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Biology: Human Nervous System and Action Potential


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About this Lesson

  • Type: Video Tutorial
  • Length: 11:17
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 122 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Animal Systems and Homeostasis (63 lessons, $84.15)
Biology: The Nerve Impulse (6 lessons, $11.88)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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Let's establish what we have to get accomplished here. We have to send an impulse down a nerve cell. We have to take this very strange look cell that you know all about and we somehow have to get this impulse and move it in this direction. And what we've seen is that this happens apparently in some kind of electrochemical series of changes. Electrochemical, meaning something to do with ions, like a battery, and therefore, conducting electricity. So we're going to have an electrochemical movement, impulse. How are we going to do that?
To really understand that, let's take a look at a small section of the neuron and see what it looks like beforehand. Now you remember that the neuron has literally a membrane that is permeable to some ions and impermeable to others, but what I haven't told you about is that it also has protein channels or gates. You know about cells and you know that cells have proteins embedded in the membranes and if you've been paying attention to this course, you know that these membrane proteins are literally the secret to live. Without these proteins embedded in the cell, we couldn't have intercellular transport and therefore, things couldn't get in and out of cells because cell membranes are made out of lipids. Lipids are pretty impermeable things except to things like other lipids. But there are gates. There are protein gates. I want to show you two gates right now that in the resting stage of the neuron look something like this. This is the resting phase. This is when, you're like - think of a muscle right now that is not contracting, its nerve is in the resting phase. So now it's at rest. Let's see what these gates look like. These gates, which are called voltage-gated ion channels look like this at rest. The sodium gate seems to have like swinging door, two of them; one on the exterior of the cell surface, one on the interior. You'll notice that this one at the bottom or on the interior is open but that's okay, sodium can't get in here because sodium is being prevented from pouring through and this would pour through. We're not talking diffusion through a membrane anymore. We're talking like a gate. So this sodium could like rush through if that were only open. On the other hand, there is another potassium-gate where ions can go shooting through if this gate were only open. But it's not. So the two gates are closed in the resting neuron. Then we get to something, which is called threshold. Now, threshold is the minimum - the thing about nerve impulses is that they are all or none. In other words, you can't just like kind of open the gate and let a few little ions through. These gates either open or they shut, and threshold, the stimulus that causes this, opens them all the way or shuts them all the way. So one particular neuron is not going to have any kind of variety in how it conducts an impulse. Really, the strength of your sensing relates to how many impulses are coming up. How many neurons are being established. Or, how many depolarizations are being established. Now in one neuron. So, this all or none - one neuron, all or none.
So what's the first thing that happens? The first thing that is going to happen is as follows. We hit threshold and the sodium gate is open. What does that cause? What's going to happen? Sodium rushes in. Now you have your two little volt meters in there, your millivolt meters. You think about this. What is going to happen when this sodium rushes in there? Let's take a look. When the sodium rushes in there, take a look at what happens to the voltage. We're going along at minus seventy millivolts and along comes the stimulus and, Bam, we get a depolarization that happens - I'll show you how fast - unbelievably quickly. Measured in like milliseconds. I'm not talking like hundredths of milliseconds. Trust me. So, Boom, in rushes the sodium and look at what happens to my potential across the membrane. It immediately becomes up to like plus forty. So in other words, you have just established a much more positive interior.
Now, just let me diverge for a second. Remember my neuron? Remember that I said, here's my axon? Remember that I said that I want to look right here? Well, what about over here? And what about over here? And what about over here? Well, remember when I said it was a wave of depolarization? Well, guess what? We'll talk about sectioning this thing later. But this depolarization is going to trigger this depolarization, which is going to trigger this depolarization, which is going to trigger that depolarization. So, it's going to be a wave of depolarization, right across the neuron. I'll come back to that. I knew you were thinking that, so I had to answer that question.,
So the sodium has rushed in, and as soon as it rushes in, within milliseconds now, the gate starts to close. Let's see what's going to happen here. This gate, the deactivation gate, is going to start to close. So, after the sodiums rush in, this gate swings shut, stopping the depolarization. But wait. How are you going to repolarize it? You have to repolarize it. Why? Because you're going to want to send another impulse down that thing within a few milliseconds probably. You don't want to tie your neuron up for the rest of the day. So you want to somehow repolarize that. So, once the depolarization happens, we have to get to repolarization. So, this inactivation gate, which started to close, and almost immediately upon the depolarization, is a little bit slower than this one. So finally it gets up there. Now, the sodium can't get in. Meanwhile over in potassium-land, as soon as that threshold was reached, there was a gate that started to open then, too, and it was opening real slowly, and so by the time this gate is closed, this gate is open and what can start to happen? Now, remember it is specific to potassium. Now the potassium, this activation gate for potassium, allows the potassium to start going out of the cell.
Now, even though you're saying to me, "Now wait a minute, we're really messing our ions up here", who cares? We're reestablishing the charge. I don't care how you get it, just give me negative inside and give me positive outside. We'll worry about balancing it back to the resting stage later. But let's get the potassiums out there. And so, sure enough, what happens next is that as the potassium comes out, we get down back to minus seventy. Have our artists messed up? No, because at this point, by the time that potassium gate finally gets shut back down, we actually get what is called a hyperpolarization. It actually goes below the resting phase. But then, no problem, because the potassium gate finally shuts down and is going to close this off, and then we go into what is called the refractory period where the original balance is restored. You know how that is restored? Do you remember how it is restored? The sodium-potassium ATP pump will restore that. So you're saying, "Well, wait a minute, this took so long, how can this be?"
How long did it take? From the beginning of depolarization to the end of hyperpolarization takes about five to seven thousandths of a second. That's pretty quick. And one last thing - where does this myelin sheath fit into all of this? And here's the coolest thing of all. Myelinated neurons, if you remember, go a little bit faster than nonmyelinated neurons, so it's obviously an adaptation for speed. Here's why. Remember those waves of depolarization? And I said, this chunk, this chunk, this chunk. Well, those chunks that I showed you would have been really teeny tiny pieces in here. But what a myelinated fiber allows you to do is the following. You get a depolarization here, and this triggers a depolarization here, which triggers a depolarization there. So, instead of a wave of depolarization, you get what is called a saltatory propagation. Saltatory, Latin, to jump. It's a jumping propagation, so it's boom, boom, boom, boom. It moves even faster than a regular one. Whoever would have thought that something like nerves can move so quickly and yet, be so explainable? Milliseconds. Muscles. Wait until you see muscles. They do the same thing. Very cool.
Animal Systems and Homeostasis
The Nerve Impulse
Human Regulation: The Nervous System and the Action Potential Page [1 of 2]

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