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MCAT
Course: MCAT > Unit 7
Lesson 3: Neuron membrane potentials- Neuron membrane potentials questions
- Neuron membrane potentials questions 2
- Mini MCAT passage: Demyelinating disease and aging
- Mini MCAT passage: In vitro membrane potential studies
- Neuron graded potential description
- Neuron resting potential description
- Neuron resting potential mechanism
- Neuron graded potential mechanism
- Neuron action potential description
- Neuron action potential mechanism
- Sodium-potassium pump
- Effects of axon diameter and myelination
- Action potential patterns
- Neuron action potentials: The creation of a brain signal
- Action potential velocity
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Sodium-potassium pump
How a sodium potassium pump can maintain a voltage gradient across a cell or neuron's membrane.
The sodium-potassium pump goes through cycles of shape changes to help maintain a negative membrane potential. In each cycle, three sodium ions exit the cell, while two potassium ions enter the cell. These ions travel against the concentration gradient, so this process requires ATP. Created by Sal Khan.
The sodium-potassium pump goes through cycles of shape changes to help maintain a negative membrane potential. In each cycle, three sodium ions exit the cell, while two potassium ions enter the cell. These ions travel against the concentration gradient, so this process requires ATP. Created by Sal Khan.
Want to join the conversation?
- does the sodium potassaum pump use any energy if so how much does it recive any energy back(12 votes)
- I've read that 1/3 of the body's ATP is used to power Na/K pumps(4 votes)
- I am confused about what ATP is, it was said that it was mentioned in the previous videos on pulmonary and circulatory but i don't recall that. All this information pertaining to ATP is getting confusing. What is ATP?(10 votes)
- Here my friends! Another awesome video made by Sal explaining what ATP really is:
https://www.khanacademy.org/test-prep/mcat/biomolecules/overview-metabolism/v/atp
You also can find more videos about ATP here:
https://www.khanacademy.org/test-prep/mcat/biomolecules/overview-metabolism
Hope that helps! :)(12 votes)
- Ok, so why is ATP(adesine triphosphate) more energetic than TDP(adesine diphosphate) the only differens is that ATP har one more phosphate, and how does it get transported to the brain. I there capillaires inside the brain or what?
PS. sorry for not being on topic(8 votes)- Great question!
The answer is very straightforward because the energy comes from the unstable high-energy bonds between the phosphates. Therefore, with more phosphates, more high-energy bonds, more energy is released when those bonds are broken.
Blood glucose goes to the brain. Neurons depend on a constant supply of glucose for energyfrom which to make ATP!
There are many blood vessels of the brain, including the vertebrobasilar system, the anterior, middle, and posterior arteries, the choroidal arteries, and more. You can Google this if you're curious or I will happily provide more information.
There are also many veins draining deoxygenated (used) blood from the brain, including but not limited to the superior sagittal sinus, vein of Galen, internal cerebral vein, superior and inferior anastomotic veins, the basal vein of Rosenthal, and the anterior, middle, and posterior cerebral veins.
I voted up your question.
You are on topic - everyone is here to learn and to teach. Questions are more than welcome, they are encouraged.
Any further questions? I'd love to help!(6 votes)
- Well, why is even this necessary? Why suddenly would the Na-K pump start using energy to do work against concentration gradient? Isn't that against the fundamental rule that things want to be in a more stable state??
Also, why is it mandatory of the outside of a cell to be more positive?(5 votes)- This is like charging a battery — once the battery is charged we can use it to do work.
Yes, things want to be in a more stable state, but living organisms harnesses that tendency and use it to drive their metabolisms. While this results in a decrease in entropy within the organism, the environment has an even larger increase in entropy — remember the laws of entropy only state that the total entropy must increase.
You may find this article helpful:
https://www.khanacademy.org/science/biology/energy-and-enzymes/the-laws-of-thermodynamics/a/the-laws-of-thermodynamics(5 votes)
- Atsal says "some of this sodium gets shoved through. Would that release energy? Common sense says that would give of some energy. 13:45(5 votes)
- Most of your cells use an inward gradient for Na+ to power lots of secondary active transport. Because the Na+ wants to flow down its gradient and into the cell, the energy provided by the inward electrochemical gradient can be used to transport larger molecules like sugars and amino acids.
