Membrane potentials

Dr G Bhanu Prakash

ObjectivesTo understand the shape and form of the action potential and understand how it arises in terms of the changes in the underlying Na+ and K+ channelsTo explore how action potentials are conducted in axons and how this is affected by myelin

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（静息电位）

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.

（极化）

（去极化） （超极化）

（复极化）

（超射）

chemical driving force

electrical driving force

++++++++++++++++

- - - - - - - - - - - - - - - - -electrochemic

al balance

The Nernst Equation:

K+ equilibrium potential (EK) (37oC) i

o

IonIon

ZFRTE

][][log

R=Gas constantT=TemperatureZ=ValenceF=Faraday’s constant

)(][][log60 mV

KKEk

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 mVFrog skeletal muscle cell -105 mV -90 mVSquid giant axon -96 mV -70 mV

Ek Observed RP

Difference between EK and directly measured resting potential

Goldman-Hodgkin-Katz equation

•Electrogenic•Hyperpolarizing

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 potentialNa+ diffusionNa+-K+ pump

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

Negative after-

potential

Positive after-

potential

Spike potential After-potential

A typical neuron action potential

Electrotonic Potential

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.

Local response

•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)

Membrane potentials

ObjectivesTo understand the shape and form of the action potential and understand how it arises in terms of the changes in the underlying Na+ and K+ channelsTo explore how action potentials are conducted in axons and how this is affected by myelin

ReviewIntracellular and extracellular recordingResting membrane potential (definition and mechanism)Action potential (definition)Local response (Graded potential)Threshold potential

Ionic basis of action potential

Voltage Clamp

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

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

Figure 2 Instantaneous I–V data reveal that IK has a more hyperpolarised reversal potential than IA

A, tail current family for IK, recorded in 5 mM 4-AP. Following a 100 ms step to +53 mV, the membrane potential was stepped to a level ranging from +53 to -117 mV in 10 mV increments. Each trace is an average of 12 interleaved episodes. Leak currents have been subtracted. B, plot of peak instantaneous IK, from extrapolated exponential fits to the tail currents (Methods), versus tail potential for this patch. The superimposed curve is a quadratic polynomial. The reversal potential for this patch was -86.4 mV. C, tail current family for IA, recorded in 30 mM TEA and shown expanded in the inset. The pulse protocol was as in A, except the duration of the prepulse to +53 mV was 1.5 ms. Each trace is an average of 6 interleaved episodes. Leak currents have been subtracted. The slowly rising trace in the inset is the estimated time course of the contaminating IK at +53 mV in this patch. At 1.5 ms the contamination is about 10 % of IA. D, plot of peak instantaneous IAversus tail potential for this patch. The fitted quadratic polynomial gives a reversal potential of -68.7 mV.

From:doi:10.1111/j.1469-7793.2000.t01-1-00593.xJune 15, 2000 The Journal of Physiology, 525, 593-609

(1) Depolarization:Activation of Na+ channel

Blocker:

Tetrodotoxin (TTX)

(2) Repolarization:Inactivation of Na+ channelActivation of K+ channel

Blocker:Tetraethylammonium(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)

Mechanism of the initiation and termination of AP

How to re-establish Na+ and K+ gradients after action potential ?

Concentration gradient of Na+ and K+

Extracellular (mmol/L) Intracellular (mmol/L)

Na+ 150.015.0K+ 5.0 150.0

For a television game show, 16 contestants volunteer to be stranded on a deserted island in the middle of the South China Sea. They must rely on their own survival instincts and skills. During one of the challenges, one team wins a fishing spear. They catch a puffer fish and cook it over the open flames of their barbecue. None of them are very skilled in cooking, but they enjoy the fish anyway. One of the contestants, a worldwide traveler, comments that it tastes like Fugu. After dinner, they all develop a strange tingling around their lips and tongue. They all become weak, and their frailty progresses to paralysis. They all die.

