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Transcript Provided by YouTube:
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What if everything you did, and thought, and felt could be communicated by pushing a button?
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It’d be like using the world’s simplest app — one that just sends out a little ping,
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always at the same volume and length — to communicate everything from, “It sure is
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cold in here,” to, “I love churros,” to, “Boy, I sure would like to breathe sometime soon.”
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Well, that is actually exactly how your neurons send ALL the impulses responsible for every
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one of your actions, thoughts, and emotions.
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When a neuron is stimulated enough, it fires an electrical impulse that zips down its axon
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to its neighboring neurons.
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But they’ve only got one signal that they can send, and it only transmits at one uniform
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strength and speed.
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What they can vary is the frequency or number of pulses — like this [buzz buzz buzz] is
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distinct from this [buzz buzz buzz buzz buzz buzz buzz].
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And your brain can translate these signals, reading them like binary code, organizing
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them by location, sensation, magnitude, and importance, so that you know the difference
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between “turn up the thermostat” and “Oh my gosh I’m on fire.”
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That buzz, that nerve impulse, is called the action potential.
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It’s one of the most fundamental aspects of anatomy and physiology, and really life in general.
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It’s happening inside of you right now. And we want to make sure that you understand
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what all that buzz is about.
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Before we delve into how neurons communicate, we’ve first got to understand a little bit
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of our old friend electricity.
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Basically, think of your body as a sack of batteries.
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NO, I mean, you don’t look like a sack of batteries, I’m just saying that, your body
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as a whole is electrically neutral, with equal amounts of positive and negative charges floating
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around. But certain areas are more positively or negatively charged than others.
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And because opposite charges attract, we need barriers, or membranes, to keep positive and
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negative charges separate until we’re ready to use the energy that their attraction creates.
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In other words, we keep ‘em separated to build potential.
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A battery just sitting on its own has both a positive and negative end, and the potential
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to release energy. But it doesn’t do anything until it’s hooked up to a flashlight or
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a phone or a kids’ toy that lets those charges move toward each other, on the way converting
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electricity into light, or sound, or children’s laughter.
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In much the same way, each neuron in your body is like its own little battery with its
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own separated charges.
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It just needs an event to trigger the action that brings those charges together.
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If you’re thinking that this sounds more like engineering than anatomy, that might
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not be a bad thing. It might even help to think of your neurons in the same terms an
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electrician might use.
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Voltage, for example, is the measure of potential energy generated by separated charges. It’s
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measured in volts, but in the case of your body, we use millivolts because it’s a pretty small amount.
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In a cell, we refer to this difference in charge as the membrane potential. The bigger
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the difference between the positive and negative areas, the higher the voltage, and the larger the potential.
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And just like there’s voltage in your body, there’s also current — the flow of electricity
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from one point to another. The amount of charge in a current is related both to its voltage and its resistance.
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Resistance is just whatever’s getting in the way of the current. Something with a high
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resistance is an insulator, like plastic, and something with a low resistance is a conductor, like metal.
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Now, when we talk about these concepts in terms of you, we’re typically talking about
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how currents indicate the flow of positively or negatively charged ions across the resistance
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of your cells’ membranes.
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And again, these membranes separate the charges, so they’re what provide the potential to
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convert the electricity into something useful.
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K, now that we’ve got Electricity 101 down, let’s see how it works inside your nervous system.
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A resting neuron is like a battery just sitting in that sack that is you. When it’s just
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sitting there, it’s more negative on the inside of the cell, relative to the extracellular
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space around it.
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This difference is known as the neuron’s resting membrane potential, and it sits at
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around -70 millivolts.
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Where do those charges come from?
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Outside of a resting neuron, there’s a bunch of positive sodium ions floating around, just
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lingering outside the membrane.
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Inside, the neuron holds potassium ions that are positive as well, but they’re mingled
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with bigger, negatively-charged proteins. And since there are more sodium ions outside
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than there are potassium ions inside, the cell’s interior has an overall negative charge.
