Physio lecture 9 Membrane and Action Potentials
You don’t need to know the Nernst equation. I will tell you how to think about it.
You see body fluid compartments, separated by a membrane.
Will the solute diffuse down the concentration gradient? Yes, if it can get through.
Well, now I am adding that the solute also needs to be able to go through an electrical gradient.
Cell Membranes
• What two conditions must be met for diffusion of a species across a semipermeable membrane?
– Is the membrane permeable to it?
– Does it have a concentration gradient?
• If the answer is yes to both questions, then the species will diffuse (Which way? Down its gradient)
Remember to ask:
Is there a gradient?
Can it diffuse?
If allowed to diffuse, which way would it go?
There is not just separation of solutes, but separation of charges, too!
• Inside of the cell is negative due to :
– Abundance of negatively charged proteins
– Na+/K+ ATPase (net loss of positive charges~ 4mV)
– Membrane is 100x more permeable (“leaky”) to K+
“Sidedness” of the membrane
Sidedness means that the electrical charges on one side of the membrane (positive or negative) are different
than on the other side.
Why does sidedness exist?
Different permeability
Pumps
Protein channels
How does a membrane become sided?
Primary and secondary active transport, or pores that allow only one particular solute to move. These things
make a higher concentration on one side. Therefore, sidedness is caused by proteins.
Take an electrode, pierce the membrane, attach to voltometer, compares the charges inside and outside of
the cell. Inside of cell is more negative for three reasons:
1. Proteins are abundant inside cell, and are negatively charged at your normal pH.
2. Sodium-potassium-ATPase mechanism contributes toward the electronegativity inside the cell. Two
K are pulled in while 3 Na go out. They are all +1, so net loss of one positive charge, so the inside is
more negative than the outside.
3. Most important reason: potassium leak channels. If men are K and women are Na, ladies are most
abundant outside of cell, more guys inside room. After class, guys want to leave, they can use the
door. Ladies want to get into the room, but to get in, they have to come under the crack of the door.
Not likely to get in. Sodium does not have leak channels, which are created by integral proteins.
Potassium leaves by leak channels, contributing to negativity.
How negative is the inside of the cell?
Minus 70 mV (milivolts), depends on the cell. Heart cells are minus 90, some are minus 50-60.
We have chemical and electrical sidedness on a cell membrane. The membrane has membrane potential
(separation of charges). You can calculate voltage. If the charges on a battery reach equilibrium on both
sides, the battery will be dead. That can happen to our cells, too. We need to use the electrical energy in our
cells to do kinetic work. You have to turn on a machine to engage its electrodes. The cells don’t all allow the
diffusion of electrical current to do work with. The ones that can increase resistance have something special
that other cells don’t have. Cells that can increase resistance have proteins that create channels. Examples
of this type of cell are neurons and muscle cells; they allow charges to move across the membrane, because
they express genes that make the integral proteins that create these channels. Charged ions, K+, Na+, Ca++
are ions, so they are called electrolytes. When they move, they carry an electrical charge. What do
electricians use to reduce resistance of wires? They use Insulation, in the form of plastic coating on wires. We
need resistance (block flow of charges). A cell creates this resistance by blocking the channel.
Myelinated neurons carry current faster (the current skips over the Nodes of Ranvier and just has to travel
down the bare portions of the axon). Another thing that affects speed of electrical transmission is the size of
neuron: bigger neurons carry current faster (expand the freeway to add extra lanes, you will get home
faster).
Conductivity means the permeability. If conductivity increases, it means that permeability increased.
Ions diffuse at a faster rate when there is less resistance. The more resistance there is, the less conductivity,
and less resistance will cause more conductivity. Myelin reduces resistance because it makes electrical
charges move faster.
Electricity
• Current: the flow of charge
• Voltage: separation of opposite charges (mV). Same as membrane potential.
• Membrane potential is how negative or positive the charge is in the membrane.
• The terms below define the degree of greatness of the separation of charges on each side of the
membrane.
