Lect 5 Nerve Impulse Part
Transcript of Lect 5 Nerve Impulse Part
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The nerve
impulse
SMS1084
Dr. Mohanad R. Alwan
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stimulus
The passage of an impulse
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stimulus
The passage of an impulse
+ + + + + -
+ + + + + -
- - - - - +
- - - - - +
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stimulus
The passage of an impulse
+ + + + + -
+ + + + + -
- - - - - +
- - - - - +
Na+
Na+
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stimulus
The passage of an impulse
+ + + + + -
+ + + + + -
- - - - - +
- - - - - +
Na+
Na+
local electricalcircuit
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The all or nothing law
An AP can only be generated if the stimulusreaches 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 greateraction potential.
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successive stimuli
increasingintensity of
stimulation
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successive stimuli
increasingintensity of
stimulation
threshold intensity
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successive stimuli
increasingintensity of
stimulation
threshold intensity
below threshold intensity:no action potentials
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successive stimuli
increasingintensity of
stimulation
below threshold intensity:no action potentials
threshold intensity
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successive stimuli
increasingintensity of
stimulation
below threshold intensity:no action potentials
threshold intensity
action potentials generated
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Speed of transmission
In myelinated neuronesspeed of transmissionis up to 100 metres per
millisecond. In unmyelinated
neurones it is muchslower at about
2 m ms-1.
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Speed of transmission
Myelin speeds up the speedof the impulse by insulatingthe axon.
Myelin is fatty and does not
allow Na+
or K+
to passthrough it.
So depolarisation (and APs)can only occur at the nodesof Ranvier.
So the AP „jumps‟ from onenode to the next.
This is known as salatoryconduction.
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Advantages Increase speed of
transmission 100 fold.
Conserve energy as
sodium-potassiumpump only has tooperate at the nodesand fewer ions have to
be transported
Nerve fibres growing through
cylindrical Schwann cell formation .
Salatory conduction
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axon
myelin sheath
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axon
myelin sheathdirection of impulse
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axon
myelin sheathdirection of impulse
+ -
+ -
+
+
-
-
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axon
myelin sheathdirection of impulse
+ -
+ -
+
+
-
-
polarised depolarised
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axon
myelin sheathdirection of impulse
+ -
+ -
+
+
-
-
polarised depolarisedlocal circuit
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Cont.
Any thing that affects the rate of respiration,such as temperature, will affect thetransmission rate in a nerve.
This is because the restoration of the restingpotential is an energy-requiring processrelying upon ATP
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Axon diameter
The thicker the axon,the faster the rate oftransmission.
Probably due to thegreater surface area ofthe membrane over
which ion exchangecan occur
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Axon diameter
Giant axons found insome invertebrates(earthworms, marine
annelids) are thought tobe associated withrapid escape responses
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The Threshold
–55mV represents the threshold potential
Beyond this we get a full action potential The membrane potential rises to +35mV
this is the peak of the action potential
The cells are almost at the equilibrium(balance) for Na+ ions.
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Three states of a neuron
Resting potential The state during which no nerve impulse is being
conducted although the neuron is capable of doingso
Action potential The state during which the neuron is actively involved
in conducting a nerve impulse
Recovery/Refractory potential
The state during which the neuron is unable toconduct a nerve impulse since the neuron must“recover” following the last nerve impulse
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1. Resting Potential
The state of the neuron when no nerve impulse is beingconducted
During resting potential there is an ion displacement betweenthe inside and the outside of the neuron (i.e. on either side ofthe neuron cell membrane) as follows:
There are more Na+ ions on the outside than on the inside
There are more K+ ions on the inside than on the outside
There are many large organic anions (-ve charged ions)locked inside since they are too big to pass through theneuron‟s cell membrane
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Due to this difference in ion displacement there is a NETCHARGE difference across the cell membrane = MembranePotential.
This membrane potential when the neuron is at rest is calledthe Resting Potential=-70mv.
This difference in ion displacement and thus the restingpotential is largely maintained by a protein channel called theNa+/K+ PUMP
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Na+/K+ PUMP
Powered by ATP this pump actively “pumps” Na+ ions out of the cell and K+ ions into the cell.
As a result of this active transport, the cytoplasm of the neuron
contains more K+ ions and fewer Na+ ions than thesurrounding medium.
The cell membrane also has 2 other separate protein channels,one that “leaks” K+ ions and one that “leaks” Na+ ions down
their electrochemical gradient (combo of concentration andelectrical).
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Na+/K+ PUMP
There are more K+ channels than Na+ channels whichmeans more K+ ions leak out of the cell as opposed toNa+ leaking into the cell.
As a result, K+ ions leak out of the cell to produce a
negative charge on the inside of the membrane.
This charge difference is known as the resting potential ofthe neuron.
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At rest, the inside of a
neuron's membrane hasa negative charge.
