Action Potentials (is it reall a "potential")

[crazycooljoel]

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I'm wondering if there is someone who can help me understand this part. For action potentials, I don't get the "potential" part of it, it sounds misleading to me, at least the way I understand it. I understand the All or None principle of it, that if it begins to depolarize, that it doesn't create an action if threshold isn't met. With the Na+/K+ ion pump, If an action potential passes (or reaches) threshold, the depolarization phase will occur and then repolarize after it reachs it's peak, and return to a resting membrane potential. My question is where is the "potential" in all of this? It would make more sense to me if it had to do with not reaching threshold but how is it a potential if it reaches threshold and completes a cycle? Why is it called an Action Potential if an action was accomplished... Does this make sense to anyone or am I just looking too deep? Thanks in advance for the info.

 

[DrYoda]

It's electrical potential.

 

[andrewnwi]

The word potential can be looked at two different ways. Basically, in an action potential it means it has the potential and it is expressing it. Look at it this way, you have a friend and you say he/she has potential at drawing. This can mean that they act on the potential they have, and draw, or they have the potential, but don't act on it.

 

[crazycooljoel]

Thats a good way to look at it... Thanks for the info. I was really scratchin my head on this one.

 

[Excelsius]

I'm wondering if there is someone who can help me understand this part. For action potentials, I don't get the "potential" part of it, it sounds misleading to me, at least the way I understand it. I understand the All or None principle of it, that if it begins to depolarize, that it doesn't create an action if threshold isn't met. With the Na+/K+ ion pump, If an action potential passes (or reaches) threshold, the depolarization phase will occur and then repolarize after it reachs it's peak, and return to a resting membrane potential. My question is where is the "potential" in all of this? It would make more sense to me if it had to do with not reaching threshold but how is it a potential if it reaches threshold and completes a cycle? Why is it called an Action Potential if an action was accomplished... Does this make sense to anyone or am I just looking too deep? Thanks in advance for the info.

I think you are looking at the word "potential" in a strictly grammatical sense. You can look at it as a force that particles must generate to move against the electric field. This is the same as gravitational potential. Now a membrane always has some potential, even at rest. That's called resting potential. Therefore, logically you need a different name to specify the potential related to conduction of impulses down an axon. This is why they call it an action potential. Also, while the immediate location of depolarization reaches threshold, the area right in front of the a.p. has just barely opened its Na channels to reach threshold (can take a millisecond or two). That means that even grammatically the word "potential" is correct since while the a.p. is digital, it's path along the axon is actually a wave meaning that each area around the crest of the wave has a graded level of activation and hence a "potential" to get activated or inactivated.

You need to understand physics to grasp the minutiae of neuroscience, but not everyone does this, not even some PIs.

 

[nick_carraway]

It's electrical potential.

Right. OP, you're overthinking this. It's called an action potential simply because an electrical potential (difference in charge) exists along the neuron.

 

[Brent8927]

It's called an action potential because the potential voltage (an electrical potential) is actively propagated down the length of the axon. This is to be contrasted with a passive potential.

It may help to remember the Nerst equation, which calculates the membrane potential, that is, the potential difference between the outside and inside of the cell membrane.

Basically, think back to physics where potential is synonymous with voltage and electrical potential. It's the voltage between two points

 

[Brent8927]

Looks like two people beat me...

Hopefully one of us explained things well enough for the term to make sense!

 

[crazycooljoel]

I think I understand it now, but I haven't taken physics yet (looking forward to it), but between y'alls explainations, it makes much more sense... Thanks again! Now I won't be thinking about it as i'm reading it in A&P.

 

[atomi]

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[atomi]

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[mmagnett]

its voltage.

maybe think about it similarly to how you think about an imbalance of salt in water divided by a semi-permeable (by salt not water) membrane. Thermodynamics wants the salt CONCENTRATIONS to be equal on each side -- so there is a potential for the water to move across the membrane accomplishing this goal.

A similar phenomena is happening inside nerve cells and actually many other cells too. By pumping positively charged ions outside of the cell, an electrical potential is created (i.e. there is a gradient between concentrations of positively charged ions outside of the cell to the inside of the cell -- making the cell negatively charged compared to the outside environment). The cell needs to be careful to not make the ION concentration (i.e. both the +/-) gradient too different otherwise the cell may either burst or whither away (by similar logic to the semipermeable membrane analogy).

When the cell is DEpolarized, the positvely charged ions that were pumped out of the cell rush in, making the cell appear to be more positvely charged compared with the surroundings. I beleive this happens at the nodes of ranvier (but I may be mistakin') which allows the signal to "jump" between nodes as the potential behind it changes. Remember, there is some refractory period too where the neuron cannot fire again -- I think becuase the Na/K pump needs a bit of time to escort Na/K outside of the cell again.