– Voltage
– Voltage difference
– Potential difference
– Potential
• Resistance: opposition to charge movement (friction)
• Conductance: allowing a charge to move (permeability)
• What are the charged things that run through our body fluids? Electrolytes!
•
ions: Na+ K+ Cl- Ca++
Therefore, when dealing with things that are charged, you must ask an additional question!
1. Is the membrane permeable to it?
2. Is there a chemical gradient for it?
– Things tend to move from high to low concentration
3. Is there an electrical gradient for it?
– Things tend to move to regions of opposite charge
Sometimes, the chemical gradient is favors one ion to go in one direction, and the electrical gradient favors
it to go in the other direction. The stronger pull will win.
Only then, can you predict!
• How do you measure each of these gradients or forces?
• Which force is greater?
• The larger force wins!
Every cell has a separation of charge; the #1 reason is the leakiness of K. It leaks out all the time, and the Na
pushes K back in. We use this electricity to do work. Blood pressure, peristalsis of intestines, muscles, etc,
use this electricity for work.
Because of this separation of chemicals and electrical charges, every cell has a Resting Membrane “Potential”
• A difference in electrical charge across the membrane (a potential difference)
• More negative inside; more positive outside
• Our cells are like batteries and some cells can tap into this “potential energy” to do work (“kinetic
energy”)
• What generates resting membrane potential?
– Mainly, ion concentration gradients and differences in membrane permeability (leaky to K+
but not to protein)
•
•
•
•
Resting Membrane Potential is -70-90mV
Why is the resting membrane potential negative? Because K has leakiness, so it escapes with its
positive charge, leaving the inside of the cell more negative.
Which force is “winning” at rest? Potassium
How can simple diffusion cause this potential? There is not much Na inside the cell, so sodium wants
to diffuse in with its positive charge.
The inside of the cell is negative because there are K leak channels, that means there is greater permeability
for K, so it will diffuse out of the cell down its concentration gradient.
The membrane potential (how negative or positive is) is a number that is a reflection of the ion with the
greatest permeability. If our cells are minus 70 mV, it’s because they are most permeable to K. Therefore, K
will diffuse out its concentration gradient, taking its positive charges with it, leaving the inside of the cell
more negative. What if the cell was more permeable to Na? Sodium would diffuse down its concentration
gradient to the inside of the cell, taking its positive charges with it, making the inside of the cell more
positive.
Membranes are leaky!
Solutes diffuse down their EC gradients.
Most leaky to K+
Not permeable to proteins (too big!)
So, we have a battle: diffusion of a chemical gradient and the diffusion of the charges (Electrical potential )
In the above picture, just look at the number of blue-dot particles (ignore the charges). There are
more blue dots outside of the cell, so there is a chemical gradient for the blue dots to move inside of
the cells. Insert a voltage rod to inject positive charges into the cell. The negative charge of the blue
dots will want to enter the cell because they are attracted to the positive charges there. Thus, there
are two reasons why these blue dots will quickly enter the cell. But when will the blue dots stop going
into the cell? They will be attracted to the positive charge, but if they are all inside, the chemical
concentration gradient makes them want to diffuse out of the cell. When the electrical and chemical
gradient is equally powerful (in opposite directions), that is the Nernst potential: No net gain or
loss. Cells with resting membrane potential are at minus 70mV. They are not at their resting K
potential. If you open more K channels than just the leak channels, there will be more movement of
K out of the cell, and the potential will get closer to minus 94 mV (at which time, the cell will reach
equilibrium, and the cell will die; but the body does not let it get that far).
The Nernst potential (equilibrium potential) is the theoretical intracellular electrical potential that would be
equal in magnitude but opposite in direction to the concentration force.
In other words: when does the attraction between opposite charges oppose the diffusion of a chemical
gradient?
In this case we’re changing the electrical potential across the membrane to see what happens to the
concentration gradient of the ion. In reality, the concentration gradient is changed during various cellular
processes which determine the Nernst potential.
Membrane Potential (Vm):
- Charge difference across the membrane Question: How can passive diffusion of potassium lead to development of negative membrane potential?
