what is a synapse? a synapse is the junction between 2 neurones. there is a very narrow gap of about...
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What is a synapse?
A synapse is the junction between 2 neurones.
There is a very narrow gap of about 20nm between neurones called the synaptic cleft.
An action potential cannot cross the synaptic cleft, so nerve impulses are carried by chemicals called neurotransmitters.
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A Synapse Pre-synaptic neurone = neurone sending impulse Post-synaptic neurone = neurone receiving impulse
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Neurotransmitter Neurotransmitter is made by
the pre-synaptic neurone and is stored in synaptic vessels at the end of the axon.
The membrane of the post-synaptic neurone has chemical-gated ion channels called neuroreceptors. These have specific binding sites for neurotransmitters.
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Cholinergic Synapses
Acetylcholine is a common transmitter.
Synapses that have acetylcholine transmitter are called cholinergic synapses.
Some neurones form more than 1 synapse.
This is an electron micrograph of synapses between nerve fibres and a neurone cell body.
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What happens at a cholinergic synapse? Stage 1
An action potential arrives at presynaptic membrane. Voltage gated calcium channels in the presynaptic membrane open, calcium ions enter the presynaptic neurone.
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What happens at a cholinergic synapse? Stage 2
Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft.
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What happens at a cholinergic synapse? Stage 3
Acetylcholine diffuses cross the synaptic cleft and binds to specific neuroreceptor sites in the post synaptic membrane.
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What happens at a cholinergic synapse? Stage 4
Sodium channels open. Sodium ions diffuse into the postsynaptic membrane causing depolarisation, which may initiate an action potential.
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What happens at a cholinergic synapse? Stage 5
Acetylcholinesterase breaks down acetylcholine. The products diffuse back into the presynaptic neurone where acetycholine is resynthesised using ATP from the mitochondria.
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Neuromuscular Junctions Same stages as
cholinergic synapses, but in this case the postsynaptic membrane is the muscle fibre membrane, (Sarcolemma). Depolarisation of the sarcolemma leads to contraction of muscle fibre.
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Drugs Drugs which have molecules of
similar shape to transmitter substances can affect protein receptors in postsynaptic membranes.
Drugs that stimulate a nervous system are called AGONISTS
Drugs that inhibit a nervous system are called ANTAGONISTS.
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Various effects of drugs on synapses:
DRUG ACTION EFFECT
Mimic a neurotransmitter Switch on a synapse
Stimulate the release of a neurotransmitter
Switch on a synapse
Open a neuroreceptor channel
Switch on a synapse
Block a neuroreceptor channel
Switch off a synapse
Inhibit the breakdown enzyme
Switch on a synapse
Inhibit the Na+K+ATPase pump
Stop action potentials
Block the Na+ or K+ channels
Stop action potentials
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Effect of nicotine and atropine
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Summary A synapse is the point where 2 nerve cells
meet. Tiny gap = synaptic cleft. Chemical transmitter released from
presynaptic neurone diffuses across synaptic cleft & fits into receptors on postsynaptic membrane. May cause postsynaptic neurone to depolarise & set up action potential.
Neuromuscular junction = motor neurone connects with muscle fibre – similar to a synapse.
Many drugs affect synapses.