(that long explanation is a couple years old, so take it with a grain of salt.. but i think its mostly right)

 

[atomi]

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[Excelsius]

Not islets of langerhans. Those are in the pancreas and totally unrelated. You are thinking of nodes of ranvier. Both similar in that they have weird names.

Everybody should know this for the MCAT.

Here is how an action potential works. Sodium and Potassium channels open. Sodium flows into the cell. The inside of the cell becomes more positive than the outside and crosses a threshold at which the inrush of sodium dominates the outrush of potassium. This is called depolarization. The potential difference has now been reversed from about -50mV to about +45mV. The sodium channels close more quickly than the potassium channels, so the sodium stops coming in, but the potassium keeps going out. This is called repolarization (the potential difference is becoming more negative). The potassium channels still don't close and the potential drops below the cells resting potential. Now there is a of potassium outside the cell and a less sodium inside. This is called hyperpolarization. Then the Na+/k+ pump enzyme action occurs, which pumps 2 k+ ions in and 3 Na+ ions out, therefore depolarizing the cell just a little bit to get it back to the resting potential.

This potential is transmitted down along the axon, and is protected from loosing steam due to the myelin sheath. The nodes of ranvier are like signal boosters that start the thing all over again so it can keep on traveling down the axon without running out of steam. Eventually it branches out and may trigger a neurotransmitter release at a synaptic cleft, thereby enabling a muscle cell contraction.

This all occurs over a period of 5-10 ms.

A few notes here: First, the reason the action potential ends is because of V-gated Na channel inactivation (the loop between the domains III and IV blocks the pore until the membrane is repolarized). K channels are never inactivated. V-gated K channels only close once voltage reaches close to resting potential. Still, that is just one of at least four types of K channels, most of which are always open, regardless of voltage.

Second, hyperpolarization occurs when there is a MORE K inside and less outside, not the other way around.

Third, Na-K pump is not depolarizing. Quite the opposite - it is hyperpolarizing. It takes out +3 and gets in +2. The net is a -1.

 

[mmagnett]

Not islets of langerhans. Those are in the pancreas and totally unrelated. You are thinking of nodes of ranvier. Both similar in that they have weird names.

Everybody should know this for the MCAT.

Here is how an action potential works. Sodium and Potassium channels open. Sodium flows into the cell. The inside of the cell becomes more positive than the outside and crosses a threshold at which the inrush of sodium dominates the outrush of potassium. This is called depolarization. The potential difference has now been reversed from about -50mV to about +45mV. The sodium channels close more quickly than the potassium channels, so the sodium stops coming in, but the potassium keeps going out. This is called repolarization (the potential difference is becoming more negative). The potassium channels still don't close and the potential drops below the cells resting potential. Now there is a of potassium outside the cell and a less sodium inside. This is called hyperpolarization. Then the Na+/k+ pump enzyme action occurs, which pumps 2 k+ ions in and 3 Na+ ions out, therefore depolarizing the cell just a little bit to get it back to the resting potential.

This potential is transmitted down along the axon, and is protected from loosing steam due to the myelin sheath. The nodes of ranvier are like signal boosters that start the thing all over again so it can keep on traveling down the axon without running out of steam. Eventually it branches out and may trigger a neurotransmitter release at a synaptic cleft, thereby enabling a muscle cell contraction.

This all occurs over a period of 5-10 ms.

thnx for the correction

 

[bnleong]

I'm wondering if there is someone who can help me understand this part. For action potentials, I don't get the "potential" part of it, it sounds misleading to me, at least the way I understand it. I understand the All or None principle of it, that if it begins to depolarize, that it doesn't create an action if threshold isn't met. With the Na+/K+ ion pump, If an action potential passes (or reaches) threshold, the depolarization phase will occur and then repolarize after it reachs it's peak, and return to a resting membrane potential. My question is where is the "potential" in all of this? It would make more sense to me if it had to do with not reaching threshold but how is it a potential if it reaches threshold and completes a cycle? Why is it called an Action Potential if an action was accomplished... Does this make sense to anyone or am I just looking too deep? Thanks in advance for the info.

Actually when they say potential, they are referring to potential difference, or the voltage. During an action potential, there is a change in resting membrane potential. Typically, a cell at resting membrane potential ( -70mv) will go to something like +50mv. Thats an action potential.

I think a lot of basic biology courses tend to oversimplify things to a point where you get confused. Alot of things dont really make sense unless you have a good grasp of whats actually going on.

If your really interested in this stuff, I recommend reading up on resting membrane potentials. You also need to understand the goldman and nerst equations.

 

[Raryn]

Shouldn't this thread really be in the MCAT study question forum?

 

[Excelsius]

thnx for the correction

See my above post. Some of the things in the post you are referring to were incorrect. Also, an action potential lasts about 1-2ms. Good luck.

 

[Hyperstudyosis]

Potential difference= voltage
There is a voltage across the membrane.