1. Membrane Potential 101 R. Low- 08/26/14 DRAFT
The purpose of this module is to provide an introduction to the basis of membrane electrical potentials. Often considered a difficult concept, understanding its origins and the basis it forms for communication between nerves and nerves and muscles is pivotal to understanding communication in the central nervous system and between nerves and tissues.
This module is meant to be an INTRODUCTION. Many important details are left out. These will be covered most particularly in HSF and Neural Sciences.

2. 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.
Slide peaks for itself.

3. The Plasma Membrane Morielli: CMB-2012
Our story begins with a reminder from Gary Ward’s lecture on membrane structure. A lipid bilayer, by itself, allows uncharged molecules to pass. Ions are a different matter however, their charge disallowing passage.
Morielli: CMB-2012

4. The Plasma Membrane as a Fluid Mosaic
As you know, membranes, including the plasma membrane, simply are not simply lipid bi-layers. They contain all sorts of proteins whose functions range from immune identity, to hormone receptors, to ion channels.
Ward: CMB - Membranes

5. Ion Channels Allow Passage of Specific Ions
Ion channels allow the passage of molecules such as sodium, potassium, chloride, calcium and others.
Morielli: CMB-2012

6. Ion Channels
Ion channels do not move ions. They simply provide a passive path for them to move according to their electrochemical gradients
Of most concern to us today are ion channels. These are membrane proteins that allow the movement of specific ions, such as sodium and potassium, down a gradient, ether a concentration or an electric gradient, or both. (More shortly about electrical gradients.) They do not “move” ions per se: rather they simply provide a simple path for ion movement.
Morielli: CMB-2012

7. Ion Channels Permit Rapid Movement
Channels can move ions 100,000 times faster than the fastest rate of “carrier” proteins.
A property of major importance is that ion channels allow the extremely rapid movement of ions down their concentration and / or electric gradient. Transport in the membrane that acts as carriers are MUCH slower. (More about carrier proteins in the lecture on membrane transport.)
Morielli: CMB-2012

8. Ion Channels Are Selective
Ion channels also can be very specific, allowing passage of a particular or in some cases subsets of ions.
Ion channels can be specific for certain ions
Morielli: CMB-2012

9. 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
Ion channels come in the several flavors listed here. Each will be discussed throughout your Foundations courses, beginning with the membrane transport lecture in CMB.

10. Ion Channels and Membrane Potential
Non-gated channels / Leakage Channels
Open at Rest / all the time.
Gated Channels
Open on demand
Two definitions of importance. Non-gated channels, also known as leakage channels, are available all the time. They are unregulated.
Gated channels only open on demand. They require some particular signal for them to be open. Three such signals indicated on the previous slide are membrane voltage (the subject of this module) mechanical forces and chemical signals.

11. Most Cells Have Membrane Potentials Membrane Potential (mV)
Cell type
Membrane Potential (mV)
Neuron
-60
Skeletal muscle
-85
Cardiac muscle
-90
Adipose cell
-40
Thyroid cell
Fibroblast
-10
Yeast
-120
Neurospora. crassa
-200
E. coli
-140
Mitochondria
As this slide suggests, most cells have electrical potentials across their plasma membranes. Though these potentials of are great importance to cell function in all cases, three cell types of special concern here and in later courses are neurons, skeletal muscle and cardiac muscle cells.
Morielli: CMB-2012

12. K+ Na+ Ion Channels of Special Concern
Two ion channels we will be focusing on here are those specific for sodium and potassium. Other absolutely do contribute to membrane potentials, but that story will come later.

13. The “Na/K pump” splits ATP to make a Na+ and K+
concentration gradient as well as an electrical gradient
(the electrochemical gradient) 3
There is an additional transport molecule that is of overwhelming importance, one you already have heard about. This is the Na+ / K+ ATPase pump, found in virtually all cells, a pump as you have learned accounts for use of some 30% of total body ATP.
As you know, the Na+ / K+ ATPase pump is responsible for maintaining a high concentration of sodium out side the cell while at the same time maintaining a high potassium concentration inside.
You see, at once how this relates to membrane potentials: it creates an electrochemical gradient. 2
A transporter protein moves a few ions for each conformational change
Ward: CMB - Membrane Transport

14. 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)
The Na+ / K+ ATPase pump explains the gradients for sodium and potassium across the plasma membrane. There are other differences as well but our main concern at this point is with Na+ and K+.
Non-permeable anions will enter the picture later. They include things like ADP, ATP, Nucleic Acids and negatively charged proteins.

