04.02 Membrane Potentials

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Cell Membrane Potential (Image)
Saltatory Conduction (Image)
Neuron Resting Potential (Picmonic)

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In this lesson I want to talk about membrane potentials and the role they play in nerve transmission.

So first - what is a membrane potential. Anytime you hear ‘potential’ related to nerves - think “electrical charge”. So the membrane potential is just the electrical charge across a membrane. In the nerves, we have a resting potential and an action potential. This should be pretty self-explanatory. The resting potential is when nothing is happening - the nerve is just resting. We’ll look at this more in a minute, but know that the resting membrane potential - or the resting electrical charge is negative 70 mV - meaning it’s more negative on the inside than the outside. The other type we have is an Action Potential - as you can see - this is when there is an ACTION happening - so an action potential is sending a stimulus to create a response somewhere in the body. The charge in an Action Potential is more positive - we’ll look at specifics in a minute.

So when I say the resting potential is about -70mV, again what I mean is that it’s more negative inside than outside. But since everything equals out based on concentrations, not charges, it’s actually at equilibrium that way. So let’s look at some of the factors that contribute to this negative membrane potential. First is the sodium potassium pump - this pump uses ATP (or energy) to force sodium (Na) and potassium (K) to move against the concentration gradient. For every 3 sodiums we kick out, we bring in 2 potassiums. Both of these ions are positively charged, so what you’re seeing is that MORE positive charges are leaving the cell than coming in - that makes it more positive on the outside than on the inside. We also have ungated leakage channels in the nerve membrane that allow sodium and potassium to enter and leave based on the concentration gradient. And what we see is that potassium diffuses OUT about 100 times faster than sodium diffuses IN. So, again, we have MORE positive on the outside - which is what gives us the net negative charge of about -70mV or so. And these ions are constantly moving back and forth to keep it there. So, again - this is our resting potential - this means nothing is happening. No signals are being sent - the nerve is just sitting there waiting. When it’s time for an action potential - we see these GATED channels come into play. Gated means they have to have certain conditions in order to be open. So, let’s look at what happens...

So it starts with just some local depolarization - so if this is our axon, it’s just in one little spot, but not moving down the whole thing. We have some kind of stimulus - like a chemical signal, heat, cold, etc. - that causes some of the sodium gated channels in this one spot to open up. So now we actually have more sodium coming in than we did before. So instead of being SUPER positive out here, it’s only moderately more because there’s more positive ions on the inside now. If nothing else happens - these gated channels close and the body will just kick some potassium out to make up for it and it will re-polarize back to that resting potential. However… if we have more stimuli or a stronger stimulus, these gated channels STAY open and sodium starts to flood in. That allows our membrane potential to get more and more positive until we hit what’s called the threshold voltage. THAT is what starts an action potential - and it starts to move down the axon - instead of just a local depolarization, it’s a full action potential down the length of the axon.

Now let’s look at what happens in an action potential once that threshold stimulus is reached. Once we reach that threshold, it causes those sodium gated channels to fly open and sodium rushes in - that means the inside gets more and more and more positive - this process is called depolarization.

Once the membrane is fully depolarized, the sodium channels close and the potassium channels open - so that allows potassium to rush OUT of the cell. So the membrane gets less and less positive and the potential starts to return towards that resting potential. That process is called repolarization.

But, what happens is that before those potassium channels close, more potassium LEAVES the cell than sodium is coming in, so the membrane actually goes MORE negative and goes PAST the resting potential. This process is called hyperpolarization. Hyper means MORE - so it goes BEYOND where it should have.

Then the sodium and potassium pump will help restore everything to normal concentrations and returns everything to its normal resting potential. Let’s visualize this all together.

So here’s our resting potential at -70 mV. Remember, we have to get enough stimulus to bring the membrane up to threshold. If it doesn’t get to threshold, nothing happens. We never get an action potential - we only get that local depolarization. That’s the all or none law - you either get a FULL action potential - or nothing. So - if we get enough stimulus and the membrane potential gets to threshold - which in the nerves is about -55 mV - then we see that action potential start. We see depolarization as the sodium rushes into the axon and makes it more positive, then we see repolarization as the sodium channels close and potassium channels open to let potassium out and bring that potential back down towards the resting potential. But we see more potassium out than sodium in, which causes this hyperpolarization phase before the sodium potassium pump returns us back to resting. Now you’ll notice here that during this hyperpolarization phase, there’s what’s called a Refractory period. During this time, no other stimulus would be strong enough to start a new action potential - because it can’t get us up to this threshold. So - resting, depolarization, repolarization, hyperpolarization, and back to resting. Now, this is just a picture of what’s happening at one spot on the axon - but in order to send a signal, we actually have to see this process continue and repeat itself down the length of the axon.

That process is called propagation or conduction. This action potential moves like a wave down the length of the axon. You can see in this neuron on the left, it just waves down the axon. Now - the difference between these two is whether or not a myelin sheath is present. Remember from the neuro anatomy lesson that schwann cells wrap around the axon and create the myelin sheath - and there are nodes of ranvier between each cells. These allow the action potential to move much faster and smoother down the axon. You can even see the difference side to side here. When the conduction happens like this and jumps from node to node - it’s called “saltatory” conduction - it just means jumping.

So let’s recap what happens with membrane potentials. Remember that just means the electrical charge across the membrane. So the resting potential in a nerve cell is when nothing is happening, and it’s usually about -70 mV. The things that control these charges are basically the movement of sodium and potassium across the membrane, both with the sodium potassium pump as well as with the gated and ungated channels. Once it reaches threshold, it will start an action potential, which is when the signal is actually being sent down the axon. That process involves depolarization, repolarization, a brief period of hyperpolarization, and then the return to resting potential. And that process propagates or conducts down the axon so that we can send the signal where it needs to go.
So that is the basics of membrane potentials in the nervous system. Make sure you check out all the resources attached to this lesson. Now, go out and be your best self today. And, as always, happy nursing!
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