How is the impulse passed along an axon

the empty boxes in the document are so you can print it off and draw the diagram (page 207 in the text book), I find this helps to learn it

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  • Created on: 04-02-16 13:25
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How is the impulse passed along an axon?
When a neurone is stimulated a sequence of action potentials is triggered along the length
of the axon
As part of the membrane becomes depolarised at the site of an action potential = a local
electrical current is created as the charged sodium ions flow between the depolarised part
of the membrane and adjacent the resting region
The depolarisation spreads to the adjacent region and the nearby Na+ gates open
o This triggers another action potential
These events are then repeated along the membrane
As a result a wave of depolarisation will pass along the membrane
o This is the nerve impulse
The refractory period = A new action potential cannot be generated in the same section of
membrane for about 5 milliseconds
The refractory period lasts until all the voltage dependent sodium and potassium channels
have returned to their normal resting state (closed) and the resting potential is restored
The refractory period ensures that impulses only travel in one direction
How does the refractory period ensure that the action potential will not be propagated
back the way it came?
o A new action potential will only be generated at the leading edge of the previous
one; because the membrane behind it will be recovering/ incapable of
transmitting an impulse; the membrane has to be repolarised and return to resting
potential before another action potential can be generated
Are impulses different sizes?
A stimulus must be above threshold level to generate an action potential
The allornothing effect for action potentials means that the size of the stimulus has no
effect on the size of the action potential (assuming that it is above threshold)
Different mechanisms are used to communicate the intensity of a stimulus. The size of the
stimulus effects:
The frequency of impulses
The number of neurones in a nerve that are conducting impulses
A high frequency of firing and the firing of many neurones are associated with a strong
stimulus

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At resting potential there is a positive charge
on outside of membrane and negative charge
on the inside
High Na+ conc on outside and high K+ conc on
inside
When stimulated, sodium ion channels opens =
sodium ions flow into the axon
This depolarises the membrane
Localised electric currents are generated in
the membrane
Na+ ions move to the adjacent polarised
region= causing a change in the electrical
charge (potential difference) across this part
of the membrane
The second action potential is initiated
This is…read more

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Speed of conduction
The speed of conduction can be determined by the diameter of the axon
The wider the diameter the faster the nerve impulse travels
The myelin sheath acts as an electrical insulator along most of the axon= preventing flow
of ions across the membrane
Nodes of Ranvier= gaps between the myelin sheath at regular intervals
These are only at places where depolarisation can occur
As ions flow across the membrane at one node during depolarisation, a circuit is set up
which reduces the…read more

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