15. 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.
This slide speaks for itself, indicating where this whole discussion is going.
P = permeability

16. Creating the Membrane Potential
Let’s start here with an idealized cell surrounded by a plasma membrane.
First, note that there is an asymmetric distribution of ions, as you have seen before. We start here with the membrane being IMPERMEABLE to every ion.
The device at the top is a volt meter which allows us to measure if there is a voltage across the membrane. There is no voltage / electrical potential difference in this situation: no permeability = no voltage.
No permeability to ions: no voltage (potential difference) across the membrane.

17. Plasma Membrane is Selectively Permeable to Potassium
Recall, however, that in real life, there is a substantial and selective permeability for potassium (slide 14 above). This is indicated by the red cylinder.
That being the case, potassium, of course will be able to move down its concentration gradient into the cell. We can measure this movement as a voltage / potential difference across the membrane, inside the cell negative.

18. Yin / Yang – Opposing Forces
Potassium cannot continue to move down its concentration gradient to steady state with equal concentrations on both sides of the membrane, because as it does move it creates an increasing a negative charge at the inside of the plasma membrane relative to the outside. This will be registered by a negative deflection in our voltmeter. This “potential difference” is the result of the diffusion-dependent flow of K+ ions from inside to outside.
Yin: Concentration Gradient / Yang – Electrical Gradient

19. Opposing Forces: Chemical gradient vs electrical gradient
This cartoon illustrates the two opposing forces: concentration gradient causing potassium to move out of the cell OPPOSED by electrical gradient that favors movement of potassium in the opposite direction. When these are out of balance, you have a NET electrochemical driving force. In this case the net force will cause potassium still to move out of the cell.
Morielli: CMB

20. Chemical and electrical forces exactly balanced
Steady State
Chemical and electrical forces exactly balanced
When the charge difference across the membrane becomes great enough, it will cause NET potassium movement to stop. The membrane has reached Steady State (often called Equilibrium). Potassium STILL is moving into and out of the cell but at the same rate – no net movement.

21. Get out of your minds that the movement of potassium out of the cell changes the intracellular potassium in a measureable way. The amount of potassium that moves to establish equilibrium amounts to some one ten thousandth of the concentration of potassium inside.
The concentration of ions creating the potential difference is very small (10-17M) compared to bulk concentrations of ions (10-3 M).

22. Generation of membrane Potential
No Permeability
Selective K+ Permeability
This slide illustrates what has happened.
Exact balance of charge
Electrical charge across the
membrane: inside negative

23. Ohm’s Law
V = IR
There is no way around it – one must consider Ohm’s law which relates voltage to current and resistance.
In barnyard terms:
V = membrane potential
I = current carried by an ion – think potassium gradient
R = Resistance – Think ion channel
When there is no permeability (slides 15) resistance is infinite: no voltage
When there is no ion gradient (slide 13, 25): no voltage
When there is permeability – think potassium channel - resistance is NOT infinite and there can be current if there is a concentration gradient down which an ion (potassium) will flow in proportion to the concentration gradient.

24. Equilibrium Potential can be easily calculated the Nernst Equation
Walther Nernst
Walther Nearnst in fact developed a formula for determining what the equilibrium for an ion like potassium will be. Lots of constants here.
The important thing to note is that the Equilibrium Potential is dependent on the ion (potassium) gradient.
To iterate, the Nearnst equation the electrical potential at which the electrical driving force for movement in one direction exactly equals the concentration driving force for movement in the other. Another way to think about it is that when an ion channel opens, it “pulls” the membrane potential towards its Nernst potential which is dependent on the concentration gradient for the ion.
You will NOT be asked to use this equation to calculate an equilibrium potential. You DO, however, need to understand the relation ship between Equilibrium Potential and concentration gradient. Example, if the concentration gradient gets smaller, the Equilibrium Potential will … (Answer at the end.)
Morielli: CMB

25. Membrane potential For an Ion With Available Channels, what If:
1/ Permeability DEcreases?
2/ Permeability Increases?
3/ Concentration Gradient Decreases?
4/ Concentration Gradient Increases?
So to test yourselves., what happens to membrane potential if permeability Decreases? Or if it Increases? If Concentration Gradient Decreases or Increases?
Answers at end of Module.

