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Nervous System

Saturday, 21 June 2014

2:22 PM

Mostly STQs

Describe and explain the transmission of an action potential along a myelinated neurone
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○ Under the resting state of a myelinated neurone, the axoplasm has a higher K+ concentration and a lower
Na+ concentration compared with the external tissue fluid, maintaining a resting membrane potential
of -65mV to -70mV
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ATP is required for this active transport process, which brings out 3 Na+ for
every 2 K+ taken in
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○ During the action potential, there is a rapid change in the potential difference across the neurone,
consisting of three phases: the depolarisation stage, repolarisation stage and the hyperpolarisation
stage
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This is triggered by the opening of voltage-gated sodium channels, allowing Na+ to flow into
the cell down the electrochemical gradient, thus increasing its membrane potential
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 An action potential can only be triggered when the membrane potential crosses the -55mV
threshold value
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Voltage-gated potassium channels open to
allow K+ ions to diffuse out of the neurone
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○ A slight delay in the closing of the voltage-gated potassium channels allows more K+ ions to diffuse
out of the cell, causing the membrane potential to fall below the resting potential momentarily and
be hyperpolarised
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○ A nerve impulse is propagated in a unidirectional manner across the neurone in a wave of depolarisation
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The area
adjacent to it is still at the negative resting potential
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○ As a new action potential occurs in one region, the previous region is undergoing repolarisation and
hyperpolarisation
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Hence, nerve impulses do not get transmitted backwards due to the
presence of a refractory period
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This means that local electrical circuits formed during nerve impulses can only be
formed at the nodes of Ranvier
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○ Temperature
Increased temperature --> Increased KE --> Increased movement of ions --> Increased speed of
Bio Notes Page 1

○ Increased temperature --> Increased KE --> Increased movement of ions --> Increased speed of
transmission
○ Refractory period
○ For ensuring unidirectionality of impulses and to prevent overstimulation of nerves
○ Absolute refractory period
 Axon membrane absolutely cannot initiate another action potential due to Na+ channels
already being opened from depolarisation or are inactivated by inactivation gates
○ Relative refractory period
 Action potential activation requires a larger stimulation than normal due to
hyperpolarisation of membrane, hence requiring a larger stimulation than normal to hit
membrane threshold
Describe the structure of a cholinergic synapse and explain how it functions, including the role of Ca 2+ ions
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The space between the presynaptic and postsynaptic neurones is
known as the synaptic cleft
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This diffusion
process creates a brief synaptic delay
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On the postsynaptic membrane, acetylcholine binds with a receptor protein on the membrane
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6
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If the depolarisation exceeds the threshold value, an action potential is generated and
travels down the axon to the next synapse or motor end plate
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At the same time, acetylcholine is being hydrolysed by acetylcholinesterase into choline and acetate,
which diffuses back into the presynaptic knob to be recombined to form acetylcholine once again
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The loss of acetylcholine from the receptors causes the postsynaptic membrane to repolarise

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