Action Potential and Synapses

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  • Created by: syaqub18
  • Created on: 07-05-15 10:54
What are the stages of an action potential?
Stimulus, depolarisation, repolarisation, hyperpolarisation and resting potential
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What happens in the stimulus stage?
Excitation of neurone membrane, Na+ ion channels open, membrane permeable to Na+ ions which diffuse down electrochemical gradient, makes inside of neurone less negative
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What happens in the depolarisation stage?
p.d. reaches threshold -55mV, more Na+ ions channels open, more sodium ions move in to neurone
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What happens in the repolarisation?
At p.d. of about +30mV, Na+ ion channels close and K+ ion channels open, membrane more permeable to K+ ions so these diffuse down electrochemical gradient out of the neurone
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What happens in the hyperpolarisation stage?
K+ ion channels slow to close so slight overshoot where too many K+ diffuse out, p.d. becomes more negative than resting potential (less than -70mV)
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What happens in resting potential stage?
Ion channels reset, sodium-potassium pump returns membrane to resting potential by pumping 3 sodium ions out and 2 potassium ions in, this is maintained until next stimulus.
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What is the refractory period?
After action potential, neurone cell membrane can’t be stimulated straight away, ion channels are recovering, Na+ ion channel closed during repolarisation and K+ ion channel closed during hyperpolarisation
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What is the purpose of the refractory period?
Acts as time delay between one action potential and another, makes sure they don’t overlap, also makes sure they are unidirectional
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What are waves of depolarisation?
Sodium ions diffuse sideways through neurone, causes sodium ion channels in the next part to open for Na+ to diffuse through, wave moves along as channels before are in refractory period
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What is the all or nothing principle?
All action potentials are same size, they are only sent when generator potential reaches threshold, a bigger stimulus would cause more frequent action potentials
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Which is a myelin sheath?
Electrical insulator made from Schwann cell
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What are nodes of ranvier?
Tiny patches of bare membrane which has concentrated sodium ion channels
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How does mylenation affect speed of conduction of action potentials?
Causes saltatory conduction, depolarisation only at nodes of Ranvier, neurones cytoplasm conducts enough electrical charge to depolarise next node, impulse jumps from node to node
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How does axon diameter affect speed of conduction of action potentials?
Big axon diameter means less resistance to flow of ions so quicker in big diameters
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How does temperature affect speed of conduction of action potentials?
Speed of conduction increases with temperature as ions diffuse faster but only up to 40 degrees as proteins start to denature
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What is a synapse?
Junction between a neurone and another neurone/between neurone and effector
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How do synapses ensure impulses are unidirectional?
Receptors only on postsynaptic membrane
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What is acetylcholine?
A neurotransmitter which binds to cholinergic receptors
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What are the stages of a cholinergic synapse?
Arrival of action potential, fusion of vesicles, diffusion of ACh
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What happens on arrival of an action potential in cholinergic synapses?
Action potential arrives and stimulates voltage gated calcium ion channels to open, calcium ions diffuse through and later pumped out by active transport
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What happens in the fusion of vesicles?
Calcium ion influx in presynaptic nob causes synaptic vesicles to fuse with presynaptic membrane, vesicles release acetylcholine in to synaptic cleft by exocytosis
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What happens in the diffusion of Ach?
Ach diffuses across synaptic cleft and binds to receptors on postsynaptic nob, causes Na+ ion channels to open and Na+ to diffuse, causes action potential at postsynaptic nob, acetylcholine broken down by acetylcholinesterase
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What are the differences between cholinergic synapses and neuromuscular junctions?
Postsynaptic membrane has clefts which store acetylcholinesterase to break down Ach, has more receptors than synapse, when motor neurone fires action potential it triggers response
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What is spatial summation?
Two or more presynaptic neurones release neurotransmitters at same time in to postsynaptic neurone, small amount of neurotransmitter build up and can reach threshold
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What is temporal summation?
Two or more nerve impulses arrive in quick succession from same presynaptic neurone, makes action potential more likely as there is more neurotransmitter in synaptic cleft
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How can drugs affect synapses?
Block receptors so fewer activated, inhibit enzyme which breaks down neurotransmitter so they remain their longer, stimulate release of neurotransmitter, and inhibit release of neurotransmitter
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Card 2

Front

What happens in the stimulus stage?

Back

Excitation of neurone membrane, Na+ ion channels open, membrane permeable to Na+ ions which diffuse down electrochemical gradient, makes inside of neurone less negative

Card 3

Front

What happens in the depolarisation stage?

Back

Preview of the front of card 3

Card 4

Front

What happens in the repolarisation?

Back

Preview of the front of card 4

Card 5

Front

What happens in the hyperpolarisation stage?

Back

Preview of the front of card 5
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