The Action Potential

hi it's Paul Andersen and in this video I'm going to talk about the action potential that characteristic depolarization and repolarization of an excitable cell I'm going to be talking about neurons but we could also have action potentials in muscle cells coordinating movement or in cells of the endocrine system for example coordinating the release of insulin so if we think about a neuron a neuron is getting information from its dendrites and cell body remember they're connected to a lot of other neurons and based on the summation of all of that information in neurotransmitters it's either going to fire an action potential down the axon or not and so in this video we're going to think about three different potentials the resting potential that's the resting potential of a neuron at rest it's not really at rest we then have a graded potential based on the stimulus that that neuron is going to get and then we may or may not have an action potential no that can lead to synaptic activity on the next neuron I'm going to go through it in the following order first we're going to talk about what a resting potential is and how the sodium potassium pump and the permeability of different ions in the membrane is contributing to the resting potential well then skip forward to the action potential and the importance of voltage-gated channels and then we'll come back and talk about how chemically gated channels contribute to either the firing of an action potential or not and so what is potential potential in physics is simply a separation of charge and so if we think of a battery a 9-volt battery having potential that means that there's a separation of the charge within the battery itself and it also means that we have potential to do work and so if we're thinking about a cross-section of a neuron it also has resting potential what that means is there's a separation of the charges now that separation of the charges is going to be across the membrane just on the surface of the neuron but we could measure the voltage it's negative 70 millivolts in a typical human neuron and so when you're seeing pictures of an action potential like this on either side of the action potential we can see that that neuron is at rest now it doesn't mean that it doesn't require energy to do that and so let's zoom into the surface of that neuron and see what's going on now I've got a little mnemonic that I use with my students to remember the ion's on either side of that membrane I say the neuron is like a salty banana most people remember that a banana has a high amount of potassium on the inside doesn't mean that it doesn't have potassium on the outside there's just a higher percent of potassium on the inside they're also going to be negatively charged anions for example proteins on the inside of the neuron are gonna have a negative charge and then since it's a salty banana on the outside we're going to have salt we're gonna have sodium and chloride ions on the outside as well they're going to be at a higher percentage we still have them on the inside but a higher percentage on the outside now why do we have more sodium on the outside more potassium on the inside I'll show you in a little bit how the sodium potassium pump contributes to that but as you look at this right now there's still no potential we don't see a separation in charges and to really figure out how that works let's remove a lot of it and just focus on the potassium itself because it's really the potassium and the permeability of that potassium that's establishing much of that membrane potential and so we've got a channel on the membrane surface it's called the leak Channel and it's a leak channel that allows potassium to move out and so if we think about where is the potassium going to go there is a chemical gradient from the inside to the outside in other words there's a higher concentration of potassium on the inside than on the outside and since some of it can leak through it's going to go in that direction but watch what happens as the potassium moves out we all of a sudden see a separation in charge since this is a positive charge here it's moving now we have a separation of that charge and it's a relatively negative charge on the inside we also see the arrival of a new gradient that's the electrical gradient since we've got all these positive charges here they're going to repel each other they're going to want to move towards the negative charge and so as more potassium leaves you can see that electric charge is increasing and so eventually what could happen is it could start to flow back in and so we're gonna have movement of potassium inside and outside but we've got an electrochemical gradient outside now why doesn't all the potassium just flow out it's because these are leak channels they don't allow much of that potassium to flow out now if we look at a Phe T simulation and I'll put a link to that in the description down below they've simulated the put the passing of potassium in and outside of the cell you can see that we also have another leak channel right here it's not as busy but that is a sodium leak channel as well and so we have an electrochemical gradient of sodium remember there's more sodium on the outside that's coming in and so we've got these two gradients that are set up you can see the chlorides not moving you can see that the the proteins are not moving as well and that's because we don't have a channel for them now if we were to just let this go eventually we would lose that potential because all the sodium would come in and so we have a sodium potassium pump where we cash in a little bit of ATP and that's going to move that sodium out three sodium ions for every two potassium ions and so as this is moving and establishing that membrane potential again it's the permeability of these two ions that's establishing it that sodium potassium pump is essentially keeping up with the sodium that's flowing in and the potassium that's flowing out and so that is our resting potential negative 70 millivolts is established because we have a separation of those ions on the inside and the outside and they're different electrochemical gradients are establishing that difference in the charge and so we have this resting potential now it requires energy to do that but what can that resting potential eventually lead to an action potential now let's look at what an action potential looks like this is that pH ET simulation again and you can see there's a huge amount of motion between the ions on that surface and as we zoom in we see our sodium leak channel we see our potassium leak channel but we see another couple of channels that are really really important and those are the voltage-gated channels how do they work well let's look at the sodium voltage-gated channel so it looks like this it's got a little polypeptide ball on the bottom and so when the voltage approaches negative 55 millivolts it will open up and it opens up wide and that allows a huge amount of sodium to come into the cell you can see that rush of sodium in that simulation eventually as we move away from that voltage it's going to be inactivated and then it's eventually going to close there's also potassium voltage-gated channels as well and they're going to open when the voltage becomes around 30 millivolts they'll open wide open is that going to do it only allows the passage of potassium potassium is going to move outside of the cell and so as we look at that action potential once we hit that threshold at negative 55 all of those sodium ion channels the voltage-gated channels are going to open wide open what does that do it allows a rush of sodium into the cell what does that do to the voltage it's going to depolarize that neuron the voltage is going to increase radically because we have an influx