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?
The membrane potential (Vm) is defined as
Vm = Vin – Vout
Vout = 0
extracellular
Vin
intracellular
Resting membrane potential
Depolarization
Hyperpolarization
-60 to –70 mV for neurons
become more positive
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 forces
determine the passive distribution
of ions
C
C1
C2
Diffusion
down
Chemical
Gradient
J
I
R
V
P
R
R
J = C/R
V
.
Q
I = V/R
’s LAW
.
Q = P/R
C1+
C2+ Diffusion
+
+
+
+
+
+
+
+
I
+
+
+
down
Electrical
Gradient
I = C/R
+
R
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+
Na+
Na+
Na+
Na+
Na+
Na+
-90 mV
Out of Equilibrium
Na+
Na+
Na+
+55 mV
In Equilibrium
Extracellular
Intracellular
-
K
K
K
K
K
K
K
Concentration gradient
voltage gradient
K
K
K
K
K
K
K
K
K
+
+
+
+
+
+
+
+
+
Negative to EK Positive to EK
Maintaining the resting membrane potential
gK
K+ moves out of cell
Reversal potential
(No net movement of K+)
EK
K+ moves into cell
gK
Time
Normal
current injection
Increased gK
Voltage response
K+ moves out of cell
EK
-102 mV
-102
gK
Time
K+ moves into cell
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 potentials
Nernst equations for biological ions: Anions: Cl- and proteins
Cations: K+ diffusion potential
RT [K]o
Na+ diffusion potential
Ek =
ln
F
[K]i
Ca2+ diffusion potential
Na+/K+-ATPase
RT [Na]o
ENa =
ln
F
[Na]i
RT [Ca]o
ECa =
ln
2F [Ca]i
RT [Cl]o
ECl =
ln
-F
[Cl]i
-60 to -75 mV
extracellular
ENa = +56
Na+ (150)
intracellular
Na+ (18)
EK = -102
K+ (3)
K+ (135)
ECl = -76
Cl- (120)
ECl = +125
Ca2+ (1.2)
Cl- (7)
Ca2+ (0.1 µM)
NSCC
Na+,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
Nernst Equation
E (ion) = RT/zF ln ([ion]outside/[ion]inside)
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”
+72
ENa+ = 63 log [142] o/[ 10 ] i = +73+ mv
ENa
0
ECl- = 63 log [103] o/[ 4 ] i = -89 mv
-1
-89
-90
EclVr (i.e. resting Vm)
-97
EK+
EK+ = 63 log [ 4 ] o/[140] i = -97 mv
Time
The “Voltage Diagram”
ENa+ (Equilibrium Potential for Na+)
+72
- -+ +
- +
- +
0
-89
-90
ECl- (Equilibrium Potential for Cl- )
Vr (i.e. resting Vm)
-97
EK+ (Equilibrium Potential for K+)
Time
Maintaining the resting membrane potential
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
RT pK[K+]o + pNa[Na+]o + pCl[Cl-]i
Vm =
ln
F
pK[K+]i + pNa[Na+]i + pCl[Cl-]o
-60 to -75 mV
extracellular
ENa = +56
Na+ (150)
intracellular
Na+ (18)
EK = -102
K+ (3)
K+ (135)
ECl = -76
Cl- (120)
ECl = +125
Ca2+ (1.2)
Cl- (7)
Ca2+ (0.1 µM)
NSCC
Na+,K+-ATPase
Three Primary reasons for a net negative potential across the membrane
(1)
(2)
Electrongenic Na/K ATPase
High IC [Anions]
(3)
Relative Permeabilities of
dominant cations
3 Na+
EXTRACELLULAR SPACE
Na+
+++++++++++++++++++++++++++++++++++++++++++
ATPase
Lipid Bilayer
-------------------------------------------------------------Anion2 K+
K+
AnionINTRACELLULAR SPACE
Anion-
Anion-
Anion-
Anion-
Changes in membrane potential due to ion movement
Depolarization:
Initiators: Na+ channels
nonselective cation channels (NSCC)
Na+,K+-ATPase
Terminators: K+ channels
Cl- channels
-60 to -75 mV
extracellular
ENa = +56
Na+ (150)
intracellular
Na+ (18)
EK = -102
K+ (3)
K+ (135)
ECl = -76
Cl- (120)
ECl = +125
Ca2+ (1.2)
Cl- (7)
Ca2+ (0.1 µM)
NSCC
Na+,K+-ATPase
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 signals
• Graded potentials: Variable-strength signals that lose strength as they travel
through the cell.
a.
b.
c.
d.
e.
f.
