

Biology: Human Regulation: Nerve Impulse



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About this Lesson

  • Type: Video Tutorial
  • Length: 9:23
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 101 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: The Nerve Impulse (6 lessons, $11.88)



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We've already talked about - you guys know all about neurons, but the real question is, how do they work? How does a cell - I mean, we're talking cells here. We're talking something with this very strange shape, and we've already established the fact that it's a very bizarre looking cell and many of them have this myelin sheath. How does this cell somehow pick up a stimulus? We won't even worry about it picks up the stimulus - Holy Mackerel. But how does the cell pick up a stimulus and somehow convert that stimulus into an impulse, send the impulse down here - almost like I'm talking about an electrical connection - and then send it through here? Then, as I've said, somehow in a synapse - which is where there's a space between these neurons, remember - it gets it to another neuron. How does that happen? It sure sounds an awful lot like electricity because we're talking about something being conveyed. It could be chemical or it could be a combination of both. We know that in cells, we have cytoplasm and we know that in cytoplasm, we have water, and we know that in water, we have ions dissolved, and we know that ions are charged atoms. We've certainly seen that ions have raised their heads before in biological processes. We know that hydrogen ions have been unbelievably important in setting up mitochondrial gradients, so that we can make a battery. We've seen the importance of hydrogen ions and chloroplast gradients, chemosmosis in both chloroplasts and mitochondria. Could this be an ion phenomena? And, if so, how might it work? Because if you're going to have electricity, you have to have something called voltage.
What is voltage? Some of you who have taken physics know all about voltage. Voltage is defined as a potential difference, a difference in potential. I like to put that in kind of a vernacular and say it's a difference of potential energy. So, I like to picture voltage as a place where there's a lot of one charge, compared to a place where there's a little charge, and that charge could be - we generally say negative to positive by convention, but the bottom line is there's a difference of potential. Just like if I say, right now, with this pen sitting here, there is a potential gravitational energy, gravitational potential energy and when I put some energy into it and I lift it, I have now added potential to it. So, in essence there's a gravitational voltage here. I just made that up, but you get what I mean. There's a gravitational difference in potential energy which is released when I drop it. So, could I be setting up a voltage inside of this neuron? And if I said, "No," you'd say, "Man, you just wasted like five of my minutes," so I'm going to say, "Yes." And, indeed, there is a voltage inside of neurons. I want to talk to you about how that is established. And then, later on, we'll talk about how it works.
So, what I want to do I want to take a look at a neuron and I want to look randomly - we'll put it all together later. I want to look randomly how a voltage might be established and where a voltage might be established. So, we're going to take this neuron and just take a little chunk of it and enlarge it right here. And what we're going to do is we're going to take - we can do this, we can take small little volt meters - and measure the millivoltage, thousandths of a volt across a membrane. You take one of the prongs and you put it in the inside of the membrane - obviously it's got to be a pretty small prong. Then you put one of the prongs on the outside of the cell membrane and you measure the voltage. What we're going to find out is that, indeed, there is a voltage across the membranes of most membranes. By convention, we're going to say it's negative, because remember, it's relative. I mean it's negative inside as you'll see in a second and positive outside. But relatively speaking, we're going to say that it's negative seventy millivolts. That's a range. It could be forty to eighty to a hundred, but generally, typically, it's minus seventy millivolts. That being said, how is this voltage established. Well, it turns out that if I were to take this hunk of neuron and look inside of it, I would find ions. And I would find some things about those ions that are going to set us up for learning about a nerve impulse works.
So, here we have a resting neuron, a neuron just sitting there. And it has this minus seventy millivolts. What's establishing this? Well, it's got to be positive outside and negative inside. Ions. On the outside, there's a large amount of sodium. Sodium ions, lots of them. And at any given time, the sodium is not real diffusible to the inside, but it can leak through a little bit. The membrane is relatively impermeable to sodium. I'll show you. We'll put a sodium in there, but there's a whole lot more sodium outside than inside. There's also potassium inside. And the membrane is permeable to potassium, so you can get some potassium outside, too. So, that's all positive. Where's the negative? Well, there is a little bit of chlorine in here. And ACL, sound familiar? But mostly, it's because there are these very large stable non-movable proteins that have negative charges inside of here. So, really, the major part of the negative charge is established by these proteins. Now, this is established - and you may want to link back to our lecture on active transport - but this is established by a sodium potassium ATP pump. And without going into a lot of detail because it's a simple link to see that, basically what the sodium potassium ATP pump does is it's constantly pumping and maintaining sodium to the outside and, when it pumps the sodium outside, it then kicks around and pumps potassium back inside.
Now why does it do that? You'll see that its very important for the sodium to be on the outside in greater concentration than it is on the inside. We want to establish more potassium on the inside than on the outside. Since it's free to diffuse potassium, we want to make sure the potassium constantly gets put back in here. So, we continue this gradient with the sodium potassium ATP pump.
But I want to talk about what happens when this thing is at rest. Negative seventy millivolts is called its resting potential. What you can do is you can mess up that resting potential very easily by starting what is called a depolarization. A depolarization begins in the sensory portion of the neuron. What can start a depolarization? Obviously, there must be some kind of cell surface receptors in these and obviously they must be responsive to things. For example, there might be a photoreceptor. Where would a photoreceptor be? In your eye. You're not going to have photoreceptors in your skin. It may be a pressure receptor. It could be any kind of receptor at all. But the bottom line is, when you start this depolarization, we get what is called an action potential. And the action potential is going to be a depolarization that starts at one end and goes to that end. Or, in other words, starts here and by sequential depolarization, moves this thing down here. That's how the nerve impulse works. We'll see the details of this at another time.
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