Action Potential: Propagation and Myelin

00:00 An Action Potential.

00:03 This is what our goal was remember? To reach threshold so we have an action potential.

00:09 Once you have an action potential, there are couple other properties we need to discuss.

00:15 To do that, let’s bring in resting membrane potential, which on our example here will be -70 millivolt.

00:22 How this graph is set up is that you’re gonna have time on the X-axis and you’re gonna have voltage or conductance on the Y.

00:32 And we have two different variables because action potentials and membrane potentials are in voltages.

00:38 And the various ions that travel through are measured in conductance.

00:42 So let’s look at what will happens first.

00:44 This is our traditional action potential.

00:48 Where you have an increase that occurs initially. And then, you have a decrease that happens to bring it down towards resting membrane potential a little bit below and then finally it come back up to normal resting potential.

01:06 Sodium conductance is the main contributor to that initial spot of increasing in membrane potential or depolarizing.

01:16 So it’s opening up sodium channels that cause this effect.

01:21 What brings down the action potential? Is the potassium conductance.

01:29 Why are they travelling in the directions they are? Because the membrane potential will want to go towards the equilibrium potential of which ever ion we’re looking at.

01:41 So if we open up sodium channels, it wants to travel towards to the sodium equilibrium potential.

01:48 If we open up potassium channels, it wants to travel towards the potassium equilibrium potential.

01:55 Besides equilibrium potentials, there’s one other thing we need to discuss.

01:59 And that is refractory periods.

02:03 Now refractory periods can either be absolute or relative.

02:08 And what is a refractory period? It is the time at which you can’t start another action potential yet.

02:16 If you’re in absolute refractory period, there’s no way to get another action potential.

02:22 No matter how hard you tried. No matter how many EPSPs you gave.

02:27 You will not get another action potential.

02:30 During the relative refractory period, it is reduced.

02:36 Meaning you’re at the hyperpolarized state.

02:39 So it’s going to take more effort to be able to reach threshold.

02:42 So you would need a greater number of EPSPs than normal to try to reach threshold.

02:49 And that is the difference between absolute and a relative refractory period.

02:57 Now, how do you get an action potential that started off at the soma or at the axon hillock all the way down to the next axon terminal? This is a propagation component.

03:14 So once you reach the axon hillock here, you were going to have to jump it down the axon till you reach the next axon terminal.

03:24 And it’s these action potentials that are going to be moving down that axon.

03:29 How does that process work? So how you get those axons to propagate? You need to utilize saltatory conduction.

03:39 Now, saltatory conduction allows for a speedy travel down the axon.

03:45 So what is the slow way down the axon? That is you would do an action potential.

03:50 action potential, action potential, action potential, action potential, action potential all the way down to the end.

03:55 I mean, I can hardly say that many action potentials.

03:58 The other way to do it, is have the action potentials jump.

04:02 And that is what salutatory conduction is.

04:06 The jumping of an action potential from one node of Ranvier to another.

04:12 These nodes are the spots that you can see here that have a high sodium channel density.

04:20 So when the sodium channels open in one node of Ranvier, they can then transmitted all the way to the next.

04:28 This is an important process of not needing to do as many action potentials down the axon.

04:35 So this is a fast propagation.

04:39 You notice that there are a lot of space in between each of these sodium channels.

04:47 That space in this particular diagram looks blue.

04:51 In fact, it is another cell that’s wrapping around that axon.

04:57 So let’s talk through what that might be.

05:03 Myelin is going to be a insulator that is allowed to wrap around an axon.

05:12 Its form and function is going to be primarily as an insulator.

05:18 It’s so you have a less current that is lost across the axon.

05:26 If we look at any individual form, you will see that there is a cell.

05:32 You’ll see the axon.

05:34 You’ll see it grows by wrapping itself around till it’s very tightly wound.

05:40 A little bit like electrical tape.

05:43 If you were to tape up a wire, you were simply wrapping a number of different iterations around the wire.

05:53 Its function again is to allow for jumping of action potentials from one node of Ranvier to another.