Origins of Membrane Potential in Cells

Biophysics 702

Chen Gu

What is membrane potential? Why is it important?

How is membrane potential generated?

How do we calculate membrane potential?

How does membrane potential encode signals?

What are the carriers for membrane potentials?

Vin

Vout = 0

The membrane potential (Vm) is defined as

Vm = Vin – Vout

intracellular

extracellular

Resting membrane potential -60 to –70 mV for neuronsDepolarization become more positive Hyperpolarization become more negative

What is membrane potential? Why is it important?

How is membrane potential generated?

How do we calculate membrane potential?

How does membrane potential encode signals?

What are the carriers for membrane potential?

The membrane potential results from a separation of positive and negative charges across the cell membrane

Electrical and thermodynamic forcesdetermine the passive distributionof ions

R

I

V

I = V/R

’s LAW

P

Q.

R

Q = P/R

.

C1 C2

C

R

J

J = C/R

C1+

V

R

I

++

+

+

+ +

+ +

+

++

+

C2+

I = C/R

Diffusiondown

Chemical Gradient

Diffusion down

Electrical Gradient

Concept of an Equilibrium Potential for an ionic species:

The potential at which the movement of ions across the membrane is in electrochemical equilibrium, i.e. the voltage necessary to result in no net movement of the ionic species across the membrane.

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

-90 mV

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

+55 mV

Out of Equilibrium In Equilibrium

Maintaining the resting membrane potentialMaintaining the resting membrane potential

K

K

KK

KK

K

K

KK K

K

K

K

K

K

+++++++++

Concentration gradient

voltage gradient

---------

Intracellular Extracellular

-102 mV

EK

Ne

ga

tive

to

EK

Po

sitiv

e t

o E

K

gK

K+ moves into cell

gKK+ moves out of cell

Reversal potential(No net movement of K+)

Time

Normalcurrent injection

Voltage response

gK

Increased gK

K+ moves out of cell

K+ moves into cell

EK

-102

Time

What is membrane potential? Why is it important?

How is membrane potential generated?

How do we calculate membrane potential?

How does membrane potential encode signals?

What are the carriers for membrane potential?

Resting membrane potentialsResting membrane potentials

Nernst equations for biological ions:Nernst equations for biological ions:

extracellular

intracellular

ENa = +56Na+ (150)

EK = -102K+ (3)

ECl = -76Cl- (120)

ECl = +125Ca2+ (1.2)

Na+ (18) K+ (135) Cl- (7) Ca2+ (0.1 µM)Na+,K+-ATPase

-60 to -75 mVNSCC

Ek = lnRTF

[K]o

[K]i

ENa = lnRTF

[Na]o

[Na]i

ECa = lnRT2F

[Ca]o

[Ca]i

ECl = lnRT-F

[Cl]o

[Cl]i

Anions: Cl- and proteinsCations: K+ diffusion potential Na+ diffusion potential

Ca2+ diffusion potentialNa+/K+-ATPase

Important: Ionic concentration differences across cell membranes determines the membrane potential

The concentration differences of ions are due to the biophysics of the channels and pumps

Guyton, Textbook of Physiology

E (ion) = RT/zF ln ([ion]outside/[ion]inside)

Nernst Equation

Na+Na+ Na+

Na+Na+

Na+ Na+

Na+

Na+

@ 370 C RT/F= (27/z) Convert to log 2.3 x 27/z = 63

@ 0o C = 54@ 24oC = 59@ 37oC = 63

E (ion) = 63 log ([ion]outside/[ion]inside)

The “Voltage Diagram”

-90 Vr (i.e. resting Vm)

ENa+ = 63 log [ ]o/[ ]i142 10 = +73 mv

ECl- = 63 log [ ]o/[ ]i 103 4 = 89 mv

-1

-

EK+ = 63 log [ ]o/[ ]i

Vm

Mem

bran

e V

olt a

ge o

r P

oten

t ial (

mV

)

Time

0

4 140 = -97 mv

+72 ENa+

Ecl--89

-97 EK+

The “Voltage Diagram”

+

-90 Vr (i.e. resting Vm)

-97 EK+ (Equilibrium Potential for K+)