As the figure shows, aNa+ / K+ pump in the cell
membrane pumpssodium out of the celland potassium into it.
As a result, the inside ofthe membrane builds upa net negative chargerelative to the outside.
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-70
-55
0
+35
Threshold
mV
Time
Resting potential Action potential
Na+ channels close
and K+ channels
open, K+ floods out
of neurone
Resting potential
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Potassium takes over
After Na+ moves in passively until the Na+ channels start to close
At the same time K+ permeability increases as
voltage-gated K
+
channels open – they are abit slower to respond to the depolarisation thanthe Na+ channels
The K+ ions move out
This makes the cell negative inside with respectto outside again
The membrane potential falls
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Action Potential An action potential occurs when a neuron is conducting a
nerve impulse In order for an action potential to occur, the neuron must
receive sufficient stimulation to open enough Na gates toreach the threshold level.
If sufficient Na gates are opened to reach the threshold level,other Na and K gates will be stimulated to open.
This results in a self-propagating wave of action potentialsand Na and K gates opening along the entire length of theneuron and an action potential and nerve impulse occur
Since an action potential will only occur if the membranethreshold level is reached, an action potential can also be
described as an all or none response.
Action potential can be divided into 2 phases:depolarization & repolarization
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Depolarization (upswing) If a neuron received sufficient stimulation to reach the
membrane threshold, successive Na gates along the entireneuron membrane will open
The opening of the Na gates allows Na ions to move into the
neuron
The movement of Na ions into the neruon causes themembrane potential to change from -70mV to +40mV
As the membrane potential becomes more positive, Na
gates begin to close.
At the end of depolarization, the Na gates are all closed.
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Repolarization (Down-swing)
At the end of the depolarization phase, K gates begin toopen, allowing K to leave the neuron.
These K gates are activated at the +ve membrane potentialvalue of about +40mV.
The movement of K ions out of the neuron produces achange in membrane potential such that the potentialbecomes more –ve.
Following repolarization, the K gates close slowly
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During the conduction of a nerve impulse, each
successive section of a neuron’s membrane willundergo an action potential consisting of
depolarization followed by repolarization
Thus the nerve impulse is the movement of the
action potential along the neuron cell
membrane
R /R f t P t ti l
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Recovery/Refractory Potential
Immediately following an action potential, a neuronis unable to conduct a nerve impulse until it has
recovered because its Na gates won’t open
A neuron which is undergoing recovery is said to be
refractory since it cannot conduct a nerve impulse.
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During the recovery phase the following events are
occurring:1. The K gates are closing
2. The Na/K pump is returning the Na ions to the outside and Kions to the inside of the neuron
3. The membrane potential is returning to its resting value of -70mV
Once the recovery phase is complete, the neuron is nolonger in its refractory period and is ready to conduct another
nerve impulse.
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Action Potential Stages: Overview
Figure 8-9: The action potential
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The refractory period
The membrane starts to recover and thepotassium channels open
Even before it is completely repolarised anAP can occur if the stimulus is more intensethan the normal threshold level
This period is known as the relative
refractory period and lasts about 5 ms.
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The refractory period
The refractory periodmeans that impulsescan only travel one way
down the axon as theregion behind theimpulse can not bedepolarised.
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The refractory period
It also limits thefrequency at whichsuccessive impulses
can pass along theaxon
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The refractory period
Following the passage of an AP, there is atime delay before the next one can pass
This is called the refractory period
During this time sodium channels in themembrane are closed, preventing the inwardmovement of Na+ ions
This is known as the absolute refractoryperiod (about 1 ms).
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n e u r o n e
e x c
i t a b i l i t y
0 1 2 3 4 5 6 7 8time / ms
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n e u r o n e
e x c
i t a b i l i t y
0 1 2 3 4 5 6 7 8time / ms
restingexcitability
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n e u r o n e
e x c
i t a b i l i t y
0 1 2 3 4 5 6 7 8time / ms
restingexcitability
stimulus
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n e u r o n e
e x c
i t a b i l i t y
0 1 2 3 4 5 6 7 8time / ms
restingexcitability
stimulus
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n e u r o n e
e x c
i t a b i l i t y
0 1 2 3 4 5 6 7 8time / ms
restingexcitability
stimulus
absoluterefractoryperiod
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n e u r o n e
e x c
i t a b i l i t y
0 1 2 3 4 5 6 7 8time / ms
restingexcitability
stimulus
absoluterefractoryperiod
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n e u r o n e
e x c
i t a b i l i t y
0 1 2 3 4 5 6 7 8time / ms
restingexcitability
stimulus
absoluterefractoryperiod
normalrestingexcitability
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n e u r o n e
e x c
i t a b i l i t y
0 1 2 3 4 5 6 7 8time / ms
restingexcitability
stimulus
absoluterefractoryperiod
relative refractory periodnormalrestingexcitability
refractory period
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