You can review content from Crash  Course Anatomy & Physiology with   the Crash Course App, available  now for Android and iOS devices.

Hello, everybody. I think that we have begun the livestream now.

I'm Hank Green, this is Office Hours. I was once the host of Crash  Course Anatomy & Physiology. And for the next hour, we're going to  be answering your questions about A&P to maybe help you study for  finals or whatever you're up to.

And I'm joined by a person who actually knows  stuff about anatomy and physiology, our script, our consultant on that project who helped  us make sure we got everything right. It's Brandon. Hello, Brandon Jackson. <Hi, Hank. >Brandon, tell us a little bit  about who you are, what you do. <I'm now an associate professor  at Longwood University.

I've been here for about seven years. I used to live in Missoula where we  first met and, I was thinking about it, I've taught Anatomy & Physiology  or Comparative Anatomy  for almost 18 years now.  So it's been quite a ride. >That's great. Well, you're  the right person to have here.

Here's how it's going to go, we've got people to send in  their questions ahead of time so we've got some prepared  that we know we're going to do. Then we're going to talk a little bit about some   study tips for specifically how  to study for Anatomy & Physiology, which I found very helpful  learning about from Brandon. And then we're going to end with  some questions from the chat.

So if you have any, put them in there, appreciate all of you for doing that. Before we get to your questions, I want to  talk a little bit about our  partner for Office Hours. We're very lucky to have a partner.

It's Flipgrid, which is a free video discussion app from Microsoft, and they got a mission to make learning fun and empowering for all. It's been used in the classroom for nearly a decade and as we talk about preparing for exams, Flipgrid is a convenient way to host study groups so that having to coordinate around a class schedule or  after-school commitments. You can create a group, start a topic and send the link to anyone you want to join.

You can record video or audio responses, discuss specific in detail, quiz each other, prep for group presentations, all of that. We hear from Crash Course viewers all the time, how helpful video is as a learning tool, it's one of the reasons we made Crash Course and connecting with peers and learning in groups with your peers in a community is a wonderful thing. We use Flipgrid to collect some of the questions that we're going to be asking on the livestream.

So let's start with some questions for the livestream. Brandon, are you ready? Do you know enough about Anatomy & Physiology to answer these questions? I'm pretty sure you do.

This first one comes from Drew who asks is the heart a muscle or an organ? This is great, because now we get to talk about muscles, organs, tissue, cells.
It seems kind of simple at first, and it's not just a yes or no answer, this is going to be kind of a long-winded answer unfortunately I think, but it's kind of cool. But really we have to get down to definitions and the hierarchy of organizations that we talk about in Anatomy & Physiology,   and most of Biology really, right? So we can take atoms and make molecules, we can take molecules and if we arrange them in just the right way, we get cells.

If we take a bunch of cells that all look alike and function together and organize them in the right way in a body, that's what we call a tissue. And this is where we kind of start. Now, if we take multiple tissues and combine them together, and we get a thing in the body, a structure in the body that has more or less  a single function, or  sometimes multiple functions, that's an organ.

So an organ has multiple tissues and at least one obvious function. >Now, see, I think this is what confused me about this and maybe what is confusing Drew about this is that I hear that muscle is a tissue type? But /a/ muscle is not a tissue type?
It's one of four tissue types. So we have epithelial tissue, muscle tissue, nervous tissue, and connective tissue. >I mean, I love that there's only four, that's way easier than almost everything.
So of muscle tissue, there's actually three kinds of muscle tissue and you can tell the difference if you look just  down at the cellular level and then there's some other  functional differences. But really the ones we're talking about here, there's two, there's skeletal muscle tissue, and that's muscle, the tissue you find in your favorite skeletal muscle. Hank, what's your favorite skeletal muscle? >My favorite skeletal muscle has got to be the butt, right? <Okay.

So the gluteus maximus, that one. Yeah, we'll call it the gluteus maximus. There's a medias too, there's some other muscles in there.

But okay. So the gluteus maximus, now that is a skeletal muscle that has skeletal muscle tissue in it, as opposed to the heart, which has cardiac muscle tissue in it. So those are multiple muscle tissue types.

Now are they an organ? And this is kind of the other part of the question. So let's take the gluteus maximus first.

And is that an organ? It actually is, because remember the definition of an organ is multiple tissue types. So we have the skeletal muscle tissue in there and that's the bulk of it, that's the thing that does the work. >Does the work, but you can't do the work, let me see if I can name a couple others.

I can feel my butt so it's got nervous tissue in it and my butt is alive so it's got to have some vasculature, there's got to be some delivery of oxygen so it's got veins and stuff. Epithelial tissue.
We didn't talk about connective tissue in there, but you have the tendons connected to the end, that's connective tissue, dense connective tissue. And then kind of through the rest of the muscle, we have all these different layers, like the epimysium and the perimysium and those are also connective tissue. So there's your organ, all four tissue types.

