Membrane Potential 101
R. Low- 08/26/14
DRAFT
1
Outline
 Membrane Structure in Review.
 Ion Channels.
 Na+ / K+ ATPase and Intracellular / Extracellular
ion concentrations.
 Building a membrane Potential.
 Electrochemical gradients.
 Physics: Ohm’s Law / Nearnst Equation
 Potassium vs Sodium.
 The Action Potential: a primer.
2
The Plasma Membrane
Morielli: CMB-2012
3
The Plasma Membrane as a Fluid Mosaic
Ward: CMB - Membranes
4
Ion Channels Allow Passage of Specific Ions
Morielli: CMB-2012
5
Ion Channels
Ion channels do not move ions. They simply provide
a passive path for them to move according to their
electrochemical gradients
Morielli: CMB-2012
6
Ion Channels Permit Rapid Movement
Channels can move ions 100,000 times faster than
the fastest rate of “carrier” proteins.
Morielli: CMB-2012
7
Ion Channels Are Selective
Ion channels can be specific for certain ions
Morielli: CMB-2012
8
Types of Membrane Ionic Channels
Non-gated channels: leakage channels open at rest
Gated Channels:
– Voltage-gated channels
– Mechanically-gated channels
– Chemically-gated channels (from outside or
inside of the membrane)
• Neurotransmitter-activated
• Calcium-gated
• ATP-gated
• Cyclic nucleotide-gated
• About 100 different kinds of channels
9
Ion Channels and Membrane Potential
Non-gated channels / Leakage Channels
 Open at Rest / all the time.
Gated Channels
 Open on demand
10
Most Cells Have Membrane Potentials
Cell type
Membrane
Potential (mV)
Neuron
-60
Skeletal muscle
-85
Cardiac muscle
-90
Adipose cell
-40
Thyroid cell
-40
Fibroblast
-10
Yeast
-120
Neurospora. crassa
-200
E. coli
-140
Mitochondria
-140
Morielli: CMB-2012
11
Ion Channels of Special Concern
 K+
 Na+
12
The “Na/K pump” splits ATP to make a Na+ and K+
concentration gradient as well as an electrical gradient
(the electrochemical gradient)
3
2
A transporter protein moves a few ions for each conformational change
Ward: CMB - Membrane Transport
13
Intra- and Extra-cellular ionic
compositions are different
Intracellular
Concentration
Extracellular
Concentration
Low Na+ (15 mM)
High Na+ (140 mM)
High K+ (130 mM)
Low K+ (4 mM)
Low Cl- (5 mM)
High Cl- (120 mM)
Non-permeable
Organic anions (128
mM)
HCO3 (12 mM)
HCO3 (24 mM)
14
Membrane Potential: Where we are going
 There is a metabolic pump which maintains these major
ionic gradients: the Na+-K+ - ATPase.
 In the Steady State condition, the membrane has selective
ion permeability through ion channels: (PK>>PNa).
 As a result, ionic gradients exist across the “resting”
membrane.
 The combination of ionic gradients and SELECTIVE permeability
to potassium creates a resting membrane potential.
P = permeability
15
Creating the Membrane Potential
No permeability to ions: no voltage (potential
difference) across the membrane.
16
Plasma Membrane is Selectively Permeable to Potassium
17
Yin / Yang – Opposing Forces
Yin: Concentration Gradient / Yang – Electrical Gradient
18
Opposing Forces: Chemical gradient vs electrical gradient
Morielli: CMB - 2013
19
Steady State
Chemical and electrical forces exactly balanced
20
The concentration of ions creating the potential difference is very
small (10-17M) compared to bulk concentrations of ions (10-3 M).
21
Generation of membrane Potential
No Permeability
Selective K+ Permeability
Exact balance of charge
Electrical charge across the
membrane: inside negative
22
Ohm’s Law
V = IR
23
Equilibrium Potential can be easily calculated
the Nernst Equation
Walther Nernst
Morielli: CMB - 2013
24
Membrane potential
For an Ion With Available Channels, what If:
1/
2/
3/
4/
Permeability DEcreases?
Permeability Increases?
Concentration Gradient Decreases?
Concentration Gradient Increases?
