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Membrane potentials. XIA Qiang, MD & PhD Department of Physiology Room 518, Block C, Research Building School of Medicine, Zijingang Campus Email: [email protected] Tel: 88206417 (Undergraduate school), 88208252 (Medical school). OUTLINE. Resting potential Graded potential - PowerPoint PPT Presentation
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Membrane potentials
XIA Qiang, MD & PhD
Department of PhysiologyRoom 518, Block C, Research BuildingSchool of Medicine, Zijingang Campus
Email: [email protected]: 88206417 (Undergraduate school),
88208252 (Medical school)
OUTLINE
Resting potentialGraded potentialAction potentialRefractory period
ElectrocardiogramECG
ElectroencephalogramEEG
ElectromyogramEMG
Extracellular Recording
Intracellular Recording
Opposite charges attract each other andwill move toward each other if not separatedby some barrier.
Only a very thin shell of charge differenceis needed to establish a membrane potential.
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
Depolarization occurs when ion movement reduces the charge imbalance.
A cell is “polarized” because its interior is more negative than its exterior.
Overshoot refers to the development of a charge reversal.
Repolarization is movement back toward the resting potential.
Hyperpolarization is the development of even more negative charge inside the cell.
•unequal ion distribution (chemical gradient) across the membrane
•selective membrane permeability (cell membrane is more permeable to K+)
•Na+-K+ pump
chemical driving force
electrical driving force
++++++++++++++++
- - - - - - - - - - - - - - - - -electrochemic
al balance
The Nernst Equation:
K+ equilibrium potential (EK) (37oC)i
o
Ion
Ion
ZF
RTE
][
][log
R=Gas constantT=TemperatureZ=ValenceF=Faraday’s constant
)(][
][log60 mV
K
KEk
i
o
Begin: K+ in Compartment 2, Na+ in Compartment 1; BUT only K+ can move.
Ion movement: K+ crosses into Compartment 1; Na+ stays in Compartment 1.
buildup of positive charge in Compartment 1 produces an electrical potential that exactly offsets the K+ chemical concentration gradient.
At the potassium equilibrium potential:
Begin: K+ in Compartment 2, Na+ in Compartment 1; BUT only Na+ can move.
Ion movement: Na+ crosses into Compartment 2; but K+ stays in Compartment 2.
buildup of positive charge in Compartment 2 produces an electrical potential that exactly offsets the Na+ chemical concentration gradient.
At the sodium equilibrium potential:
Mammalian skeletal muscle cell -95 mV-90 mV
Frog skeletal muscle cell -105 mV -90 mV
Squid giant axon -96 mV -70 mV
Ek Observed RP
Difference between EK and directly
measured resting potential
Goldman-Hodgkin-Katz equation
)(][][][
][][][60 mV
PClPKPNa
PClPKPNaEm
CloKiNai
CliKoNao
•Electrogenic
•Hyperpolarizi
ng
Role of Na+-K+ pump:
Establishment of resting membrane potential:Na+/K+ pump establishes concentration gradientgenerating a small negative potential; pump uses up to 40% of the ATP produced by that cell!
Origin of the normal resting membrane potential
K+ diffusion potential
Na+ diffusion
Na+-K+ pump
Electrotonic Potential
Graded potential
Graded potentials are changes in membrane potential that are confined to a relative small region of the plasma membrane
The size of a graded potential(here, graded depolarizations) is proportionate to the intensity of the stimulus.
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.
Graded potentials decay as they move over distance.
Graded potentials(Local response, local excitation, local
potential)
•Not “all-or-none”
•Electrotonic
propagation: spreading
with decrement
•Summation: spatial &
temporal
Threshold Potential: level of depolarization needed to trigger an
action potential (most neurons have a threshold at -50 mV)
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
Excitable cells: a cell in which the membrane response to depolarisations is nonlinear, causing amplification and propagation of the depolarisation (an action potential).
Negative after-
potential
Positive after-
potential
Spike potential After-potential
A typical neuron action potential
Ionic basis of action potential
(1) Depolarization:
Activation of voltage-gated Na+ channel
Blocker:
Tetrodotoxin
(TTX)
(2) Repolarization:
Inactivation of Na+ channel
Activation of K+ channel
Blocker:
Tetraethylammoni
um
(TEA)
The rapid opening of voltage-gated Na+ channels explains the rapid-depolarization phase at the beginning of the action potential.
The slower opening of voltage-gated K+ channels explains the repolarization and after hyperpolarization phases that complete the action potential.
An action potentialis an “all-or-none”sequence of changesin membrane potential.
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 rapid opening of voltage-gated Na+ channels allows rapid entry of Na+, moving membrane potential closer to the sodium equilibrium potential (+60 mv)
The slower opening of voltage-gated K+ channels allows K+ exit, moving membrane potential closer to the potassium equilibrium potential (-90 mv)
Click here to play theVoltage Gated Channels
and Action PotentialFlash Animation
Mechanism of the initiation and termination of AP
Re-establishing Na+ and K+ gradients after AP
Na+-K+ pump
“Recharging”
process
Properties of action potential (AP)
•Depolarization must exceed threshold
value to trigger AP
•AP is all-or-none
•AP propagates without decrement
Voltage Clamp
How to study ?
Nobel Prize in Physiology or Medicine 1963
"for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane"
Eccles Hodgkin
Huxley
Currents recorded under voltage
clamp condition
Patch Clamp
Nobel Prize in Physiology or Medicine 1991
"for their discoveries concerning the function of single ion channels in cells"
Erwin Neher Bert
Sakmann
Conduction of action potentialContinuous propagation in the unmyelinated axon
Click here to play theAction Potential Propagationin an Unmyelinated Neuron
Flash Animation
Saltatory propagation in the myelinated axon
http://www.brainviews.com/abFiles/AniSalt.htm
Saltatorial Conduction: Action potentials jump from one node to thenext as they propagate along a myelinated axon.
Click here to play theAction Potential Propagation
in Myelinated NeuronsFlash Animation
Excitation and Excitability
To initiate excitation (AP) Excitable cells Stimulation
Intensity
Duration
dV/dt
Excitability: the ability to generate action potentials is known as EXCITABILITY
Strength-duration Curve
Four action potentials, each the result of a stimulus strong enough to cause depolarization, are shown in the right half of the figure.
Threshold intensity & Threshold stimulus
Refractory period following an AP:
1. Absolute Refractory Period: inactivation of Na+ channel
2. Relative Refractory Period: some Na+ channels open
A refractory period is a period of time during which an organ or cell is incapable of repeating a particular action potential
Factors affecting excitability
Resting potential
Threshold
Channel status
The propagation of the action potential from the dendritic to the axon-terminal end is typically one-way because the absolute refractory period follows along in the “wake” of the moving action potential.
SUMMARY
Resting potential: K+ diffusion potential Na+ diffusion Na+ -K+ pump
Graded potential Not “all-or-none” Electrotonic propagation Spatial and temporal summation
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
THANK YOU FOR YOUR ATTENTION!