26. Some Equilibrium Potentials
Ion
Outside mM
Inside
Ratio out:in
Ex at 37 oC mV K+ 5
100
1:20
-80
Na+
150 15
10:1 62
Ca2+ 2
2x10-4
10,000:1
123
Cl- 13
11.5:1
-65
Here are some Equilibrium Potentials to consider. The ONLY TWO we will need to be concerned with here in Membrane potential 101 are for potassium and, as we shall see shortly, for Sodium.
Morielli: CMB

27. 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
Fibroblast
-10
Yeast
-120
Neurospora. crassa
-200
E. coli
-140
Mitochondria
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.
You saw the table on the left hand side of this slide before. It indicates that the Steady State / Equilibrium for cells can be strikingly different from one another. The reason for this is that, in real life, cells have a number of different ion channels, each variously open in the sense that their respective ions can move across the membrane.
Morielli: CMB

28. Calculation of Electrochemical Equilibrium Potentials
EK = 2.3 RT log [K+]o = 62 log = mV
ZF [K+]i
ENa = 2.3 RT log [Na+]o = 62 log 140= mV
ZF [Na+]i
ECl = 2.3 RT log [Cl-]i = 62 log = mV
ZF [Cl-]o
NOT to
worry about
Nothing to memorize here. This simply shows how to calculate equilibrium potentials.
Equilibrium potentials for each ion calculated according to the internal and external ionic concentrations described for skeletal muscle.
Morielli: CMB

29. Membrane Potential (mV)
Which Ions Dictate the Steady State Membrane Potential (Ex)
Ion
Outside
Inside Ex mM mV K+ 5
100
-80
Na+
150 15 62
Ca2+ 2
2x10-4
123
Cl- 13
-65
Cell type
Membrane Potential (mV)
Neuron
-60
Skeletal muscle
-85
Cardiac muscle
-90
Adipose cell
-40
Thyroid cell
Fibroblast
-10
Yeast
-120
Neurospora. crassa
-200
E. coli
-140
Mitochondria
Putting these last two slides together allows some important conclusions. Even though many ions contribute to the actual Ex of a cell, One can figure out which ion(s) is most important by comparing a cell’s membrane potential (right) with ion equilibrium potentials (Left)
Let’s take the neuron. The two numbers match closest for potassium and chloride. Hence the conclusion that these are the most permeable of the ions listed – the most able to travel down their concentration gradient to establish a potential.
As you might have guessed from previous slides, when measurements are done it turns out that in the neuron, chloride permeability (channel openness) is very low; while potassium permeability is very high.
Let’s put this concept to work.
The resting potential of the fibroblast is much less negative. That turns out to mean that fibroblasts generally not only have a high potassium permeability but they also have a quite high sodium permeability “dragging” the membrane potential away from potassium equilibrium towards the sodium equilibrium.
So what might be the approximate equilibrium potential for a cell ONLY permeable to sodium? 60 mV, inside positive!

30. Sodium Enters the Game
Here we have added the capacity for sodium to cross the cell membrane.
Does it make sense that, therefore, the equilibrium potential will be less negative that when just potassium could cross?
How much less negative will depend on how “open” are the sodium channels to sodium traffic. Indeed, as suggested by the cartoon, as a constriction in the red cylinder for sodium. Though sodium can cross, it can’t cross much.
This limited “openness” of sodium channels in resting cells is called sodium “leak”, the channels “leakage channels.”
Addition of a Na-selective channel. A steady-state equilibrium is established

31. Membrane Potential (mV)
Which Ions Dictate the Steady State Membrane Potential (Ex)
Ion
Outside
Inside Ex mM mV K+ 5
100
-80
Na+
150 15 62
Ca2+ 2
2x10-4
123
Cl- 13
-65
Cell type
Membrane Potential (mV)
Neuron
-60
Skeletal muscle
-85
Cardiac muscle
-90
Adipose cell
-40
Thyroid cell
Fibroblast
-10
Yeast
-120
Neurospora. crassa
-200
E. coli
-140
Mitochondria
Back to these two tables. Though it isn’t the complete story, the resting potential for neurons is less negative than the potassium equilibrium potential because sodium leakage channels “drag” the resting potential to a less negative potential.
Fortunately, the presence of the Na+ / K+ ATPase pump will pump out the sodium that has leaked in (and keep intracellular potassium concentration high) thus maintaining the sodium and potassium gradients so required for the existence of membrane potentials.