of all of this sodium ions what eventually happens is we get to 30 millivolts and those sodium voltage-gated channels are going to become inactivated and now the potassium channels are going to open wide open what's going to happen we have a rush of that potassium outside of the neuron it takes a while for those to eventually close and so we then have an undershoot and what's happened is that this point the voltage has gotten really really low but now that permeability again those leak channels are going to reestablish that gradient again to negative 70 millivolts so we could have another action potential and so if we look at what that looks like in slow motion we've got a quick opening of the sodium voltage-gated channels you can see an influx of sodium from there and adjacent that's followed quickly by an opening of the potassium channels within that of that outflow of that potassium and then you can see that permeability is going to reestablish us at that resting potential we could look at what this looks like as we graph the potential over time open up sodium we see a rapid depolarization opening of the potassium a repolarization and then we have that undershoot and during that time until we reestablish that gradient at resting potential we can't have another action potential now so far we've only been looking at one cross-section of the neuron itself how does that message move all the way down the axon well if we look at a simulation it looks like this let me slow that down what really is going on is we're having depolarization of a segment of the axon which triggers depolarization of the next segment and the next segment in the next segment so let's zoom in and look at those voltage-gated channels and see what's going on so if we look at this is near the axon hillock at the beginning of the axon itself we've got some voltage voltage gated sodium and potassium channels and so when we hit that threat told those sodium channels are going to open wide open we have an influx of sodium ions here and what they're going to do is they're going to move away from each other since they are going to repel like charges and they're going to change the voltage in this next segment what does that do it hits threshold again and opens up the next sodium channels now why isn't it going to move to the left because to the left we have that repolarization of that neck of that segment to the left so we're opening up those potassium channels and we have an outflow of those charges we're not going to be near that threshold for the sodium voltage-gated channels and so what we're really having on the leading edge of this is a positive feedback loop we have more sodium coming in which triggers more opening of those sodium channels and behind that we have the undershoot of the as we re-establish that gradient again and so it's going to travel all the way down the axon now this is a simulation of what that electrochemical signal looks like as it moves down but we have some really long axons in our body we have an axon that goes all the way from the base of our spinal cord down to our foot and so it's not going to travel fast enough and so in a lot of those neurons they're myelinated and what that means is they're wrapped with myelin which insulates that nerve fiber and so if we look at it like this what's really going on is we're putting those voltage-gated channels in between that myelin sheath and not I'm not going to get into the specifics of what's going on in here but if we look at this simulation right here what it does is it allows that electrochemical gradient you can compare the two to travel more readily as it moves farther farther down the neuron now action potentials succumb to what's called the all-or-none law and what that means is that an action potential will occur or will not and it doesn't depend on how much of the stimulus it gets it depends on if you hit that threshold or not and so once we hit that threshold we can move towards it we can move away from that voltage but once we hit negative 55 millivolts we're going to have an action potential and that action potential is going to look exactly the same now in this diagram right here we have the three different potentials that I'm talking about in this video at the beginning we have the resting potential remember that's contributed to by the permeability of the membrane to different ions we then have the action potential that rapid polarization and repolarization of the neuron but what's going on in here we have graded potentials in other words the potential is moving towards or away from that threshold and so if we think about what's going on there to really understand that we're getting information from other nerves there are synapses here and those synapses are releasing neurotransmitters those neurotransmitters are moving across the synapse and they're opening up a new form of channel that's important when we talk about graded potentials those are chemically graded channels and so these neurotransmitters are going to dock with proteins on the postsynaptic side of those neurons and what they'll do is they'll open it up what does that do well it depends on what type of neurotransmitter it is and what type of receptor it is there are some that are going to be excitatory and if we look back at our graded potentials excitatory means that they're moving us towards the threshold towards that all-or-none action potential to occur what's the easiest way to do that if we open up the channel and we have a channel that allow sodium to run in that's going to depolarize us and it's going to move us to that threshold just like those sodium gated channels are going to do now we also have inhibitory neurotransmitters and receptors what are they going to move do they're going to move away from that threshold what's the easiest way to do that is to have a receptor that's going to allow potassium to rush out or you could allow chloride to come in to that neuron itself and it's going to push us away from that threshold so we can think of a neuron as an analog portion and a digital portion so since it's all or none down here we're going to have the action potential or not so it's like a 1 or a 0 but if we look on this side of the neuron where the cell body is and the dendrites are we're getting a lot of information from neurotransmitters of neurons that are attached to the dendrites in the cell body of this neuron itself some of those are excitatory what they're doing is depolarizing that neuron they're increasing the voltage pushing us closer to that threshold and some of those are going to be inhibitory they're pushing us away from it and so it's the summation of all of those signals that's telling that neuron to either fire an action potential if we hit negative 55 or and so that is the aller non-law and so in summary what is the resting potential it's negative 70 millivolts it's where a neuron is sitting it has a potential to do work it's established by the permeability of the membranes and the sodium potassium pump we then moved on to the graded potential and that's based on all the neurotransmitters that we're getting from connected neurons and then if we hit that negative 55 we're going to have an opening of those sodium ion channels and we're gonna have an all-or-none action potential that runs down to the neuron so that's the action potential and I hope that was helpful [Music]

Channel: Bozeman Science

Views: 1,065,839

Rating: 4.9536176 out of 5

Keywords: educational videos, science videos, high school science, action potential, neuron, resting potential, sodium, potassium, all or none law, neurophysiology, graded potential, AP Biology, neurotransmitter, voltage-gated channel, leak channel, nervous system

Id: HYLyhXRp298

Length: 14min 6sec (846 seconds)

Published: Mon Jan 23 2017