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The nerve impulse
Part 2
Progress of an impulse
When an impulse reaches any point on the axon an action potential (AP) is generated
Small local currents occur at the leading edge of the AP
Sodium ions move across the membrane towards negatively charged regions.
This excites the next part of the axon so the AP progresses along its length
The local currents change the potential of the membrane, creating a new action potential ahead of the impulse.
stimulus
The passage of an impulse
stimulus
The passage of an impulse
+ + + + + -
+ + + + + -
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stimulus
The passage of an impulse
+ + + + + -
+ + + + + -
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Na+
Na+
stimulus
The passage of an impulse
+ + + + + -
+ + + + + -
- - - - - +
- - - - - +
Na+
Na+
local electrical circuit
The all or nothing law
An AP can only be generated if the stimulus reaches a certain threshold intensity
Below this threshold, no AP can be created Once the threshold level is reached, the size
of an impulse is independent of the stimulus So, a greater stimulus does not give a greater
action potential.
successive stimuli
successive stimuli
increasing intensity of stimulation
successive stimuli
increasing intensity of stimulation
threshold intensity
successive stimuli
increasing intensity of stimulation
threshold intensity
below threshold intensity: no action potentials
successive stimuli
increasing intensity of stimulation
below threshold intensity: no action potentials
threshold intensity
successive stimuli
increasing intensity of stimulation
below threshold intensity: no action potentials
threshold intensity
action potentials generated
The all or nothing law
The difference between a weak and a strong stimuli is due to the frequency of the APs
A weak stimulus gives few APs A strong stimulus gives more APs ….(and is also likely to result in APs in more
neurones)
The refractory period
Following the passage of an AP, there is a time delay before the next one can pass
This is called the refractory period During this time sodium channels in the
membrane are closed, preventing the inward movement of Na+ ions
This is known as the absolute refractory period (about 1 ms)
neur
one
exci
tabi
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0 1 2 3 4 5 6 7 8time / ms
neur
one
exci
tabi
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0 1 2 3 4 5 6 7 8time / ms
resting excitability
neur
one
exci
tabi
lity
0 1 2 3 4 5 6 7 8time / ms
resting excitability
stimulus
neur
one
exci
tabi
lity
0 1 2 3 4 5 6 7 8time / ms
resting excitability
stimulus
neur
one
exci
tabi
lity
0 1 2 3 4 5 6 7 8time / ms
resting excitability
stimulus
neur
one
exci
tabi
lity
0 1 2 3 4 5 6 7 8time / ms
resting excitability
stimulus
absolute refractory period
neur
one
exci
tabi
lity
0 1 2 3 4 5 6 7 8time / ms
resting excitability
stimulus
absolute refractory period
neur
one
exci
tabi
lity
0 1 2 3 4 5 6 7 8time / ms
resting excitability
stimulus
absolute refractory period
normal resting excitability
neur
one
exci
tabi
lity
0 1 2 3 4 5 6 7 8time / ms
resting excitability
stimulus
absolute refractory period
relative refractory periodnormal resting excitability
neur
one
exci
tabi
lity
0 1 2 3 4 5 6 7 8time / ms
resting excitability
stimulus
absolute refractory period
relative refractory periodnormal resting excitability
refractory period
The refractory period
The membrane starts to recover and the potassium channels open
Even before it is completely repolarised an AP can occur if the stimulus is more intense than the normal threshold level
This period is known as the relative refractory period and lasts about 5 ms.
The refractory period
The refractory period means that impulses can only travel one way down the axon as the region behind the impulse can not be depolarised.
The refractory period
It also limits the frequency at which successive impulses can pass along the axon
Speed of transmission
In myelinated neurones speed of transmission is up to 100 metres per millisecond.
In unmyelinated neurones it is much slower at about
2 m ms-1.
Speed of transmission Myelin speeds up the speed
of the impulse by insulating the axon.
Myelin is fatty and does not allow Na+ or K+ to pass through it.
So depolarisation (and APs) can only occur at the nodes of Ranvier.
So the AP ‘jumps’ from one node to the next.
This is known as salatory conduction.
Salatory conduction
Advantages Increase speed of
transmission 100 fold. Conserve energy as
sodium-potassium pump only has to operate at the nodes and fewer ions have to be transported
Nerve fibres growing through cylindrical Schwann cell formation.
axon
myelin sheath
axon
myelin sheathdirection of impulse
axon
myelin sheathdirection of impulse
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+ -
+
+
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-
axon
myelin sheathdirection of impulse
+ -
+ -
+
+
-
-
polarised depolarised
axon
myelin sheathdirection of impulse
+ -
+ -
+
+
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-
polarised depolarisedlocal circuit
Any thing that affects the rate of respiration, such as temperature, will affect the transmission rate in a nerve.
This is because the restoration of the resting potential is an energy-requiring process relying upon ATP
Axon diameter
The thicker the axon, the faster the rate of transmission.
Probably due to the greater surface area of the membrane over which ion exchange can occur
Axon diameter
Giant axons found in some invertebrates (earthworms, marine annelids) are thought to be associated with rapid escape responses