It's kind of an overachiever of an organ. And yet we don't- . But you don't really think of it that way.

Because I'm like, yeah, a liver is an organ when I can take out and hold it in my hand and be like, "That looks like an organ."
Okay, well, that's skeletal muscle, but what about cardiac muscle? Same thing, add up the tissue types. What we have there, cardio muscle cells, that's the cardiac muscle tissue so that's one.

We also have epithelium, the inside of the heart is the endocardium, the outside of the heart is the epicardium. Those are both epithelial tissues. So there's two.

And then there's other forms of connective tissue in and around it, there's fat tissue around it that's connective. The valves inside of the heart are a type of connective tissue. >Yeah, I've never touched one, but I've seen them and they look like cartilage almost.
So there we have multiple tissue types, an obvious function like pumping the blood. There we go, it's an organ. So here's the question.

Is it a muscle? >It's a muscle. That's my answer for you. <Right, so in Anatomy & Physiology, we have very specific language. So we don't just say "a muscle," we say a   "skeletal muscle." So is it a skeletal muscle?

No. >No. Skeletal muscles are organs,  just blown everybody's  minds.

Okay, got another question for you. It's from Maggie. This one came in from Flipgrid and Maggie asks, "I'm in my first year of college, my first year taking anatomy.

I had a question about skin cells. How are they organized throughout the layers of the skin?" So she goes on talk about a bunch of different types of skin cells and are they like, spread out? So you've got melanocytes,  you've got keratinocytes,  Langerhans cells, which  are, I think, immune cells.

Am I wrong about that? <Nope, that's correct. >And so they're in the skin. Are they peppered throughout? Are they in layers?

As the skin, like, it sort of builds up  at the bottom and then pushes higher, do these things move up with it  or do they stay in the same place? How are they doing this? What are they doing? <Yeah, so some of these cells are related to other and some aren't and so we can start with that.

And actually, the idea of the tissues will come back into play here. So the main cell that we talk about with the epidermis at least are the keratinocytes, these are what make the keratin that make your skin kind of dry and tough and yeah, they do the job they say. >Impermeable, yeah. <Yeah, exactly. And so these are formed in the stratum basale, the deepest layer of the epidermis.

And that's where the new ones are formed from a thin layer of stem cells. So the stem cell divides,  it creates one keratinocyte  and then the other one is still going to stay down there as the stem cell. And so that keratinocyte then gets kind of pushed higher and higher as new, younger ones are made behind it.

And I mean, it's kind of dark to think about this, but these skin cells are almost like us says we age, right? We start up young and plump and happy and healthy and then as we age, we start getting some spots and that bit- >Harder. Life happens.
You get wrinkly. That's the stratum spinosum. Yeah, you get beat up, withered, dried up, you end up literally a shell of your former self.

And at that point -- if you're keratinocyte,  at least -- you're dead  and you're just the keratin and wax that you kind of aged with. And then you're in the stratum corneum, the top layer. Eventually, you get pushed off  at lost as dust, basically. > Yes.

Which is all of our  eventual fates, just lost as dust.
And melanocytes are actually  related to the keratinocyte. So the keratinocytes are an epithelial cell, stratified squamous epithelial cell, and the melanocytes are also epithelial. They are kind of distant  cousins of the keratinocytes.

So the melanocytes come from a different stem cell, but the keratinocyte stem cell and the melanocytes stem cell come  from the same stem cell. >It's like a taxonomic tree happening here, but just our body cells.
And so the melanocytes, that stem cell is usually found near hair follicles, but then the melanocyte kind of migrates through, sets up shop in the lower levels of the keratinocyte and with the younger ones and creates melanin and then kind of distributes that melanin further up in the skin. And they can be much longer lived. > So it never moves up, it just sort of like hangs out there and they move past it? <Correct, correct. The cells kind of move past and pick up these melanin granules and carry them up and then lose them eventually.

Let's see. Where were we? So then that's the melanocyte, that's kind of a cousin, still epithelial.

And then we had the Langerhans cell and the Langerhans, like you said, is an immune cell. And so the immune cells are actually essentially blood cells, right? We've heard of white blood cells. >Totally different cell lineage, not the same stem cells.
That's connective tissue, blood is actually connective tissue. And these forms- >You say this, it will never make sense to me. What's blood connect to?

We don't have to talk about it. <Everything? No-- [laughs] I mean, it's- >I don't think that's what they meant when they originally came up with the term connective tissue that connects skeletal stuff together. But hey.
There's some embryology that supports blood  in this group and we won't  get into that right now. > Haha, okay.
So at these Langerhans cells are also called dendritic cells because they have lots of branches and dendrite means branches. But really they are a macrophage. So macrophage is this big functional description. >Like white blood cells, yeah.
That's what they're doing there. >Yeah. Right, right. And so they're staying there,  they're not moving up with everything? <No, they're also not getting moved up. >So it's just, like, there's like  the conveyor belt of keratinocytes,  but nothing else goes up the conveyor belt? <Correct, correct. And then the last one are the Merkel discs, or the Merkel cells, and they're really nervous function, they're part of our sensory system, they're part of how we sense touch and one of the types of touch.