32. Critical Nomenclature
+60
Depolarization
Em mV
Resting Em
We need to be clear about certain terms related to membrane potentials. The slide is self-explanatory.
-60
Hyperpolarization
-100
Time

33. Potassium Permeability Dominates
For a Membrane where
Potassium Permeability Dominates
What will Happen if Sodium Channels Open?
Nothing
Depolarization
Hyperpolarization
More Clicker Question fodder. Answers at the end.

34. 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.
Here is where we are at.
*PK>˃>PNa

35. Once Again … What will happen to Membrane Potential if:
1/ Extracellular Potassium Concentration Rises?
2/ Intracellular Potassium Concentration Rises?
3/ Potassium Permeability Decreases?
More Clicker Question fodder. Again, answers at the end.

36. The Magic of the Action Potential
As all of you certainly know, electrical communication between cells requires what are called action potentials. These amount to rapid changes in resting potential away from the negative potassium equilibrium potential in a self-sustaining way.
Not all cells are capable of developing Action Potentials. Three of special importance in Physiology and Medicine are neurons, skeletal muscle and cardiac muscle.
Let’s now see what causes the action potential.
Only certain KEY CELLS: e.g. Neurons, Skeletal Muscle, Cardiac Muscle

37. The Action Potential Silverthorn, Human Physiology, 5th edition
Let’s say this is a neuron in the Central Nervous system. At Steady State / Equilibrium (1) the membrane potential is -70. Potassium is the dominant determinant. The membrane is NOT quite at the potassium equilibrium (the resting potential is less negative) because there is some permeability to other ions such as sodium.
Certain events take place that cause the resting potential to begin to become less negative (2-3). There can be several different causes of this as you shall learn – not important to know at this point.
Magic occurs at (3). The membrane reaches what is called threshold potential. At exactly this point, voltage gated sodium channels open rapidly. Hopefully from the previous slides, no surprise as to why this causes depolarization – the membrane is now moving towards to sodium equilibrium potential.
Sodium channels remain open (4) for a time.
Next what happens is that the Voltage-gated sodium channels begin to close (5). AND voltage-gated potassium channels open. Hopefully no surprises here either, based on what you have learned above.
Membrane potential becomes progressively more negative (6)
There is an overshoot past the negative resting potential (7) because some of the voltage-gated potassium channels are still open, though they are beginning to close. The “overshoot” is approaching the true potassium equilibrium potential because of so many open potassium channels.
And then recovery as voltage-gated potassium channels all close (8).
Resting potential
Depolarizing stimulus (don’t worry about this now)
Threshold: Voltage-Gated Sodium Channels Open
Rapid Sodium Entry Depolarizes the Cell
Sodium Channels Close / Voltage-Gated Potassium Channels Open
Potassium Leaves the Cell: Re-polarization.
Voltage Gated potassium Channels Still Open: potassium equilibrium potential approached
Voltage Gated Potassium Channels close
Cell returns to resting potential.
Silverthorn, Human Physiology, 5th edition

38. The Action Potential and the Currents
This slide shows the same thing , this time including a graph of showing the time course of change of sodium and potassium permeability.
Silverthorn Human Physiology Fifth Ed.

39. Receptors as Ion Channels The Synapse between Neurons
Putting ion Channels
to work
Axon
Where does this sort of thing happen? This example is of the synapse between two neurons. Action Potentials are generated along the axon of the nerve cell on the left which, by opening sodium channels of the cell on the right will cause the membrane of the latter to depolarize.
DO NOT WORRY ABOUT THE DETAILS HERE.
Cell Body 39

40. Receptors as Ion Channels Neuromusclar Junction
Putting ion Channels
to work
Axon
Terminus
Neuromuscular
Junction
And this is the neuromuscular junction. Action potentials are generated at the muscle cell membrane.
DO NOT WORRY ABOUT THE DETAILS HERE.
Muscle
Membrane
Case #1
Low: CMB-Cell Signaling

41. Summary: Action Potential
See Slide #38

42. That’s It!
That’s it.
Be sure to Bob Low if you have comments / suggestions:

43. 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
Here are the answers to Clicker Fodder.