Membrane potentials

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Membrane potentials

• Xia Qiang, MD PhD
• Department of Physiology
• Rm C518, Block C, Research Building, ZJU School
  of Medicine
• Tel [phone]
• Email xiaqiang_at_zju.edu.cn

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OUTLINE

• Resting potential
• Graded potential
• Action potential
• Refractory period

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Electrocardiogram ECG
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Electroencephalogram EEG
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Electromyogram EMG
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Extracellular Recording
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Intracellular Recording
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Opposite charges attract each other and will move
toward each other if not separated by some
barrier.
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Only a very thin shell of charge difference is
needed to establish a membrane potential.
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Resting membrane potential
A potential difference across the membranes of
inactive cells, with the inside of the cell
negative relative to the outside of the
cell Ranging from 10 to 100 mV
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Overshoot refers to the development of a charge
reversal.
A cell is polarized because its interior is
more negative than its exterior.
Repolarization is movement back toward the
resting potential.
Depolarization occurs when ion movement
reduces the charge imbalance.
Hyperpolarization is the development of even
more negative charge inside the cell.
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• unequal ion distribution (chemical gradient)
  across the membrane
• selective membrane permeability (cell membrane is
  more permeable to K)
• Na-K pump

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chemical driving force
electrical driving force
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• The Nernst Equation
• K equilibrium potential (EK) (37oC)

RGas constant TTemperature ZValence FFaradays
constant
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Difference between EK and directly measured
resting potential
Ek Observed RP

• Mammalian skeletal muscle cell -95
  mV -90 mV
• Frog skeletal muscle cell -105 mV -90 mV
• Squid giant axon -96 mV -70 mV

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Goldman-Hodgkin-Katz equation
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Role of Na-K pump

• Electrogenic
• Hyperpolarizing

Establishment of resting membrane
potential Na/K pump establishes concentration
gradient generating a small negative potential
pump uses up to 40 of the ATP produced by that
cell!
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Origin of the normal resting membrane potential

• K diffusion potential
• Na diffusion
• Na-K pump

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• Electrotonic Potential

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Graded potential

• Graded potentials are changes in membrane
  potential that are confined to a relative small
  region of the plasma membrane

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The size of a graded potential (here, graded
depolarizations) is proportionate to the
intensity of the stimulus.
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Graded potentials can be EXCITATORY or INHIBITORY
(action potential (action potential
is more likely) is less likely)
The size of a graded potential is proportional to
the size of the stimulus.
Graded potentials decay as they move over
distance.
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Graded potentials decay as they move over
distance.
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• Graded potentials
• (Local response, local excitation, local
  potential)

• Not all-or-none
• Electrotonic propagation spreading with
  decrement
• Summation spatial temporal

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• Threshold Potential level of depolarization
  needed to trigger an action potential (most
  neurons have a threshold at -50 mV)

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Action potential
Excitable cells a cell in which the membrane
response to depolarisations is nonlinear, causing
amplification and propagation of the
depolarisation (an action potential).
Some of the cells (excitable cells) are capable
to rapidly reverse their resting membrane
potential from negative resting values to
slightly positive values. This transient and
rapid change in membrane potential is called an
action potential
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A typical neuron action potential
Positive after-potential
Negative after-potential
Spike potential After-potential
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Ionic basis of action potential
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• (1) Depolarization
• Activation of voltage-gated Na channel

Blocker Tetrodotoxin (TTX)
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• (2) Repolarization
• Inactivation of Na channel
• Activation of K channel

Blocker Tetraethylammonium (TEA)
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An action potential is an all-or-none sequence
of changes in membrane potential.
The rapid opening of voltage-gated Na channels
allows rapid entry of Na, moving membrane
potential closer to the sodium equilibrium
potential (60 mv)
Action potentials result from an all-or-none
sequence of changes in ion permeability due to
the operation of voltage-gated Na and K
channels.
The slower opening of voltage-gated K channels
allows K exit, moving membrane potential
closer to the potassium equilibrium potential
(-90 mv)
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Click here to play the Voltage Gated Channels and
Action Potential Flash Animation
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Mechanism of the initiation and termination of AP
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Re-establishing Na and K gradients after AP

• Na-K pump
• Recharging process

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Properties of action potential (AP)

• Depolarization must exceed threshold value to
  trigger AP
• AP is all-or-none
• AP propagates without decrement

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How to study ?

• Voltage Clamp

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• Currents recorded under voltage clamp condition

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• Patch Clamp

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Conduction of action potential
Continuous propagation in the unmyelinated axon
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Click here to play the Action Potential
Propagation in an Unmyelinated Neuron Flash
Animation
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Saltatory propagation in the myelinated axon
http//www.brainviews.com/abFiles/AniSalt.htm
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Saltatorial Conduction Action potentials jump
from one node to the next as they propagate along
a myelinated axon.
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Click here to play the Action Potential
Propagation in Myelinated Neurons Flash Animation
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Excitation and Excitability
Excitability the ability to generate action
potentials is known as EXCITABILITY

• To initiate excitation (AP)
• Excitable cells
• Stimulation
• Intensity
• Duration
• dV/dt

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• Strength-duration Curve

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Threshold intensity Threshold stimulus
Four action potentials, each the result of a
stimulus strong enough to cause depolarization,
are shown in the right half of the figure.
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A refractory period is a period of time during
which an organ or cell is incapable of repeating
a particular action potential

• Refractory period following an AP
• 1. Absolute Refractory Period inactivation of
  Na channel
• 2. Relative Refractory Period some Na channels
  open

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Factors affecting excitability

• Resting potential
• Threshold
• Channel status

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SUMMARY

• Resting potential
• K diffusion potential
• Na diffusion
• Na -K pump
• Graded potential
• Not all-or-none
• Electrotonic propagation
• Spatial and temporal summation

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• Action potential
• Depolarization Activation of voltage-gated Na
  channel
• Repolarization Inactivation of Na channel, and
  activation of K channel
• Refractory period
• Absolute refractory period
• Relative refractory period

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THANK YOU FOR YOUR ATTENTION!