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When a neuron has a negative membrane potential like this, it is said to be polarized.
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Now, these ions didn’t just show up in this arrangement on their own. This is all orchestrated
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by one of the most important bits of machinery in your nervous system, the sodium-potassium pump.
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This little protein straddles the membrane of the neuron, and there are tons of them
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all along the axon. For every two potassium ions it pumps into the cell, it pumps out
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three sodium ions.
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This creates a difference in the concentration of sodium and potassium, and a difference
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in charges — making it more positive outside the neuron.
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This difference is an electrochemical gradient, and you probably know enough about biology
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by now to know that NATURE HATES GRADIENTS! It wants to even out all of those inequalities,
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in concentration and in charge, to restore balance.
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But the only way to even out that gradient, is for the ions to pass across the membrane.
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Thankfully, the sodium-potassium pump isn’t the only way in or out of the cell — the
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membrane is also riddled with ion channels, large proteins that can provide safe passage
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across the membrane, when their respective gates are open.
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And these gates open and close for different reasons, depending on their structure and purpose.
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Most are voltage-gated channels, which open at certain membrane potentials, and close
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at others. For example, sodium channels in your neurons like to open around -55 mV.
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But some others are ligand gated channels — they only open up when a specific neurotransmitter,
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like serotonin, or a hormone latches on to it.
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And then we also have mechanically gated channels, which open in response to
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physically stretching the membrane.
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In any case, when the gates do open, ions quickly diffuse across that membrane down
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their electrochemical gradient, evening out the concentrations, and running away from
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other positively charged ions.
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This movement of ions is the key to all electrical events in neurons, and thus is the force behind
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every. single. thing. we think, do, and feel.
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Of course, not all of your body’s electrical responses are the same. And neither are the
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flows of ions going in and out of your neurons.
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If only a few channels open, and only a bit of sodium enters the cell, that causes just
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a little change in the membrane potential in a localized part of the cell. This is called
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a graded potential.
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But in order to send long-distance signals all the way along an axon, you need a bigger
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change — one big enough to trigger those voltage-gated channels.
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That is an action potential!
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And your best bet for making that happen is to depolarize that resting neuron — I mean,
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cause a big enough change in its membrane potential that it’ll trigger the voltage-gated
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channels to open.
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It all starts with your neuron sitting there at resting state. All of the ion channels
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are closed, and the inner voltage is resting at -70 mV.
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And then something happens! Some environmental stimulus occurs — say like a spider brushes
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up against a tiny hair on your knee — triggering those sodium channels to open, increasing
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the charge inside the membrane.
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Now, the stimulus — and the resulting change — have to be strong enough to cross a threshold
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for the true action potential to kick in and that threshold is about -55 mV.
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Remember that number. Because this is an all-or- nothing phenomenon. If the stimulus is too weak, and
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the change doesn’t hit that level, it’s like a false alarm — the neuron just returns
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to its resting state.
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But kind of like Doc Brown hitting 1.21 gigawatts in the Delorean, once it hits that threshold
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— you’re not going to travel in time, but you are going to see some serious action potential.
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At that threshold, the voltage-gated sodium channels open, and there are tons of these,
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so all of the positive sodium ions rush in, making the cell massively depolarized — so
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much so that it actually goes positive, up to about positive 40 mV.
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This is action potential in … action.
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It’s really just a temporary reversal of a membrane potential — a brief depolarization
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caused by changes in currents.
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And unlike graded potentials, which are small and localized, an action potential kicks off
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a biological chain reaction, which sends that electrical signal down the axon.
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Because each of your neurons has lots of voltage-gated sodium channels. So when a few in one area
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open, that local current is strong enough to change the voltage around them. And that
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triggers their neighbors, which triggers the voltage around them, and so on down the line.