And as far as I can tell, we don't actually know exactly what they come from in terms of their stem cell lineage. They function with the nervous system, some people say from what I've read, they say that they come from skin cells or they say that they come from the nervous system. It's actually kind of cool because both the skin and the nervous system come from the ectoderm embryological, so they're at least distant cousins in that manner. > So they're all friends  and they hang out together,  but only there's only one conveyor belt and it's keratinocytes? <Correct, yes. >Alright, we have another question.

We have got a bunch of people who ask questions about the nervous system and gated channels and action potentials. Kit and Diana and Allie and Allen and Wazi. So can you tell me just in general about ion channels, I guess, and action potentials.
But it's actually really interesting because if you get down the basics, and I'll try to boil this down to just a few rules here, but if you can get the basics down, you actually learn about not just how neurons work, but  also how the heart works,  how skeletal muscle works. There's probably something else that uses these action potentials that I can't think of right now. >Well, I mean any sensing. <Exactly, all of our senses. Our eyes, our ears.

Exactly, yeah. Okay. And this is also a very common stumbling block for students.

A lot of people have trouble when they're starting out learning this so I like to teach this boiled down to just a few pretty simple rules. It's oversimplifying a little bit, but if you get these down, then you can add on the other layers that really help you get into all the details. Okay, so first rule, there are more sodium ions outside of these cells than inside and there's more potassium inside than outside.

And the cell is making that happen? The cell is making happen with a pump called the sodium-potassium pump. So good name for it. >Pump the potassium in, sodium out? <Correct.

So rule one, sodium's out, potassium's in. And both of them are positive ions if you don't know that. Okay, now these kinds of ions, when they're dissolved in water, we call them solutes and generally, solutes want to move from areas  of high concentration to  areas of low concentration.

In other words, given the opportunity, sodium wants to come into the cell because it's outside and potassium wants to get out of the cell because it's inside. We got that, Hank? >We got that.
If we were to measure the electricity inside of the cell compared to the outside, it would show up at about -70. And depending on the book, sometimes it's listed as -65, -70, close enough. >Who cares? <Yeah, it's close enough. >Significant figures,   but why is there an electrical charge if they're both positive charged? <Oh, okay. So you want to ask about this? >Well, it seems like a logical question to ask.
So one reason is that inside of the cell,  there are large anionic  negatively charged proteins. So there's some stuff inside of the cell that has a negative charge that can't leave the cell. There's another reason that has to do with potassium trying to get out and actually being allowed out a little bit down its gradient and -70 is the balancing voltage to prevent more from leaving. >Yeah, the cell figured it out.  The cell made it so that there's  -70 milliwatts or whatever. <Right.

And this is the trick. If the book tries to get you to see why it's -70, leave that for later. You'll get it later.

It's so much easier if you leave that for that after we talk about all the movement. Okay, so we have -70, and then you often see these graphs of action potentials where you see a line, the voltage starting at -70, and then it's going to go up or down or something like that. So it always will start at -70 or -65.

And that is again always telling you the inside of the cell relative to the outside. Okay, the last rule is actually a result of all of those other rules. And so here's, Hank, where I'm going to ask you to answer this.

If the inside is -70 and sodium is allowed to come into the cell, and sodium's positively charged,  what happens to the voltage? Does it go up or down? Does it get more positive or- >It goes up. <Yeah, it goes up, it becomes  more positive or less negative. >Less negative. <Yeah, right.

So we're adding positives to the inside if sodium comes in. Now, what happens if potassium is allowed to leave? >Then it gets more negative. Okay.

Love that, up and down. It's not math, it's just a direction.
If you get those, then the rest is literally- >Just how everything works. And there's a bunch of different doors that  let the different things in  and out in different ways.
So these are protein channels. >Just a door? <Yeah, it's an open door. These are protein channels  across the cell membrane. They're specific, they only let either sodium or potassium through.

And so those things are going to go the direction that they want to go. And the leakage channels are just always open. The other one that is part of how we sense touch and hear and balance is called a mechanically-gated channel.

Basically, it opens if the cell membrane gets stretched, like the door gets stretched open. >It's actually a physical reaction.  So when we are feeling  touch, we are feeling touch. Cool.
One is called a chemically gated channel or a ligand-gated channel. And a ligand is just something that binds to a protein. This is a key in a lock kind of situation.

