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However, when that gate is opened, the sodium channel is now going to be incredibly selective and only let through sodium ions and no other type of ion.
但是，当闸门打开的时候，钠离子通道,将会有极高的选择性,不允许除钠离子以外的任何其它离子通过。
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And all ion channels are selective for a single type of ion, and we can think about how that selectivity takes place, and that's where this idea of atomic radius is going to become very important.
所有的离子通道都是仅对某一种离子具有选择性的，而我们可以来想一想这种选择性是如何发生的，这也就是原子半径这个概念将会变得，非常重要的地方。
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The ones that are most important in physiology are ones that only allow ions to go through: sodium, potassium, chloride, calcium, bicarbonate.
在生理学上这种,只能允许某种离子通过的通道十分重要,这些离子包括钠离子,钾离子,氯离子,钙离子和碳酸氢根离子
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And actually, a question that might come up, I just explained, the sodium channel, you might say, well, how do potassium channels work then, because I can understand how you can filter something big out, but how do you filter out something small.
实际上，可能大家会问一个问题，我刚解释过，钠离子通道，你可以说，好吧，那么钾离子通道该如何起作用呢，因为我可以理解你是如何把大东西过滤掉的，但是你如何把小东西过滤掉呢？
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We can already use that in terms of single ions to think about a really complex biological issue, which is to talk about ion channels.
我们已经能够仅仅凭借这个关于单个离子的原理,来讨论一个非常复杂的生物问题了，我们要讨论的就是离子通道。
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So, what happens, this is another view of a sodium channel, so this is actually looking a little bit more at the protein structure.
那么，这里发生了什么，这是钠离子通道的另一张图片，在这种蛋白质结构中,它看起来更复杂一点。
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In the presence of this ligand, this molecule, it's open, it allows transport of this ion, when the ligand is gone it doesn't.
配体存在的情况下通道才打开,允许这些离子穿过,配体离开后,这些离子就不能通过了
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Because if it was a neurotransmitter activated ion channel, what would happen when the neurotransmitter bound here?
因为假如这是一个,神经递质激活的离子通道,当神经递质结合于此的时候,会发生些什么呢
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What all of these channels have is what's called a selectivity filter, so this filter filters out the type of ion that's going to be allowed through.
这些离子通道都有的一部分,就是所谓的选择性过滤器，这个过滤器,会滤过允许穿过的离子。
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So, that is just a quick example for some of you, you might be very familiar with ion channels, others might not know what these are, so I'll just tell you quite briefly that ion channels are these very massive transmembrane proteins.
这对你们中的某些人来说仅仅是个小例子，有些人可能已经非常熟悉离子通道这个概念了，但有些人可能还不清楚，那么就让我来简单地介绍一下离子通道,就是这些巨大的跨膜蛋白。
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many of these ion channels have this size exclusion pore, it's a very important part of them.
很多离子通道也都有这样的具有尺寸排斥性的小孔，这是它们非常重要的组成部分。
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And when we talk about any type of ion channel, there are just tons of different kinds of ion channels, and you can characterize them in a few different ways.
而当我们讨论任何一种离子通道时，离子通道有非常多种，你可以用几种不同的方法来描述它们。
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It can exist in - a gated ion channel - can exist in one or two states.
它们能够存在于,门控离子通道有一种或者两种状态
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So the influx of ions from the outside of the cell to the inside of And you can think of ion channels as being gated, by gated it means the gate can be closed and no ions are going through, as in this case here.
下面以离子从细胞外部流入内部,你可以把离子通道想象成是有闸门的，也就是说闸门可以关上,这时没有离子能通过，就像这种情况。
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For these purposes think about a channel, it only allows sodium to go through for example.
基于此目的,我们来讲离子通道,拿仅允许钠离子通过的通道作为例子
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That change happens because something gets triggered in the membrane, and what gets triggered is a voltage gated sodium channel, which is shown here.
变化的原因是膜上某些东西被触发了,被触发的是钠离子电压门控通道,展示在这儿了
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That's shown functionally here but recovery means that this sodium channel becomes closed again.