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As soon as all that’s underway, the process of repolarization kicks in. This time the
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voltage-gated potassium ion channels open up, letting those potassium ions flow out,
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in an attempt to rebalance the charges.
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If anything, it goes too far at first, and the membrane briefly goes through hyperpolarization:
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Its voltage drops to -75 or so mV, before all of the gates close and the sodium-potassium
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pumps take over and bring things back to their resting level.
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Now when part of an axon is in the middle of all this, and its ion channels are open,
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it can’t respond to any other stimulus, no matter how strong. This is called the refractory
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period, and it’s there to help prevent signals from traveling in both directions down the
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axon at once.
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So that is the surprisingly simple app that your nervous system uses to let you experience the world.
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And because the voltages in this process are always pretty much the same — the initial threshold
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around -55 mV, and the peak at depolarization at +40 mV — your neurons only communicate
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in a single, monotone buzz.
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It doesn’t matter if it’s a spider on your knee or an elephant, a paper cut or stab
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wound, the strength of that action potential is always the same.
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What does change is the frequency of the buzz.
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A weak stimulus tends to trigger less frequent action potentials. And that includes if the
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stimulus is coming from you, like your brain telling your muscles to perform some task.
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If I need to do something delicate, like pick up an egg, the signal is low-frequency: [buzz…buzz…buzz…]
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But a more intense signal — like trying to crush a can — increases the frequency of
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those action potentials to tell your muscles to contract harder, and the message turns
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into something that you can’t ignore — [buzzbuzzbuzzbuzz]
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Action potentials also vary by speed, or conduction velocity.
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They’re fastest in pathways that govern things like reflexes, for example, but they’re
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slower in places like your glands, guts, and blood vessels.
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And the factor that affects a neuron’s transmission speed the most, is whether there’s a myelin
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sheath on its axon.
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Axons coated in insulating myelin conduct impulses faster than non-myelinated ones,
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partly because, instead of just triggering one channel at a time in a chain reaction,
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a current can effectively leap from one gap in the myelin to the next.
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These little gaps are the delightfully named Nodes of Ranvier, and this kind of propagation
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is known as saltatory conduction, from the Latin word for “leaping.”
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But what happens when an action potential hits the end of its axon and is ready to do
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more than leap … and jump all the way to another neuron?
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That you will find out next time!
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Today you learned how your body is kinda like a big bag o’ batteries, and how ion channels
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in your neurons regulate this electrochemistry to create an action potential, from resting
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state to depolarization to repolarization and a brief bout of hyperpolarization.
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Thanks for watching, especially to all of our Subbable subscribers, who make Crash Course
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possible for themselves and for everyone else. To find out how you can become a supporter,
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just go to subbable.com.
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This episode was written by Kathleen Yale. The script was edited by Blake de Pastino,
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and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins and Michael
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Aranda, and our graphics team is Thought Café.
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One more thing before you leave.
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We like Crash Course a lot and we hope that you like Crash Course a lot, but I kind of
10:43
feel like Crash Course is only useful for a certain segment of the population. Like,
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once you get to a certain age, then it’s good and then forever it can be helpful to people.
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But younger people, not so much.
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And so we are creating Crash Course Kids. Hosted by Sabrina Cruz from NerdyAndQuirky,
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Crash Course Kids will start out focusing on fifth grade science, but will keep expanding
11:02
to other topics as the the channel grows.
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Sabrina will be talking about food chains, and gravity, and how the sun works, and how
11:07
plants eat, and why flamingos are pink, and many other topics.
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Oh, and another note: teachers, you can rest assured that we’ve got you covered. There
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will be info about the standards we’ve used to make sure that we’re doing our very best
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to help you out. So, if you are a teacher or you know a teacher or you know a child
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or you know someone who has a child or you’ve ever seen a child, you can tell them to go
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to youtube.com/CrashCourseKids and subscribe and you can go do that as well if you would
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find that kind of content useful or interesting.
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Photo credit: Screenshot from video.