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Action Potential
Lecture 3
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Historical Figures
Hodgkin and Huxleywon Nobel Prize forVoltage clamp in 1961
Sakman and Nehr wonNobel Prize for PatchClamp in 1991
measured ion flowthrough individualchannels
shows that each
channel is either inopen or closedconfiguration with nointermediate. The sumof many recordings
gives you the shape ofsodium conductance.
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Conductance = g
How many charges (ions) enters orleaves cell (inverse of resistance)
due to: number of channels/membrane area
Highest density at axon hillock
number of open channels
ion concentration on either side ofmembrane
Measured in Siemens (S)
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Generation of Resting Membrane
Potential (-70mV) Plasma membrane
Selective permeability, permeable to K, not
Na
Unequal distribution of ions across
membrane
Due to open potassium channels and closed
sodium and chloride channels
Action of ion pumps 3Na/2K ATPase
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Ion Inside Outside
K+ 125 5
NA+ 12 120
Cl- 5 125
H2O 55,000 55,000
Anion- 108 0
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Ionic Equilibrium Potential
The membrane potential that balances the
ions concentration gradient so that there is
no net current for that ion.
No permeability factor.
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Equilibrium Potential of An Ion
The membrane potential at which the net
driving force propelling the ion in = the net
driving force propelling the ion out.
Written Eion; ENa, ECl, EK
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Nernst Equation
The diffusion potential level a cross a membrane
That exactly opposes the net diffusion of a particular ion
through the membrane is called the Nernst potential forthat ion
The magnitude of this N.potential is determined by theratio of the concentration of that specific ion on the two
sides of the membrane. The greater the ratio the greaterthe tendency to diffuse in one direction and the greaterthe Nernst potential required to prevent additional netdiffusion.
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Action Potentials
Can travel up to
100 meters/second
Usually 10-20 m/s
0.1sec delay
between muscle and
sensory neuron
action potential
Action Potential: a
transient and rapid
sequence of changes in
the membrane potential
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6 Characteristics of an Action
Potential
#1 Triggered by depolarization
a less negative membrane potential that
occurs transiently
Understand depolarization, repolarization
and hyperpolarization
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#2 Threshold
Threshold depolarization needed to trigger
the action potential
10-20 mV depolarization must occur to
trigger action potential
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#3 All or None
Are all-or- none event
Amplitude of AP is the same regardless of
whether the depolarizing event was weak(+20mV) or strong (+40mV).
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#5 Reverses Polarity
At peak of action potential the membranepotential reverses polarity
Becomes positive inside as predicted by theEna Called OVERSHOOT
Return to membrane potential to a morenegative potential than at rest
Called UNDERSHOOT
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#6 Refractory Period
Absolute refractory period follows an action
potential. Lasts 1 msec
During this time another action potential
CANNOT be fired even if there is a
transient depolarization.
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Voltage Sensitive Ion Channels
Sodium
Potassium
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Ionic Equilibrium Potential
Membrane Potential (potential difference
across the plasma membrane) at which the
net flow of an ion type = zero The number of ions moving into the cell =
the number of ions moving out of the cell
for a particular species of ion
R ti P
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Regenerative Process:
Once one Na channel
Opens, Na enters,
Depolarizes membrane,More and more Na
Channels open leading to
More sodium influx &
causes upward &depolarizing (more +)
phase of the AP
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What does a sodium
Channel look like?
It is one large protein
With 4 domains that
Each loop through the
Plasma membrane 7Times.
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Property of Voltage Dependent
Sodium Channel Sodium channel opens for 1-2 millisecond
following threshold depolarization
then inactivates and does not open even ifVm is depolarized.
This is called sodium channel inactivation
and contributes to the repolarization of Vm
N Ch l G
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M gate= activation gate
on Na channel; opensquickly when membrane
is depolarized
H gate- inactivation gateon Na channel; Closes
slowly after membrane is
depolarizedcauses the absolute
refractory period for AP
propagation
Na Channel Gates
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Potassium Channel Property
K channels open with a delay and stay open
for length of depolarization
Repolarize the Vm to Ek= -75mV which iswhy you have hyperpolarization.
Also called a delayed rectifier channel
G t th D l d R tifi
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K channels have a
single gate (n) that stays
open as long as Vm is
depolarized.
n gate on K channels
opens very slowly this
allows the Vm to
depolarize due to Nainflux; Na and K
currents do not offset
each other right away
Gate on the Delayed Rectifier
Potassium Channel
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Refractory Period
Refractory period due to Na channelinactivation and the high gk
Subsequent Action potential cannot begenerated
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2 ways to increase AP
propagation speed Increase internal diameter of axon which
decreases the internal resistance to ion flow
Increase the resistance of the plasmamembrane to charge flow by insulating it
with myelin.
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See and understand
what happens to the form
Of the action potentialWhen you add a voltage
Sensitive calcium channel
And a calcium gated
Potassium channel
Test question : think about
This and the next 2 slides
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Channel Density
Density is how many channels are in a unit
area of plasma membrane, ie how closely
they are packed together. Determines the length of the membrane that
will be depolarized at a given time
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Understand
Regenerative nature of action potential
. Voltage gates in sodium channel
Threshold potential sodium and potassium fluxesare balanced Initial segment of axon = axon hillock
Two mechanisms for increasing speed of action
potential propagation Saltatory conduction
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Understand
Action potential occurs because sodium
and potassium fluxes change the charge
on the cell membrane not because thefluxes change ion concentrations.
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Definitions
Current=net flow of
ions per unit time
1 ampere of currentrepresents movement
of 1 coulomb of
charge per second
Resistance- frictional
forces that resists
movement of ions orcharges
Measured in ohm
Current (A)= V/R
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Definitions
Conductance is the
reciprocal of
resistance andmeasures the ease with
which current flows in
an object.
Measured in siemens(S)
Capacitance refers to theability of plasmamembrane to store or
separate charges ofopposite signs.
Myelin has highcapacitance so storescharges and ions do not
move across themembrane
Measured in Farads