Answer: Potassium leaks out of the cell, taking its positive charge with it, leaving the inside of the cell more
negative.
Simplest Case Scenario:
If a membrane were permeable to only K+ then…
K+ would diffuse down its concentration gradient until the electrical potential across the membrane
countered diffusion. As K leaves the cell, it takes a positive charge outside with it, so the inside is more
negative. However, as the inside of the cell is becoming more negative, the outside of the cell is becoming
more positive, and the positive charges will want to flow back inside of the cell since they are attracted to the
negative charges. This is electrical potential that counters the net diffusion of K.
The electrical potential that counters net diffusion of K+ is called the K+ equilibrium potential (EK).
The equilibrium potential of K is minus 94 mV
So, if the membrane were permeable only to K+, Vm would be -94 mV (cell death from equilibrium)
Let’s try it with sodium:
If a membrane were permeable to only Na+ then…
Na+ would diffuse down its concentration gradient, bringing its positive charges into the cell. The inside of the
cell would become more positive (less negative). When the inside of the cell has too many positive charges in
it, the positive charges will repel each other, and leave the cell to see a less positively charged area. This is
electrical potential that counters the net diffusion of Na.
The electrical potential that counters net diffusion of Na+ is called the Na+ equilibrium potential (ENa).
So, if the membrane were permeable only to Na+, Vm would be +61 mV (cell death from equilibrium).
But the body prevents the cell from reaching equilibrium. Since the resting membrane potential is minus 70
mV, (a negative number), it tells us that potassium has the greatest membrane potential at rest. Don’t
confuse this with thinking that potassium has a negative charge. It has a +1 charge, the same as sodium. But
when potassium leaves the inside of the cell, it takes its positive charge with it, leaving the inside of the cell
more negative. The inside of the cell is negative because proteins have a negative charge, and the cell
contains so many proteins.
At resting membrane potential, cell voltage is at minus 70 mV.
Since potassium’s chemical equilibrium is minus 94, potassium’s chemical equilibrium is not met yet.
That means that it will WANT to flow out of the cell.
But the difference between the voltage of where it is (minus 70) and where it wants to be (minus 94), is only
24 mV.
This is not a very strong difference.
Since sodium’s chemical equilibrium is plus 61 mV, sodium’s chemical equilibrium is not met yet.
That means that it will WANT to flow into of the cell.
The difference between the voltage of where it is (minus 70) and where it wants to be (plus 61), is 131 mV.
This is a much stronger difference, compared to potassium. Therefore, the ion with the strongest driving
force is sodium because its equilibrium potential is much different from the resting membrane potential.
If you want to harness the electrical current to use it to do work, use the sodium driving force instead of
potassium.
Anatomy of a neuron
Axon Hillock
(Trigger zone)
Receptors on the dendrites act to bind proteins, can direct things into and out of the cells.
Neurotransmitters from one synaptic knob are released onto another dendrite; a channel opens within the
receptor on the dendrite. If the channel was for sodium only, sodium will move inside of the cell. The sodium
travels down to the axon hillock. If enough sodium is allowed to enter the soma, the sodium that reaches
the axon hillock can trigger the opening of the first voltage-regulated gated channel on the axon. Channels
distal to the hillock are locked down (bound), and will open only if electrical charges have been reduced on
inside of cell (they are voltage regulated). As the sodium gets to the axon hillock, the axon will get less
negative, so the inside of the cell is less negative, will it will open K voltage channels. The first opened gated
channel will open the next gated channel, and so on, down the axon. The axon hillock is the trigger zone
where you start to see voltage gated channels. When you are firing a gun, even if you squeeze the trigger
slowly, there is a trigger point where the bullet will fly out of the gun. Pulling the trigger slow or fast does not
change velocity of the bullet. The voltage gated channels are same; if one opens, they will all open one by
one, like a wave. If enough sodium diffused in when a ligand channel opened, the first voltage gated channel
will open. A ligand is an integral protein in a cell membrane that binds to a chemical, and then transports it
into the cell. Thus, it serves as both a receptor and the transporter.
