action potential lec # 4

8/3/2019 Action Potential Lec # 4

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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

action potential lec # 4

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