+72 ENa+ (Equilibrium Potential for Na+)

Vm

Mem

bran

e V

olt a

ge o

r P

oten

t ial (

mV

)

Time

0---

-

+++

ECl- (Equilibrium Potential for Cl- )-89

Maintaining the resting membrane potentialMaintaining the resting membrane potential

Vm = lnRTF

pK[K+]o + pNa[Na+]o + pCl[Cl-]i

pK[K+]i + pNa[Na+]i + pCl[Cl-]o

The Goldman-Hodgkin-Katz Equation:The steady state membrane potential for a given set of ionic concentrations inside and outside the cell and the relative permeability of the membrane to each ion

extracellular

intracellular

ENa = +56Na+ (150)

EK = -102K+ (3)

ECl = -76Cl- (120)

ECl = +125Ca2+ (1.2)

Na+ (18) K+ (135) Cl- (7) Ca2+ (0.1 µM)Na+,K+-ATPase

-60 to -75 mVNSCC

Lipid Bilayer

Anion-

Anion-

Anion-Anion-

Anion-

Anion-

3 Na+

2 K+

ATPase

(1)Three Primary reasons for a net negative potential across the membrane

(2)

EXTRACELLULAR SPACE

INTRACELLULAR SPACE

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

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

K+

Na+

Electrongenic Na/K ATPase High IC [Anions]

(3) Relative Permeabilities of

dominant cations

Changes in membrane potential due to ion movementChanges in membrane potential due to ion movement

Depolarization:

Initiators: Na+ channels nonselective cation channels

(NSCC) Na+,K+-ATPase

Terminators: K+ channels Cl- channels

extracellular

intracellular

ENa = +56Na+ (150)

EK = -102K+ (3)

ECl = -76Cl- (120)

ECl = +125Ca2+ (1.2)

Na+ (18) K+ (135) Cl- (7) Ca2+ (0.1 µM)Na+,K+-ATPase

-60 to -75 mVNSCC

What is membrane potential? Why is it important?

How is membrane potential formed?

How do we calculate membrane potential?

How does membrane potential encode signals?

What are the carriers for membrane potential?

Types of electrical signalsTypes of electrical signals

• Graded potentials: Variable-strength signals that lose strength as they travel through the cell.

a. Can be depolarizations (Na+ channel) or hyperpolarizations (K+ or Cl- channel)b. Begins on the cell membrane at the point where ions enter from the extracellular

fluid (local current or electrotonic current)c. The strength or amplitude is directly proportional to and is determined by the

number of charges that enter the cell, which in turn is determined by the number of receptors which are opened. (concentration of the neurotransmitters and density of the receptors)

d. The size of the graded potential decreases as it spreads out from its point of origine. Graded potentials travel through the neurons until they reach the trigger zone, the

point where an action potential is generated. Depending on the strength of the graded potential, it either triggers an action potential or dies out (threshold potential).

f. Can be summed: spatial summation and temporal summation

• Action potentials: Signals that travel for long distances through the neuron without losing strength.

a. Rapid electrical signals that pass along the axon to the axon terminal.b. Identical to each other and do not diminish in strength when traveling through the

cellc. The strength of the graded potential that initiates an action potential has no

influence on the action potential as long as it is above threshold.d. All-or-none

Comparison of graded potential and action potentialComparison of graded potential and action potentialFeature Graded Potential Action Potential

Type of signal Input signal Conduction signal

Where it occurs Usually dendrites and cell body. Axon hillock, initial segment and entire length of axon

Types of gated ion channels Mechanically or chemically gated channels

Voltage-gate channels

Ions involved Usually Na+, K+, and Cl- Na+ and K+

Type of signal Depolarizing (Na+ ) or hyperpolarizing (K+, Cl- )

Depolarizing

Strength of signal Depends on initial stimulus; can be summed

Is always the same as long as graded potential is above threshold; cannot be summed

What initiates the signal Entry of ions through chemically or mechanically gated ion channels

Above-threshold graded potential arrives at the integration zone

Unique characteristics No minimum level required to initiate a graded potential Two signals coming close together in time will sum

Threshold stimulus required to initiate action potential Refractory period: two signals too close together in time cannot sum Initial stimulus strength is indicated by frequency of a series of action potentials

What is membrane potential? Why is it important?