在此展示了这项功能,恢复代表着钠离子通道再次关闭
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And that's all that's big enough to pass through or small enough to pass through. And if we go up even just one row on the periodic table to potassium, what we actually see is now that it's going to be too large, and, in fact, a potassium solvated with one water molecule won't go through our channel.
就是这样刚好大到它通过,或者说刚好小到它通过,即使我们再沿着周期表往下走仅仅一行到钾离子，我们看到的就将是它的个头太大了，实际上，溶解于一个水分子的单个钾离子,不能穿过我们的通道。
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When an action potential is initiated these ion channels go from their closed state to their open state, when they open sodium can now pass through.
当动作电位形成时,离子通道,从关闭状态转到打开状态,当通道打开时,钠离子就可以通过
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The one that's on the top here is called ligand-gated ion channel and an ion channel is a protein that sits in the surface of a cell.
最上面的这个叫作配体门控离子通道,离子通道是一类位于细胞表面的蛋白
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In some cases it might be a ligand-gated ion channel.
有时受体可能是一个配体门控离子通道
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The cells that take an electrical signal, which is coming down your nerve and convert it into a muscle contraction work this way, so 'neuro' nerve, muscle junctions act based on ligand-gated ion channels.
依靠相同的工作原理,细胞将来自于神经的,电信号传递到细胞收缩肌中,所以神经和肌肉关节运动,都基于配体门控离子通道
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There's a local change in the membrane, that local change involves opening of channels that allow ions to pass through regions where they couldn't pass through before, that results in a change in voltage.
膜上会发生局部变化,局部变化包括离子通道开启,使离子通过,先前不能通过的区域,从而引起电压的变化
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The balance of sodium movement relative to potassium movement changes because there's this resting movement of all these molecules anyway, but that balance changes dramatically when these ion - when these gated ion channels open.
钠钾各自运动始终保持动态的平衡,因为尽管在静息状态,所有这些离子也在运动,但是平衡的剧烈变化,只发生在这些门控离子通道打开的时侯
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Now, remember we talked about these last time, voltage-gated channels are channels that would allow the passage of sodium, in this case, but they can exist in two states, a closed state and an open state.
还记得我们上节课讨论过的吧,电压门控通道是,一种可以容许钠离子通过的通道,在这一例子中 电压门控通道有两种状态,关闭状态和开放状态
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Now, it's more complicated than that because it's not just sodium channels that are involved, there are potassium channels also, and the interplay between sodium channels opening and potassium channels opening, this is described in some detail in your book.
现在 事情更复杂了,因为这一过程不仅关系着钠离子通道,也关系着钾离子通道,以及钠离子通道和钾离子通道之间开关,的相互作用,书中有详细解释
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Now, sodium and potassium don't ordinarily move across membranes, they're charged molecules, they can't dissolve, they can't permeate through a cell membrane, but they go through because there are channels that allow them to pass through in the membrane.
钠和钾通常是不能跨过细胞膜的,它们都是带电粒子,它们不能溶解在细胞膜中,不能渗透通过细胞膜,但它们之所以能通过细胞膜,全赖膜上离子通道的作用
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It would generate an electrical signal because it would - you'd open the ion channel and you would ion fluxes and you would change the membrane potential in just the way I described for the action potential.
它会产生一个电信号,因为--,离子通道会打开,产生离子流,膜电位会改变,像我讲动作电位时那样
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So, for example, if we look at sodium channels, and sodium channels are some of the particularly prevalent ones when we're talking about neurons, if you think about the cell membrane, and this little green cartoon is me trying to show a sodium channel here, and in this case, you can see that it's closed, such that no ions are getting through.
比如，让我们来看一下钠离子通道，钠离子通道是我们会经常遇到的几个之一,在讨论神经元的时候，如果大家想想细胞膜，而这个绿色的小示意图就是我用来表示,钠离子通道的，在这种情况下，大家可以看到它是关闭的，因此没有离子可以通过。
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It can also happen in other ways, it could be a G-protein coupled receptor which we talked about last time, which indirectly activates another ion channel to start the electrical signal.
还可以有其他方式,可以通过G蛋白偶联受体,这我们上次谈过,它可以间接激活,另一个离子通道以开启电信号
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