If the sodium ions enter the cell through 'leak channels', as Sal describes, there is some energy released but it isn't used by anything. Instead it reduced the electrochemical potential across the membrane, ensuring the cell can function normally.(3 votes)
- Is there another video on YouTube that could explain this to a 6th grader? Sal does a good job explaining this concept, no doubt about it, but I still don't quite understand it. Does anybody want to share a good video that I could watch on this?(2 votes)
- I watched this video last year too, in Grade 6, actually at this time of year. What I did was watched every video 3-5 times, made sure I thoroughly understood them, and then moved. Also, I took notes on the videos just in case I forgot something, I wouldn't have to go back and watch the video again, I would just look at my noted (which I still do now).
Hope this helped! :D(5 votes)
- Are neurons all over the body or just in the brain?(4 votes)
- The previous answer is incomplete.
Neurons are all over your body, letting you use all of your senses plus helping you react with every single part fo your body. (not just having afferent but efferent pathways are as important!) We have also motor neurons, not just sensory neurons.
There are parts of the body where you have relatively less concentrated neurons or thicker skin (such as the back) and those parts that are hypersensitive (such as lips).(2 votes)
- Is there an evolutionary hypothesis as to why we evolved to have a gradient with the inside of the cell being more negative rather than reversing the pump and using a more positive gradient? Nature would rush to equalise it either way.(3 votes)
- I don't know if there's a hypothesis for why the specific ions are involved the way they are, except "it worked so nature kept it and found new ways to use it".
On a more practical level, the cell has a negative resting potential because that's how we named charges and currents before we understood how they work. There's nothing intrinsically 'negative' about the charge gradient, it's just what humans called it and it would be much too confusing to change the name now.(3 votes)
- So, basically what are negative charges?
And why do they have to escape ?? why don't the positive charge repel ?
How can a +ve charge move to an area where there is more of it ? Why can't the +ve charge get binded with -ve charge ?(3 votes)- Because of the selective permeability of the cell.
Forst, the cell is generated negative charge thanks to potassium ions which leave the cell leaving a negative charge balance inside the cell.
When the neuronal membrane is at rest, the resting potential is negative due to the accumulation of more sodium ions outside the cell than potassium ions inside the cell.
And for an action potential to get generated, first depolarization must occur. For that to happen, the negative charge is required.
Hypothetically, it could have been a positive charge, but then, other ions would have played a critical role.(1 vote)
- How fast is this process? How many of these pumps happen in one second?(2 votes)
- It's difficult to quantify exactly for a single pump, because the pump and the ions are too small to directly observe as they work and because they depend on optimal conditions to work at their fastest rate. The best estimate I could find is about 135 times per second for a single pump, based on ATP usage. One paper I read estimates that all of the sodium-potassium pumps from all of your muscle cells, working together under ideal conditions, could clear all of the potassium in your body in about 25 seconds.(3 votes)
Video transcript
In the last video, I showed you
what a neuron looked like and we talked about the
different parts of a neuron, and I gave you the general
idea what a neuron does. It gets stimulated at the
dendrites-- and the stimulation we'll talk about
in future videos on what exactly that means-- and
that that impulse, that information, that signal
gets added up. If there's multiple stimulation
points on various dendrites, it gets added up and
if it meets some threshold level, it's going to create
this action potential or signal that travels across the
axon and maybe stimulates other neurons or muscles because
these terminal points of the axons might be connected
to dendrites of other neurons or to muscle
cells or who knows what. But what I want to do in this
video is kind of lay the building blocks for exactly
what this signal is or how does a neuron actually transmit
this information across the axon-- or really,
how does it go from the dendrite all the way
to the axon? Before I actually even talk
about that, we need to kind of lay the ground rules-- or a
ground understanding of the actual voltage potential
across the membrane of a neuron. And, actually, all cells have
some voltage potential difference, but it's especially
relevant when we talk about a neuron and its
ability to send signals. Let's zoom in on a
neuron's cell. I could zoom in on any point
on this cell that's not covered by a myelin sheath. I'm going to zoom in
on its membrane. So let's say that this is
the membrane of the neuron, just like that. That's the membrane. This is outside the neuron
or the cell. And then this is inside the