If you want more electrical current, open the sodium channels first (instead of the potassium channels).
When you increase sodium conductance (permeability), sodium can move down its electrical gradient as well
as its chemical gradient. Sodium’s equilibrium potential (+61), will make inside of the cell very positive,
which is the opposite of the resting membrane potential (minus 70). The reversal of the membrane
potential is called the ACTION POTENTIAL. But you cannot let sodium continue on into the cell until it
reaches equilibrium, or the cell will not be able to metabolize, and it will die. To prevent too much sodium
from entering the cell, you have to open the K channel to allow K to diffuse out by its chemical and
electrical gradients. This is called the dance of the gates. During an action potential, the sodium gate opens
first, the potassium gate opens second. Are the ions where they should be? If not, something needs to push
K back into the cell and Na out: Na-K ATPase (the mother protein, or housekeeping protein). Mom
organizes the house and the kids mess it up again. She directs the kids to take their toys and put that here,
and put that there. During action or resting potential, Na-K ATPase is active all the time, constantly trying to
reestablish the gradients. She never stops working. When is the Na-K ATPase active? ALWAYS!
Sample questions
Why is Vm so close to EK?
Ans. The membrane is far more permeable to K+ than Na+.
What is the net driving force on K+ ions?
What is the net driving force on Na+ ions?
Which way do the ions diffuse?
What effect does increasing Na+ or K+ permeability (or extracellular concentration) have on Vm?
The resting membrane potential is closest to the equilibrium potential for the ion with the highest
permeability!
What keeps the ion gradients from running down?
The sodium/potassium ATPase (“the housekeeper”) . It contributes to the resting membrane potential, but
it is not why the inside of the cell is more negative; that is from K leakage.
If equilibrium is achieved on both sides of a membrane, it will become like a dead battery.
Do we want our cells to be like a “dead battery?”
Integral membrane proteins found in all cells will “pump” (against their gradients across the membrane) Na
and K. This is fueled by ATP.
ATP  ADP + Pi + energy
Remember: sodium is pumped out of the cell, potassium is pumped in...
This pump is electrogenic, it contributes slightly to RMP
What if…..
• What if a membrane suddenly became MORE PERMEABLE to Na+?????
• What would Na+ do? (Ask yourself this question)
Which way is the electrochemical gradient for Na+?
Electrical: inward
Chemical: inward
Answer:
Most definitely INWARD
Sodium WANTS IN!
What would happen to the membrane potential of the cell when you open up a sodium channel?
If we instantly increase sodium permeability, sodium will enter the cell, creating a large electoral current,
then K permeability will increase, and the membrane potential will return to negative.
Excitable cells (neurons and muscles) are those that want this large electrical current to use for work.
They have proteins that are sodium channels. Not all cells have these proteins. All cells have the genes to
make these proteins, but only the excitable cells EXPRESS these genes, and actually make the proteins that
fuse with the cell membrane and form a sodium channel. Muscle cells use the electrical force to contract,
and neurons use it to excite the neurons touching them.
Excitable Cells
• Cells that can experience a momentary change in membrane voltage are “excitable” cells
• That temporary change in voltage is due to a momentary change in permeability
• The membrane, for only a moment, becomes more permeable to Na+ than to K+
• Called an Action Potential-it is a reversal of the membrane potential!
• Cell becomes positive inside!!!
• Question: What can allow membrane permeability to these ions? Proteins
• Why are neuronal cells and muscle cells able to change their membrane potential? They express
their genes to make the integral proteins that form the sodium channels.
Integral proteins can open and close depending on the stimulus.
Definitions: There is a potential difference (pd) across the cell membrane
(minus 70 mV) is called the “Resting Membrane Potential”
Because a charge is present (it is not zero), we say the membrane is “polarized”
If it becomes less negative, it is called depolarization (happens when sodium is entering the cell).
If it becomes more negative than minus 70, it is hyperpolarization. (happens when K leaves the cell)
In either case, when you go back towards minus 70, it is repolarization.
