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  • In this video, I want to describe

  • the neuron resting membrane potential, which we often

  • just call the resting potential for short.

  • So first, let me just draw a neuron that'll

  • be a little distorted, just so I have room to draw.

  • So we'll draw this soma here and a really big axon coming out

  • of the soma-- and normally an axon

  • is a thin, long process coming out of the soma,

  • but I just need a little room to draw.

  • So I'll draw a big, thick one.

  • And this will be the other part of the soma, or the cell body.

  • And then I'll just draw one really big dendrite.

  • And like the axon, of course, these

  • are normally just these little thin processes coming out

  • of the soma.

  • But I just need some space.

  • So most neurons at rest, meaning when they're not

  • receiving any input, have a stable separation

  • of charges across the cell membrane called

  • the resting potential.

  • And that consists of more positive charges in a layer

  • on the outside of the membrane, and more negative charges

  • in the layer along the inside of the membrane.

  • And these charges are ions.

  • So the negatively charged ions that

  • are in a layer along the inside of the membrane we also

  • call anions.

  • And the positively charged ions in a layer

  • on the outside of the membrane we call cations.

  • And this layer of anions on the inside

  • and cations on the outside goes all over the neuron cell

  • membrane.

  • All through the membrane of the dendrite,

  • and the soma, and all along the membrane of the axon.

  • And just to be clear, there is a mix

  • of anions and cations on both sides of the membrane.

  • And I've just drawn plus signs on the outside of the membrane

  • to represent that in the layer against the outside

  • of the membrane, there are more cations and anions.

  • And I have drawn negative signs on the inside of the membrane

  • to represent that in that layer, there

  • are more anions than cations.

  • And talk about the size of the difference in the separation

  • of charges, the convention is to call the outside zero.

  • So we just say the outside is zero,

  • and we just kind of set that as the reference.

  • And then we just refer to a single number

  • on the inside of the membrane, which

  • is the difference between the voltage on the outside

  • and the inside, or the difference in the strength

  • of the charge separation.

  • And this difference can vary between neurons,

  • but around negative 60 millivolts

  • would be a really common resting potential for a neuron.

  • So I'll just write a little m and a big V for millivolts.

  • That's the value we use to quantify

  • this difference in charge separation.

  • And around negative 60 would be a really common

  • resting membrane potential for a neuron.

  • The resting potential of neurons is

  • related to concentration differences, which are also

  • called gradients, of many ions across the cell membrane.

  • So there's lots of different ions

  • that have high concentrations outside the neuron compared

  • to lower concentrations inside the neuron, or vice versa.

  • But a few of these ions are the most important

  • for neuron function.

  • The cations, or the positive charged ions that are most

  • important for neuron function are potassium--

  • and I'll just write that as a K+, sodium,

  • which I'll write as an Na+, and calcium,

  • which I'll write as a Ca2+.

  • Because each calcium ion has two positive charges.

  • And the most important anions for neuron function,

  • or negatively charged ions, are chloride,

  • which I'll write as Cl-, and then there are multiple organic

  • anions.

  • And so I'll just write OA- to stand for organic anions.

  • And there a bunch of different organic

  • anions inside neurons and other cells.

  • Most of these are proteins that carry a net negative charge.

  • Now, these five kinds of ions are

  • going to have concentration differences across the cell

  • membranes, which we also call concentration gradients.

  • And it's different for the different ions

  • if they have a higher concentration

  • inside or outside the neuron.

  • The organic anions and the potassium ions

  • have a higher concentration inside the neuron than outside.

  • So I'll just represent that by having these letters written

  • large inside the neuron.

  • And then I'll write a small OA- to show

  • that there's a smaller concentration of organic anions

  • outside the neuron than inside.

  • And the same for potassium.

  • I'll write a small K+ outside the neuron compared to a large

  • K+ inside, because the concentration of potassium is

  • higher inside the neuron that outside the neuron.

  • And the opposite is true for these other three ions.

  • So the concentration of sodium is much higher

  • outside the neuron than inside the neuron,

  • as is the concentration of calcium.

  • There's much more calcium outside the neuron than inside.

  • And the concentration of chloride ions

  • is also much higher outside the neuron than inside the neuron.

  • Each of these ions, therefore, is

  • going to be acted on by two forces that

  • try to drive them into or out of the neuron.

  • The first is an electrical force from the membrane potential.

  • Because each ion will be attracted

  • to the side of the membrane with the opposite charge,

  • opposite charges attract each other and like charges

  • repel each other.

  • So if we look at each of these ions in turn,

  • the organic anions are negatively charged,

  • so they will be attracted to the outside of the neuron

  • where there are more positive charges.

  • So the electrical force acting on the organic anions

  • will try to drive them out of the neuron.

  • Potassium is the opposite.

  • It's positively charged.

  • So it will be attracted to the inside of the membrane

  • where it's more negative.

  • So it's electrical force will try

  • to drive it into the neuron.

  • Sodium is the same as potassium.

  • It's positively charged, so it will

  • be attracted to the more negative inside of the neuron.

  • Chloride is an anion like the organic anions,

  • so its electrical force will try to drive it out of the neuron.

  • Calcium is a cation like potassium and sodium,

  • so it's electrical force will also

  • try to drive it into the neuron.

  • But now the second force acting on these ions

  • can be thought of as a diffusion force,

  • or it's often called a chemical force, related

  • to the concentration gradients across the neuron membrane.

  • Because particles in solution will always

  • try to move from an area of higher concentration

  • to an area of lower concentration.

  • So if we look at the organic anions,

  • they're in a higher concentration

  • inside the neuron than outside.

  • So their diffusion force will be trying to drive them out

  • of the neuron, just like their electrical force is.

  • Now, potassium is a little confused.

  • Its electrical force is trying to drive it into the neuron,

  • but it has a higher concentration

  • inside the neuron.

  • So it's diffusion force is actually

  • trying to drive it out of the neuron.

  • Sodium has matched electrical and diffusion forces,

  • because it has a higher concentration

  • outside the neuron than inside.

  • Chloride's electrical force is trying

  • to drive it out of the neuron.

  • But because it has a higher concentration

  • outside the neuron, it's diffusion force

  • will be trying to drive it into the neuron.

  • And calcium is just like sodium.

  • Both its electrical and its diffusion force

  • are trying to drive calcium into the neuron.

  • These forces we often call electrochemical driving forces

  • for short.

  • And neurons are going to use these forces to perform

  • their functions.

  • But before we talk about that, in the next video,

  • let's talk about how the resting membrane potential is created

  • and how it's related to the concentration

  • differences of some of these key ions.

In this video, I want to describe

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B2 中上級

神経細胞の休息電位の記述 (Neuron resting potential description)

  • 117 15
    Alvin He に公開 2021 年 01 月 14 日
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