neuron or the cell. Now, you have sodium
and potassium ions floating around. I'm going to draw sodium
like this. Sodium's going to be a circle. So that's sodium and their
positively charged ions have a plus one charge and then
potassium, I'll draw them as little triangles. So let's say that's potassium--
symbol for potassium is K. It's also positively charged. And you have them just
lying around. Let's say we start off
both inside and outside of the cell. They're all positively
charged. Sodium inside, some
sodium outside. Now it turns out that cells
have more positive charge outside of their membranes than inside of their membranes. So there's actually a potential
difference that if the membrane wasn't there,
negative charges would want to escape or positive charges
or positive ions would want to get in. The outside ends up being more
positive, and we're going to talk about why. So this is an electrical
potential gradient, right? If this is less positive than
that-- if I have a positive charge here, it's going to
want to go to the less positive side. It's going to want to
go away from the other positive charges. It's repelled by the other
positive charges. Likewise, if I had a negative
charge here, it'd want to go the other side-- or a positive
charge, I guess, would be happier being here
than over here. But the question is, how
does that happen? Because left to their own
devices, the charges would disperse so you wouldn't have
this potential gradient. Somehow we have to put energy
into the system in order to produce this state where we
have more positive on the charge of the outside than
we do on the inside. And that's done by sodium
potassium pumps. I'm going to draw then
a certain way. This is obviously not how the
protein actually looks, but it'll give you a sense of how it
actually pumps things out. I'll draw that side
of the protein. Maybe it looks like this and
you'll have a sense of why I drew it like this. So that side of the protein or
the enzyme-- and then the other side, I'll draw
it like this. It looks something like this,
and of course the real protein doesn't look like this. You've seen me show you what
proteins really look like. They look like big clusters
of things, hugely complex. Different parts of the proteins
can bond to different things and when things bond to
proteins, they change shape. But I'm doing a very simple
diagram here and what I want to show you is, this is our
sodium potassium pump in its inactivated state. And what happens in this
situation is that we have these nice places where our
sodium can bind to. So in this situation, sodium can
bind to these locations on our enzyme or on our protein. And if we just had the sodiums
bind and we didn't have any energy going into the system,
nothing would happen. It would just stay in
this situation. The actual protein might look
like something crazy. The actual protein might be this
big cloud of protein and then your sodiums bond there,
there, and there. Maybe it's inside the protein
somehow, but still, nothing's going to happen just when the
sodium bonds on this side of the protein. In order for it to do anything,
in order for it to pump anything out, it uses
the energy from ATP. So we had all those videos on
respiration and I told you that ATP was the currency of
energy in the cell-- well, this is something useful
for ATP to do. ATP-- that's adenosine
triphosphate-- it might go to some other part of our enzyme,
but in this diagram maybe it goes to this part
of the enzyme. And this enzyme, it's
a type of ATPase. When I say ATPase, it breaks off
a phosphate from the ATP-- and that's just by virtue
of its shape. It's able to plunk it off. When it plunks off the
phosphate, it changes shape. So step one, we have sodium
ions-- and actually, let's keep count of them. We have three sodium-- these are
the actual ratios-- three sodium ions from inside the
cell or the neuron. They bond to pump, which is
really a protein that crosses our membrane. Now, step two, we
have also ATP. ATP gets broken into ADP plus
phosphate on the actual protein and that changes
the shape. So that also provides energy
to change pump's shape. Now this is when the
pump was before. Now after, our pump might look
something like this. Let me clear out some
space right here. I'll draw the after
pump right there. And so this is before. After the phosphate gets split
off of the ATP, it might look something like this. Instead of being in that
configuration, it opens in the other direction. So now it might look something
like this. And of course it's carrying
these phosphate groups. They have a positive charge. It's open like this. This side now looks like this. So now the phosphates are
released to the outside. So they've been pumped
to the outside. Remember, this is required
energy because it's going against the natural gradient. You're taking positive charge
and you're pushing them to an environment that is even more
positive and you're also taking it to an environment
where there's already a lot of sodium, and you're putting
more sodium there. So you're going against the
charge gradient and you're going against the
sodium gradient. But now-- I guess we call it
step three-- the sodium gets released outside the cell. And when this changes shape,