Threshold is the point at which the first voltage-regulated sodium channel opens.
Threshold voltage is the minimum voltage needed to trigger an AP. not a number, rather the “trigger” to
open voltage operated channels
Question
To depolarize a cell, what kind of charge must be put into the cell, positive or negative? Positive
When you take the mantle off the giant squid, the nerves are right there, and are so big, you can touch them.
Hodgekin and Huxley put one of these neurons in an isosmotic solution and inserted four wires along the
axon, distal to the hillock. The first wire was attached to an instrument that can inject a positive charge into
the cell (increasing its membrane potential). The next three wires (R1, R2, R3) received the signal and
measured the resulting charge. In this way, they could find out if the injected positive charge would continue
down the axon or dissipate.
With a small injection of a positive charge, they found the membrane potential was less as it got farther from
electrode (the charge dissipated). Then, they put in a lot more positive charges, there was a greater potential
in the other three receiving electrodes, but it still dissipated. Then they gave it a whopping positive charge,
RMP rose to minus 160, and they found a reproducible membrane potential, which continued through length
of axon, and they were able to maintain this reversal of the RMP. The voltage gated sodium channels
allowed the Na to come in, allowed more Na gated channels to open, one after another. This is called an
action potential. Sodium ions entering the cell is what created the action potential. You can have hundreds
of thousands of action potentials in a second, but there is not a flood of sodium coming in; only a few ions
need to go in to make an action potential.
Compare Local Potentials to Action Potentials
Local Potentials
• Generally, they reflect shape, size of voltage stimulus (similar to stimulus pulse)
• They are graded in size (i.e. bigger stimuli give bigger depolarizations)
• They “die out” (voltage grows smaller) as they move from site of stimulation (resistance!)
Parts to an action potential:
Upstroke: Na is more permeable, cell becomes less negative
Downstroke: K permeability is greater, cell returns toward negative, repolarization.
Hyperpolarization: Dips below line
As sodium channels open, it is recorded on a machine as an upstroke.
The peak of the curve shows that Na channels are deactivated, so K channels open, recorded on the machine
as a downstroke.
Actually, sodium channels do not have just two positions (open or closed); they have three gated properties,
like a stop light: red, yellow, green. If a light is green, you are conducted through intersection. When the light
is yellow, you should slow down and not go through the intersection. Likewise, in the middle position, the
sodium voltage channel prohibits further sodium from crossing the channel.
Yellow lights do not turn green. It has to turn red first. There are 2 amino acid lids (gates); an external and
internal gate. Together, they are shaped like a ball and chain. When the voltage becomes positives, the AA’s
change their charge, and their ball and chain will rock over and cover up the channel. This state (yellow
light) means inactivation. The amino acids have to change their charge to move back out of the way (red
light), meaning deactivation. Then the channel can become active again. It is a safely mechanism, insuring
that another neuron cannot fire another action pot before it is ready to receive one. When the gated channel
is yellow, it is in absolute refractory period. This means that it cannot go from yellow to green. When
enough channels are deactivated (red light), they can open again. Then neuron can fire again. After a man
ejaculates, he needs some time (a refractory period) before he can do it again! But he will need to put more
energy into to get erection than he did the first time; needs more stimulus. Likewise, if you need neurons to
fire quickly, you need greater stimulus.
Local potentials are graded. Need a greater stimulus for greater potential.
Action potentials are maintained, and every point on the neuron shows the same amount of charge.
The neuron that is responding to a small amount of tissue damage will send fewer action potentials than a
neuron that is responding to a large amount of tissue damage, but the current is the same. The brain knows
from the greater number of action potentials about the amount of damage; the frequency increases for
bigger cut. The current is the same for small or large cut: all or none, like a stop sign, instead of a traffic light.
Action Potentials
• They don’t reflect the shape, size of the stimulus, rather they are uniform in size, shape; always
identical
• They are “all-or-none” (i.e. either you trigger an AP if you reach threshold – or if subthreshold, you
don’t get an AP – get local potential.)