How is membrane potential formed?

How do we calculate membrane potential?

How does membrane potential encode signals?

What are the carriers for membrane potentials?

Voltage-gated ion channels: currentsVoltage-gated ion channels: currents

INa,t

ICa,L

ICa,N

ICa,T

-100

-10

Na+

Ca2+

Voltage (mV)

INa,p

-100

-10

K+

Voltage (mV)

IK

IC

IA

IM

-100

-10

Na+/K+

Voltage (mV)

Ih

Inward currents Outward currents

Voltage-gated ion channels: structureVoltage-gated ion channels: structure

Perez-Reyes, Cell Mol Life Sci 56, 660-669, 1999

The structure of mammalian Kv1.2/Kv2

Long et al, 2005 Science

Voltage-gated ion channels: the superfamily Voltage-gated ion channels: the superfamily

Yu et al., Pharmacol Rev 57: 387-395, 2005

Voltage-gated ion channels: structure of CaVoltage-gated ion channels: structure of Ca2+2+ channels channels

s

Bers and Perez-Reyes, Cardiovasc. Res. 42, 339-360, 1999

Skeletal muscle L-type Cardiac muscle L-type

Ligand-gated ion channels (ionotropic receptors)Ligand-gated ion channels (ionotropic receptors)

. .. .

..

...

.

ligand

ions

Bind to neurotransmittersReceptor channelsMediate fast synaptic transmission

Presynaptic terminal

IP3

ICa

mGluR1

AMPARCa2+

Ca2+Gq/11

PLC

4

DAGCa2+

Postsynaptic terminal

1BNMDAR

5 mV20 ms

ionotropic glutamate receptorsionotropic glutamate receptors

Mg2+

TM4

NR1

TM4NR2

TM4

NR1

TM4NR2

NR2NR1

NR2

TM1

TM4

TM2TM3

NR1NH2

COOH

extracellular

intracellular

NH2

COOH

TM1 TM2 TM3TM4

NMDA: NMDA: N-methyl-D-aspartateCoincidence detector because of voltage dependent

Mg2+ block

GluR1

NR1

GluR2GluR3GluR4

GluR5GluR6GluR7

NR2ANR2B

NR2CNR2D

KA1KA2

10%AMPA: AMPA: -amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid

kainatekainate

TM1TM3

TM1

TM3

TM1 TM3

TM1

TM3

Cys-loop superfamilyCys-loop superfamily

TM2TM1

TM3TM4

TM2

TM1

TM3

TM4

TM2

TM1

TM3

TM4TM2

TM1TM3

TM4

TM2

TM1

TM3

TM4

TM1

TM4TM2

TM3

NH2

COOH

extracellular

intracellular

NH2

COOH

TM1 TM2 TM3 TM4

Cation channelsCation channelsNicotinic acetylcholine receptorNicotinic acetylcholine receptor

I, epithelial 9; II, neuronal 7,8; III, neuronal 2–6 and 2–4

III-1: 2,3,4,6; III-2, 2,4 III-3, 5, 3;

IV, muscle 1, 1, , , and IV-1, 1IV-2, , , IV-3, 1.

5-HT3 serotonin receptor5-HT3 serotonin receptor5-HT3A, 5-HT3B

Anion channelsAnion channelsGABAGABAAA receptor receptor

Glycine receptorGlycine receptor

Muscle-type

homo-oligomeric 7

hetero-oligomeric 42

Neuronal type

Corringer et al., Annu. Rev. Pharmacol. Toxicol. 40:431-458, 2000

subf

amili

es

P2X receptorsP2X receptors

P2X2

P2X3

P2X5

P2X4

P2X1

P2X6

P2X7

b

Plasma membrane

a

C tail length varies

Cysteine richextracellular

loop

Each channel may contain three to six subunits

• Activated by ATP• Cation nonselective• ~6.5% of current is carried by Ca2+

Khakh et al., Pharmacol. Rev. 53, 107-118. 2001

NH2

Anion channelsAnion channels

GABAA receptor

CLCClC-5

Text books:

Chapter 6Fundamental Neuroscience

Chapter 7Principles of Neural Science