25
Some Equilibrium Potentials
Ion
Outside
mM
Inside
mM
Ratio
out:in
K+
5
100
1:20
Ex at 37
oC
mV
-80
Na+
150
15
10:1
62
Ca2+
2
2x10-4
10,000:1
123
Cl-
150
13
11.5:1
-65
Morielli: CMB - 2013
Membrane potential is influenced by multiple ions
Cell type
Membrane
Potential (mV)
Neuron
-60
Skeletal muscle
-85
Cardiac muscle
-90
Adipose cell
-40
Thyroid cell
-40
Fibroblast
-10
Yeast
-120
Neurospora. crassa
-200
E. coli
-140
Mitochondria
-140
Morielli: CMB - 2013
The Goldman-Hodgskin-Katz
equation accounts for this by
including a factor for the
permeability of each ion.
The permeability term includes
both the number of ion channels
and their individual permeabilities.
27
Calculation of Electrochemical
Equilibrium Potentials
EK = 2.3 RT log [K+]o
ZF
[K+]i
= 62 log 4 = - 94 mV
130
ENa = 2.3 RT log [Na+]o = 62 log 140= + 60 mV
ZF
[Na+]i
15
ECl = 2.3 RT log [Cl-]i
ZF
[Cl-]o
= 62 log 5 = - 86 mV
120
Equilibrium potentials for each ion calculated
according to the internal and external ionic
concentrations described for skeletal muscle.
Morielli: CMB - 2013
28
Which Ions Dictate the Steady State Membrane Potential (Ex)
Ion
+
K
Na
Ca
+
2+
Cl
-
Outside
Inside
Ex
mM
5
mM
100
mV
-80
150
15
62
2
150
2x10
13
-4
Cell type
Membrane
Potential (mV)
Neuron
-60
123
Skeletal muscle
-85
-65
Cardiac muscle
-90
Adipose cell
-40
Thyroid cell
-40
Fibroblast
-10
Yeast
-120
Neurospora.
crassa
-200
E. coli
-140
Mitochondria
-140
29
Sodium Enters the Game
Addition of a Na-selective channel. A steady-state
equilibrium is established
30
Which Ions Dictate the Steady State Membrane Potential (Ex)
Ion
+
K
Na
Ca
+
2+
Cl
-
Outside
Inside
Ex
mM
5
mM
100
mV
-80
150
15
62
2
150
2x10
13
-4
Cell type
Membrane
Potential (mV)
Neuron
-60
123
Skeletal muscle
-85
-65
Cardiac muscle
-90
Adipose cell
-40
Thyroid cell
-40
Fibroblast
-10
Yeast
-120
Neurospora.
crassa
-200
E. coli
-140
Mitochondria
-140
31
Critical Nomenclature
+60
0
Depolarization
Em
mV
Resting Em
-60
Hyperpolarization
100
Time
32
For a Membrane where
Potassium Permeability Dominates
What will Happen if Sodium Channels Open?
 Nothing
 Depolarization
 Hyperpolarization
33
Summary: Membrane potential
 “Resting” Membrane Potential REQUIRES a
Permeable Ion AND a Concentration Gradient.
 In most cells, the key ion is Potassium: high
Intracellular concentration PLUS available
open channels*
 A Steady State (Equilibrium) is reached when the
Chemical and Electrical Driving Forces are matched.
*PK>˃>PNa
34
Once Again …
What will happen to Membrane Potential if:
1/ Extracellular Potassium Concentration Rises?
2/ Intracellular Potassium Concentration Rises?
3/ Potassium Permeability Decreases?
35
The Magic of the Action Potential
Only certain KEY CELLS: e.g. Neurons, Skeletal Muscle, Cardiac Muscle
36
The Action Potential
Silverthorn, Human Physiology,
5th
edition
37
The Action Potential and the Currents
Silverthorn Human Physiology Fifth Ed.
38
Receptors as Ion Channels
The Synapse between Neurons
Axon
Cell Body
39
Receptors as Ion Channels
Neuromusclar Junction
Axon
Terminus
Neuromuscular
Junction
Muscle
Membrane
Low: CMB-Cell Signaling
Case #1
40
Summary: Action Potential
See Slide #38
41
That’s It!
42
Answers: Clicker Question Fodder
Slide #24: Get closer to zero
Slide: #25:
1/ Less Negative
2/ More Negative
3/ Less Negative
4/ More Negative
Slide #33: depolarization
Slide #35:
1/ Depolarozation
2/ Hyperpolarization
3/ Depolarization
43