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  • What if everything you did, and thought, and felt could be communicated by pushing a button?

  • It’d be like using the world’s simplest app -- one that just sends out a little ping,

  • always at the same volume and length -- to communicate everything from, “It sure is

  • cold in here,” to, “I love churros,” to, “Boy, I sure would like to breathe sometime soon.”

  • Well, that is actually exactly how your neurons send ALL the impulses responsible for every

  • one of your actions, thoughts, and emotions.

  • When a neuron is stimulated enough, it fires an electrical impulse that zips down its axon

  • to its neighboring neurons.

  • But theyve only got one signal that they can send, and it only transmits at one uniform

  • strength and speed.

  • What they can vary is the frequency or number of pulses -- like this [buzz buzz buzz] is

  • distinct from this [buzz buzz buzz buzz buzz buzz buzz].

  • And your brain can translate these signals, reading them like binary code, organizing

  • them by location, sensation, magnitude, and importance, so that you know the difference

  • betweenturn up the thermostatandOh my gosh I’m on fire.”

  • That buzz, that nerve impulse, is called the action potential.

  • It’s one of the most fundamental aspects of anatomy and physiology, and really life in general.

  • It’s happening inside of you right now. And we want to make sure that you understand

  • what all that buzz is about.

  • Before we delve into how neurons communicate, weve first got to understand a little bit

  • of our old friend electricity.

  • Basically, think of your body as a sack of batteries.

  • NO, I mean, you don’t look like a sack of batteries, I’m just saying that, your body

  • as a whole is electrically neutral, with equal amounts of positive and negative charges floating

  • around. But certain areas are more positively or negatively charged than others.

  • And because opposite charges attract, we need barriers, or membranes, to keep positive and

  • negative charges separate until were ready to use the energy that their attraction creates.

  • In other words, we keepem separated to build potential.

  • A battery just sitting on its own has both a positive and negative end, and the potential

  • to release energy. But it doesn’t do anything until it’s hooked up to a flashlight or

  • a phone or a kidstoy that lets those charges move toward each other, on the way converting

  • electricity into light, or sound, or children’s laughter.

  • In much the same way, each neuron in your body is like its own little battery with its

  • own separated charges.

  • It just needs an event to trigger the action that brings those charges together.

  • If youre thinking that this sounds more like engineering than anatomy, that might

  • not be a bad thing. It might even help to think of your neurons in the same terms an

  • electrician might use.

  • Voltage, for example, is the measure of potential energy generated by separated charges. It’s

  • measured in volts, but in the case of your body, we use millivolts because it’s a pretty small amount.

  • In a cell, we refer to this difference in charge as the membrane potential. The bigger

  • the difference between the positive and negative areas, the higher the voltage, and the larger the potential.

  • And just like there’s voltage in your body, there’s also current -- the flow of electricity

  • from one point to another. The amount of charge in a current is related both to its voltage and its resistance.

  • Resistance is just whatever’s getting in the way of the current. Something with a high

  • resistance is an insulator, like plastic, and something with a low resistance is a conductor, like metal.

  • Now, when we talk about these concepts in terms of you, were typically talking about

  • how currents indicate the flow of positively or negatively charged ions across the resistance

  • of your cellsmembranes.

  • And again, these membranes separate the charges, so theyre what provide the potential to

  • convert the electricity into something useful.

  • K, now that weve got Electricity 101 down, let's see how it works inside your nervous system.

  • A resting neuron is like a battery just sitting in that sack that is you. When it’s just

  • sitting there, it’s more negative on the inside of the cell, relative to the extracellular

  • space around it.

  • This difference is known as the neuron’s resting membrane potential, and it sits at

  • around -70 millivolts.

  • Where do those charges come from?

  • Outside of a resting neuron, there’s a bunch of positive sodium ions floating around, just

  • lingering outside the membrane.

  • Inside, the neuron holds potassium ions that are positive as well, but theyre mingled

  • with bigger, negatively-charged proteins. And since there are more sodium ions outside

  • than there are potassium ions inside, the cell’s interior has an overall negative charge.