it's not so good at bonding with the sodium anymore. So maybe these can become a
little bit different too, so that the sodium can't even bond
in this configuration now that the protein has changed
shape due to the ATP. So step three, the three Na
plusses, sodium ions-- are released outside. Now once it's in this
configuration, we have all these positive ions out here. These positive ions want to get
really as far away from each other as possible. They'd actually probably be
attracted to the cell itself because the cell is less
positive on the inside. So these positive ions-- and in
particular, the potassium-- can bond this side of the
protein when it's in this-- I guess we could call it this
activated configuration. So now, I guess we could
call it step four. We have two sodium ions bond
to-- I guess we could call it the activated pump--
or changed pump. Or maybe we could say it's
in its open form. So they come here and when they
bond, it re-changes the shape of this protein back
to this shape, back to that open shape. Now when it goes back to the
open shape, these guys aren't here anymore, but we have these
two guys sitting here and in this shape right here,
all of a sudden these divots-- maybe they're not divots. They're actually things in this
big cluster of protein. They're not as good at staying
bonded or holding onto these sodiums so these sodiums get
released into the cell. So step five, the pump-- this
changes shape of pump. So pump changes shape
to original. And then once we're in the
original, those two sodium ions released inside the cell. We're going to see in the next
few videos why it's useful to have those sodium ions
on the inside. You might say, well, why don't
we just keep pumping things on the outside in order to have
a potential difference? But we'll see these
sodium ions are actually also very useful. So what's the net effect
that's going on? We end up with a lot more sodium
ions on the outside and we end up with more potassium
ions on the inside, but I told you that the inside is less
positive than the outside. But these are both positive. I don't care if I have more
potassium or sodium, but if you paid attention to the ratios
I talked about, every time we use an ATP, we're
pumping out three sodiums and we're only pumping in two
potassiums, right? We pumped out three sodiums and
two potassiums. Each of them have a plus-1 charge, but
every time we do this, we're adding a net-1 charge to
the outside, right? 3 on the outside,
2 to the inside. We have a net-1 charge-- we have
a plus-1 to the outside. So we're making the outside
more positive, especially relative to the inside. And this is what creates that
potential difference. If you actually took a
voltmeter-- a voltmeter measures electrical potential
difference-- and you took the voltage difference between that
point and this point-- or more specifically, between this
point and that point, if you were to subtract the voltage
here from the voltage there, you will get -70
millivolts, which is generally considered the resting voltage
difference, the potential difference across the membrane
of a neuron when it's in its resting state. So in this video, I kind of laid
out the foundation of why and how a cell using ATP,
using energy, is able to maintain a potential difference
across its membrane where the outside is slightly
more positive than the inside. So we actually have a negative
potential difference if we're comparing the inside
to the outside. Positive charge would want to
move in if they were allowed to, and negative charge would
want to move out if it was allowed to. Now there might be one
last question. You might say, well, if we just
kept adding charge out here, our voltage difference
would get really negative. This would be much more negative
than the outside. Why does it stabilize at -70? To answer that question-- these
are going to come into play in a lot more detail in
future videos-- you also have channels, which are really
protein structures that in their open position will allow
sodium to go through them. And there are also channels
that are in their open position, would allow potassium
to go through them. I'm drawing it in their
closed position. And we're going to talk in the
next video about what happens when they open. But in their closed position,
they're still a little bit leaky. And if, say, the concentration
of potassium becomes too high down here-- and too high meaning
when they start to reach this threshold of -70
millivolts-- or even better, when the sodium gets too high
out there, a few of them will start to leak down. When the concentration gets
really high and this is really positive just because of the
electrical potential, some of them will just be
shoved through. So it'll keep us right around
-70 millivolts. And if we go below, maybe some
of the potassium gets leaked through the other way. So even though when these are
shut-- if it becomes too ridiculous-- if it goes to -80
millivolts or -90 millivolts, all of a sudden, there'd be a
huge incentive for some of this stuff to leak through their
respective channels. So that's what allows
us to stay at that stable voltage potential. In the next video, we're going
to see what happens to this voltage potential when the
neuron is actually stimulated.