What is the mechanism of toxicity? A Blockage of the sodium gates B Blockage of the potassium gates C Interference with the release of acetylcholine D Antibody directed against the acetylcholine receptor E Maintaining the sodium channel in an open state

For a television game show, 16 contestants volunteer to be stranded on a deserted island in the middle of the South China Sea. They must rely on their own survival instincts and skills. During one of the challenges, one team wins a fishing spear. They catch a puffer fish and cook it over the open flames of their barbecue. None of them are very skilled in cooking, but they enjoy the fish anyway. One of the contestants, a worldwide traveler, comments that it tastes like Fugu. After dinner, they all develop a strange tingling around their lips and tongue. They all become weak, and their frailty progresses to paralysis. They all die.

What is the mechanism of toxicity? A Blockage of the sodium gates B Blockage of the potassium gates C Interference with the release of acetylcholine D Antibody directed against the acetylcholine receptor E Maintaining the sodium channel in an open state

Conduction of action potentialContinuous propagation in the unmyelinated axon

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.

Excitation and Excitability

To initiate excitation (AP) Excitable cells Stimulation

Intensity Duration dV/dt

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+

channel2. Relative Refractory Period: some Na+ channels open

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.

Factors affecting excitability

Resting potentialThresholdChannel state

A well-meaning third year medical student accidentally pushes an unknown quantity of KCl IV to a patient. If the concentration of potassium outside a neuron were to increase from 4 mEq/L to 8 mEq/L, what would you expect to happen to the minimal stimulus required for initiation of an action potential?

A The minimal stimulus required for initiation of an action potential would remain the same

B The minimal stimulus required for initiation of an action potential would increase

C The minimal stimulus required for initiation of an action potential would decrease

D The minimal stimulus required for initiation of an action potential would stay the same, but the amplitude of the peak of the action potential would increase

E The minimal stimulus required for initiation of an action potential would stay the same, but the conduction velocity of the action potential down an axon would slow

A well-meaning third year medical student accidentally pushes an unknown quantity of KCl IV to a patient. If the concentration of potassium outside a neuron were to increase from 4 mEq/L to 8 mEq/L, what would you expect to happen to the minimal stimulus required for initiation of an action potential?

A The minimal stimulus required for initiation of an action potential would remain the same

B The minimal stimulus required for initiation of an action potential would increase

C The minimal stimulus required for initiation of an action potential would decrease

D The minimal stimulus required for initiation of an action potential would stay the same, but the amplitude of the peak of the action potential would increase

E The minimal stimulus required for initiation of an action potential would stay the same, but the conduction velocity of the action potential down an axon would slow

Sydney Ringer published 4 papers in the Journal of Physiology in 1882 and 1883, while working as a physician in London.

Sydney Ringer and his work on ionic composition of buffers

He found that 133mM NaCl, 1.34mM KCl, 2.76mM NaHCO3 1.25mM CaCl2 could sustain the frog heart beat.

J Physiol 2004, 555.3; 585-587Biochem J 1911, 5 (6-7).

1835-1910

He wrote “The striking contrast between potassium and sodium with respect to this modification (wrt refractoriness) is of great interest….because, from the chemical point of view, it would be quite unlooked for in two elements apparently so akin”

Ringer found that in excess potassium the period of diminished excitability is increased, and frequnecy of heart beats diminishes.

A rather somber application note: Death by lethal injectionLethal injection is used for capital punishment in some states with the death penalty.

Lethal injection consists of (1) Sodium thiopental (makes person unconscious), (2) Pancuronium/tubocurare (stops muscle movement), (3) Potassium chloride (causes cardiac arrest).

It seems a bit sick, but we can understand how this works from what we know about electrical signalling. Recall that

iNaiK

oNaoK

NaPKPNaPKPmVVm][][][][log54.61

10

ii

oomVVm]15[1]100[40]150[1]5[40log54.61 10

mVmVVm 654015350log54.61 10

ii

oomVVm]15[1]100[40]150[1]95[40log54.61 10

mVmVVm 4.040153950log54.61

10

This explains what Sydney Ringer observed in frog hearts in 1882!

SUMMARYResting potential: K+ diffusion potential Na+ diffusion Na+ -K+ pumpGraded 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+ channelRefractory period Absolute refractory period Relative refractory period

THANK YOU!