  • When a neuron has a negative membrane potential like this, it is said to be polarized.

  • Now, these ions didn’t just show up in this arrangement on their own. This is all orchestrated

  • by one of the most important bits of machinery in your nervous system, the sodium-potassium pump.

  • This little protein straddles the membrane of the neuron, and there are tons of them

  • all along the axon. For every two potassium ions it pumps into the cell, it pumps out

  • three sodium ions.

  • This creates a difference in the concentration of sodium and potassium, and a difference

  • in charges -- making it more positive outside the neuron.

  • This difference is an electrochemical gradient, and you probably know enough about biology

  • by now to know that NATURE HATES GRADIENTS! It wants to even out all of those inequalities,

  • in concentration and in charge, to restore balance.

  • But the only way to even out that gradient, is for the ions to pass across the membrane.

  • Thankfully, the sodium-potassium pump isn’t the only way in or out of the cell -- the

  • membrane is also riddled with ion channels, large proteins that can provide safe passage

  • across the membrane, when their respective gates are open.

  • And these gates open and close for different reasons, depending on their structure and purpose.

  • Most are voltage-gated channels, which open at certain membrane potentials, and close

  • at others. For example, sodium channels in your neurons like to open around -55 mV.

  • But some others are ligand gated channels -- they only open up when a specific neurotransmitter,

  • like serotonin, or a hormone latches on to it.

  • And then we also have mechanically gated channels, which open in response to

  • physically stretching the membrane.

  • In any case, when the gates do open, ions quickly diffuse across that membrane down

  • their electrochemical gradient, evening out the concentrations, and running away from

  • other positively charged ions.

  • This movement of ions is the key to all electrical events in neurons, and thus is the force behind

  • every. single. thing. we think, do, and feel.

  • Of course, not all of your body’s electrical responses are the same. And neither are the

  • flows of ions going in and out of your neurons.

  • If only a few channels open, and only a bit of sodium enters the cell, that causes just

  • a little change in the membrane potential in a localized part of the cell. This is called

  • a graded potential.

  • But in order to send long-distance signals all the way along an axon, you need a bigger

  • change -- one big enough to trigger those voltage-gated channels.

  • That is an action potential!

  • And your best bet for making that happen is to depolarize that resting neuron -- I mean,

  • cause a big enough change in its membrane potential that itll trigger the voltage-gated

  • channels to open.

  • It all starts with your neuron sitting there at resting state. All of the ion channels

  • are closed, and the inner voltage is resting at -70 mV.

  • And then something happens! Some environmental stimulus occurs -- say like a spider brushes

  • up against a tiny hair on your knee -- triggering those sodium channels to open, increasing

  • the charge inside the membrane.

  • Now, the stimulus -- and the resulting change -- have to be strong enough to cross a threshold

  • for the true action potential to kick in and that threshold is about -55 mV.

  • Remember that number. Because this is an all-or- nothing phenomenon. If the stimulus is too weak, and

  • the change doesn’t hit that level, it’s like a false alarm -- the neuron just returns

  • to its resting state.

  • But kind of like Doc Brown hitting 1.21 gigawatts in the Delorean, once it hits that threshold

  • -- youre not going to travel in time, but you are going to see some serious action potential.

  • At that threshold, the voltage-gated sodium channels open, and there are tons of these,

  • so all of the positive sodium ions rush in, making the cell massively depolarized -- so

  • much so that it actually goes positive, up to about positive 40 mV.

  • This is action potential inaction.

  • It’s really just a temporary reversal of a membrane potential -- a brief depolarization

  • caused by changes in currents.

  • And unlike graded potentials, which are small and localized, an action potential kicks off

  • a biological chain reaction, which sends that electrical signal down the axon.

  • Because each of your neurons has lots of voltage-gated sodium channels. So when a few in one area

  • open, that local current is strong enough to change the voltage around them. And that

  • triggers their neighbors, which triggers the voltage around them, and so on down the line.

  • As soon as all that’s underway, the process of repolarization kicks in. This time the

  • voltage-gated potassium ion channels open up, letting those potassium ions flow out,

  • in an attempt to rebalance the charges.