• They do not diminish in size no matter how far from the stimulus; regenerate anew at each point
along the axon
What is responsible for the change in membrane permeability during the action potential?
• Although called “action” potential, it is NOT an active (energy-consuming) event for the cell. It is
purely a passive event. It is due to diffusion of ions!
• It is dependent on
– ionic electrochemical gradients (Na+, K+) and
– The membrane’s permeability.
Excitable cells have “fickle” cell membranes…they keep changing their permeabilities.
What determines the membrane’s permeability at any moment?
Answer: GATED ion channels—these allow SIMPLE DIFFUSION of ions down their electrochemical
gradients
Gated Ion Channels
Allow simple diffusion
Can open and close
Are selective
Voltage-gated channels (VGC)
Open/close depending on the voltage across the membrane
Na+ VGC, K+ VGC, Ca++VGC
Located on the axon, at hillock and beyond
Ligand-gated channels (LGC)
Are not dependent on membrane potential but on the binding of ligands (ligand bind neurotransmitters)
Neurotransmitters bind to protein receptors on a cell membrane (on the dendrite of a neuron)
These ligand receptors are located on dendrites and cell body, above hillock
Ion channels cause the action potential
• Thus, permeability of axon membrane to ions is determined by the:
Number of open channels
• Ion channels - structure
 proteins that span the membrane
 have water filled channel that runs through protein
• Ion channel - properties
 Have conducting states (open) and non-conducting states
 Ion channels are usually selectively permeable
• some pass only Na ions and are generally called ‘Na channels’
• some pass only K ions = ‘K channels’
• some pass only Ca ions = ‘Ca channels’ (important in synaptic transmission)
• some pass only Cl ions = ‘Cl channels’
The occurrence of action potentials means the gated channels are open. When they are open, ATP is not
used, it is simple diffusion. But resting membrane potential requires ATP to keep it at steady state.
Homeostasis (steady state) requires a lot of energy (ATP). Homeostasis is the opposite of equilibrium. If the
cell voltage reaches minus 94, it is at equilibrium, no driving force is present, cell death.
Voltage gates open if there is membrane potential, or if there is a ligand.
A ligand is a type of protein in the cell membrane that binds to a chemical and this binding causes a
conformational change (change in shape and position) of the ligand, so that the channel opens.
A ligand both a receptor and a channel. It allows sodium to migrate into the cell. If it is enough, the first
sodium voltage gate opens, then the next, and the next.
The graphs shows you
Red is sodium conductance (permeability). When it is greatest, compare it to the membrane potential
Purple is the action potential (the current)
Blue is potassium conductance.
Green is sodium/K ratio. At time zero, this ratio is 0.01, very low.
Before an action potential, Na permeability is low.
K has 100x greater permeability than Na during resting membrane potential.
As Na permeability increases, Na rises to 1000x greater than K. Cell becomes more positive. Then K
channels become activated, so K increases permeability, it leaves cell, takes a long time to deactivate, so you
get hyperpolarization, and the inside of cell becomes more negative. When restored, the cell can fire an
action potential again.
You need to understand all of this for your homework. Let’s go over this again, with numbers:
Resting membrane potential indicates which ion has greater permeability: K has the greatest permeability.
Let’s say there are 1000 K channels open, and sodium only has 10 channels open.
10 divided by 1000 is 0.01, so K is 100x more permeable than Na.
During an action potential, threshold is reached, and Na channels open, lots of them.
Now the ratio is 10,000 Na channels open, and only 10 K voltage gated channels open.
Now ratio is 1000x in favor of sodium.
When membrane potential gets to +30, it reflects the permeability of sodium, striving towards its potential of
+61, but it never gets there. The ball and chain stops it. As sodium conductance is declining, K is increasing,
and membrane potential goes back down toward a negative value. This reflects the ion with greatest
permeability; in this case, it is K. It takes a while, so you see a hyperpolarization, where there is even greater
K permeability than what you had at resting membrane potential.