal
button
3.-a!.stimulates
vesicles to fuse
with membrane
4.2xocytosis of
neurotransmitter
It diffuses !"nm across
the synaptic cleft
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The passage across the
synapse
n action potential travels down an axon to thetermina buttonsor synaptic knobs at the end
The action potential depolarises the membrane ofa terminal button causing !otage"gate# $a2%channesto open
-a!.ions flood into the terminal button This stimulates hundreds of synaptic vesicles,
packed with neurotransmitter, to fuse with themembrane of the terminal button
%y exocytosis The -a!.ions are then pumped out again
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&.0ocalised
depolarisation as
ions leak in or out
of membrane+
'.eurotransmitter
receptor sites on the
postsynaptic membrane
are ion channels+They open when the
neurotransmitter binds
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The passage across the
synapse
The neurotransmitter #iffusesacross cleft topostsynaptic membrane
The neurotransmitter molecules bind withspecific receptor siteson postsynapticmembrane
The receptor sites are part of a igan#"gate# ionchanne
These channels let a.ions in or 3.ions outcausing ocaise# #epoarisationof themembrane
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(.eurotransmitterdestroyed by
en4ymes in the cleft+
Stops signal being
perpetuated+
).ction potential
generated which
travels down the
postsynaptic cell+
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new action potential
If the localised depolarisations build up to thenerve cell threshold, a full action potentiawill beproduced
This will travel away, down the postsynapticneurone
The action of the neurotransmitters stops5(i) as they dilute by #iffusionin the synaptic cleft(ii) by hy#roysisthrough the action of en*ymes
there +mportant,The signal must not be perpetuated
indefinitely
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The neuromuscular 6unction
is a synapse
The motor en# pateis the terminal button
of a motor neuronethat makes contact
with a musce ce The motor end plate releases the
neurotransmitter acetychoinethat
ultimately causes the muscle cell to
contract
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