  • If anything, it goes too far at first, and the membrane briefly goes through hyperpolarization:

  • Its voltage drops to -75 or so mV, before all of the gates close and the sodium-potassium

  • pumps take over and bring things back to their resting level.

  • Now when part of an axon is in the middle of all this, and its ion channels are open,

  • it can’t respond to any other stimulus, no matter how strong. This is called the refractory

  • period, and it’s there to help prevent signals from traveling in both directions down the

  • axon at once.

  • So that is the surprisingly simple app that your nervous system uses to let you experience the world.

  • And because the voltages in this process are always pretty much the same -- the initial threshold

  • around -55 mV, and the peak at depolarization at +40 mV -- your neurons only communicate

  • in a single, monotone buzz.

  • It doesn’t matter if it’s a spider on your knee or an elephant, a paper cut or stab

  • wound, the strength of that action potential is always the same.

  • What does change is the frequency of the buzz.

  • A weak stimulus tends to trigger less frequent action potentials. And that includes if the

  • stimulus is coming from you, like your brain telling your muscles to perform some task.

  • If I need to do something delicate, like pick up an egg, the signal is low-frequency: [buzz...buzz...buzz...]

  • But a more intense signal -- like trying to crush a can -- increases the frequency of

  • those action potentials to tell your muscles to contract harder, and the message turns

  • into something that you can’t ignore -- [buzzbuzzbuzzbuzz]

  • Action potentials also vary by speed, or conduction velocity.

  • Theyre fastest in pathways that govern things like reflexes, for example, but theyre

  • slower in places like your glands, guts, and blood vessels.

  • And the factor that affects a neuron’s transmission speed the most, is whether there’s a myelin

  • sheath on its axon.

  • Axons coated in insulating myelin conduct impulses faster than non-myelinated ones,

  • partly because, instead of just triggering one channel at a time in a chain reaction,

  • a current can effectively leap from one gap in the myelin to the next.

  • These little gaps are the delightfully named Nodes of Ranvier, and this kind of propagation

  • is known as saltatory conduction, from the Latin word forleaping.”

  • But what happens when an action potential hits the end of its axon and is ready to do

  • more than leapand jump all the way to another neuron?

  • That you will find out next time!

  • Today you learned how your body is kinda like a big bag o’ batteries, and how ion channels

  • in your neurons regulate this electrochemistry to create an action potential, from resting

  • state to depolarization to repolarization and a brief bout of hyperpolarization.

  • Thanks for watching, especially to all of our Subbable subscribers, who make Crash Course

  • possible for themselves and for everyone else. To find out how you can become a supporter,

  • just go to subbable.com.

  • This episode was written by Kathleen Yale. The script was edited by Blake de Pastino,

  • and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins and Michael

  • Aranda, and our graphics team is Thought Café.

  • One more thing before you leave.

  • We like Crash Course a lot and we hope that you like Crash Course a lot, but I kind of

  • feel like Crash Course is only useful for a certain segment of the population. Like,

  • once you get to a certain age, then it's good and then forever it can be helpful to people.

  • But younger people, not so much.

  • And so we are creating Crash Course Kids. Hosted by Sabrina Cruz from NerdyAndQuirky,

  • Crash Course Kids will start out focusing on fifth grade science, but will keep expanding

  • to other topics as the the channel grows.

  • Sabrina will be talking about food chains, and gravity, and how the sun works, and how

  • plants eat, and why flamingos are pink, and many other topics.

  • Oh, and another note: teachers, you can rest assured that we've got you covered. There

  • will be info about the standards we've used to make sure that we're doing our very best

  • to help you out. So, if you are a teacher or you know a teacher or you know a child

  • or you know someone who has a child or you've ever seen a child, you can tell them to go

  • to youtube.com/CrashCourseKids and subscribe and you can go do that as well if you would

  • find that kind of content useful or interesting.

What if everything you did, and thought, and felt could be communicated by pushing a button?

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The Nervous System, Part 2 - Action! Potential!: Crash Course A&P #9

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    bsofade posted on 2015/05/27
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