Can be depolarizations (Na+ channel) or hyperpolarizations (K+ or Cl- channel)
Begins on the cell membrane at the point where ions enter from the extracellular
fluid (local current or electrotonic current)
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)
The size of the graded potential decreases as it spreads out from its point of origin
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).
Can be summed: spatial summation and temporal summation
• Action potentials: Signals that travel for long distances through the neuron
without losing strength.
a.
b.
c.
d.
Rapid electrical signals that pass along the axon to the axon terminal.
Identical to each other and do not diminish in strength when traveling through the
cell
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.
All-or-none
Comparison of graded potential and action potential
Feature
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: currents
Inward currents
-10
-100
INa,t
Voltage (mV)
-10
-100
Na+
Outward currents
-10
Voltage (mV)
-100
Na+/K+
Ih
K+
IK
IA
INa,p
IM
ICa,L
ICa,N
ICa,T
Voltage (mV)
Ca2+
IC
Voltage-gated ion channels: structure
Perez-Reyes, Cell Mol Life Sci 56, 660-669, 1999
The structure of mammalian Kv1.2/Kvb2
Long et al, 2005 Science
Voltage-gated ion channels: the superfamily
Yu et al., Pharmacol Rev 57: 387-395, 2005
Voltage-gated ion channels: structure of Ca2+ channels
Skeletal muscle L-type
Cardiac muscle L-type
a1s
b
b
Bers and Perez-Reyes, Cardiovasc. Res. 42, 339-360, 1999
Ligand-gated ion channels (ionotropic receptors)
Bind to neurotransmitters
Receptor channels
Mediate fast synaptic transmission
. .. . .
ions
Postsynaptic
terminal
Presynaptic
terminal
ICa
a1B
Ca2+
..
.
. .
NMDAR
AMPAR
Ca2+
DAG
ligand
Ca2+
Gq/11
IP3
mGluR1
5 mV
20 ms
ionotropic glutamate receptors
GluR1
GluR2
GluR3
GluR4
10%
AMPA:
a-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid
GluR5
GluR6
GluR7
KA1
KA2
kainate
NR1
NR2A
NR2B
NR2C
NR2D
NMDA: N-methyl-D-aspartate
Coincidence detector because of voltage dependent
Mg2+ block
NH2
TM4
NR2
NR1
NH2
extracellular
TM3 NR1 TM1
NR2
TM1
NR1
TM4
TM2
TM1
intracellular
TM2
TM4
TM3
TM1
TM4
TM3
NR2
Mg2+
TM3
TM1
TM1 NR1 TM3
TM3
TM4
COOH
COOH
NR2 TM4
Cys-loop superfamily
Muscle-type
subfamilies
Cation channels
Nicotinic acetylcholine receptor
I, epithelial a9;
II, neuronal a7,8;
III, neuronal a2–6 and b2–4
III-1: a2,3,4,6;
III-2, b2,4
III-3, a5, b3;
IV, muscle a1, b1, g, d, and e
IV-1, a1
IV-2, g, d, e
IV-3, b1.
homo-oligomeric a7
NH2
COOH
extracellular
TM1 TM2 TM3 TM4
intracellular
hetero-oligomeric a4b2
Corringer et al., Annu. Rev. Pharmacol. Toxicol. 40:431-458, 2000
5-HT3 serotonin receptor
5-HT3A, 5-HT3B
Anion channels
GABAA receptor
Glycine receptor
Neuronal type
g
a
b
d
TM4
a
NH2
TM3
TM2
TM4
TM3
a
TM1
g
TM4
TM2 TM3
TM3
TM1
TM2
COOH
TM1
TM4
b
TM1
TM2
TM2
TM2
TM1
TM3
d
TM4
TM1
a
TM3
TM4
P2X receptors
a
P2X2
P2X3
P2X5
P2X6
P2X1
b
Cysteine rich
extracellular
loop
P2X4
Plasma membrane
P2X7
NH2
Each channel may
contain three to six
subunits
C tail length varies
Khakh et al., Pharmacol. Rev. 53, 107-118. 2001
• Activated by ATP
• Cation nonselective
• ~6.5% of current is carried by Ca2+
Anion channels
GABAA receptor
ClC-5
CLC
Text books:
Chapter 6
Fundamental Neuroscience
Chapter 7
Principles of Neural Science