So here's a door it's closed, it's locked, we need a key to open it. That key is usually going to be something like acetylcholine, which is a neurotransmitter, it's actually the neurotransmitter that helps trigger your muscles to contract. Okay, so acetylcholine, if it binds to that little protein, it's the key, it unlocks the door.

The one we usually talk  about with these ligand-gated  channels are sodium channels. So let's say we open a sodium channel, what happens to sodium? Which direction does it go? >Look, I forgot. <Sodium's outside and it wants to come in. >Wants to come in, okay. <Yep.

It wants to come in. And so then the sodium right, now since we're adding positives, the inside is going to get more positive and the voltage is going to start to go up. Now we could- >We should have just renamed these ions.

We should have called one of them the out ion and one of them the in ion and that would've simplified things greatly. <Well, and the abbreviation for sodium is Na and the abbreviation for potassium is K. >So we picked the hardest to remember ones? It's like mercury is a little bit harder than those, but basically everything else. <I'm glad I'm not responsible for the naming convention. So let's see. So we have these key channels.

We can open sodium ones,  we can open potassium ones. Now, the next ones are the important part for how the action potential actually travels. So the whole idea of this is to get a signal, to go from point A like your brain to point B, like your gluteus maximus muscle, and to get it to contract.

Now that's a long way for it to travel and so we want it to travel fairly quickly so that we can react to proper things like walking,  it's important to time things  well when we're walking. And that's what this next channel is, called voltage-gated channels. And they open when that voltage inside of the cell reaches a certain level and just in one location where that cell is.

So they open at about -55 millivolts. We call this the threshold voltage for these channels. So we started at -70, right?

We bring in some sodium and then the line starts to go up. If that cell reaches about -55, the voltage-gated channels will open. And the first ones that open are the sodium channels.

Did you have a question, Hank? >No, I was just imagining them expanding. <Yeah, so they open up,   sodium starts coming in now these voltage-gated channels. And as the sodium comes in, it starts to crawl along the inside of the membrane. It kind of floats in and then distributes.

And it's going to slide its way down to a little bit further on down the cell, eventually it will find another voltage-gated sodium channel. If enough sodiums are on the inside, it raises the voltage. At that point, opens that door, sodium marshes in, slides down, next gate- >Cascade.
And now we get this wave of sodium rushing in all the way down the cell in a fraction of a second, it can go a meter down your leg. So very fast reaction. >And this is why I like salt.
Now, when that gets all the way down to the end of the neuron, it does something else, it actually opens a voltage gate calcium channel and calcium is just the fine signal that tells the cell to release its neurotransmitters. Now the whole time that this has been happening, there's actually another channel, another voltage-gated channel. We kind of ignored potassium to this point, right?

And so the sodium at that threshold voltage that was opening the voltage-gated sodium channels was also opening voltage-gated potassium channels, but they are sticky doors. They don't open that quickly. So actually they're like big, thick, creaky doors, they're slowly opening, sodium's rushing in its channel.

And by the time sodium's pretty much done rushing in, potassium wants to rush out. And so they're just offset enough. So as the sodium rushes in, the voltage goes up and right at the top at about +30 then the potassium channels start to open.

And then when the potassium channels open, potassium is leaving. So what happens if we take a bunch of positive things from inside and we let them out, what happens to the inside? Does it get more positive or more negative if we remove positives? >I was looking at the Slack I  wasn't paying attention to you.  I had to check on something.
Haha it's okay, my students text in class. So the inside of the cell is going to get  more negative if those positive  potassium are leaving. And it actually is going to get so negative that we reset the voltage.

So now we've sent the signal and we've reset it. And again, there's a little bit more to it than that, but if you can get that part down and those rules that we started with, then  you can layer on the rest of  your understanding on that. >Right, right, right. Amazing.

I mean, and this is all, the great thing about understanding that stuff is that from now on and forever, you just have a totally different understanding of how your body interacts with the world around it. It's pretty cool. #9 is the question that this is on my  list, but not the number that we're on. It's from Laurel who asks, what is the best way to remember the names and locations of the bone landmarks?

I don't even know a bone landmark was a thing, but in general, there's a lot of memorization in Anatomy & Physiology. I like the part where it's conceptual, I don't like the part where I'm memorizing bones.
So for example, with the thing we just talked about, if you know a few rules of how these channels work and how cells are set up, you know how nerves work, how muscles work and how a bunch of our senses work. So find those commonalities. Now bones are kind of two parts, one is the structural part and the other part of learning them is learning the words and I think we're  going to talk about how to  learn all the words later. >Yeah.

We'll get there too.
But really, it's helpful to get your brain to process it in a different way. >This is well known that the more work you are doing with your fingers,   the better you are learning. So actually drawing, looking at a thing and then closing it and then trying to draw it. That is how-- that is how you learn things. <Right.

And I'm going to suggest something. I like what you just said, that it's really trying to draw up from memory. Now, you take a femur or something like that,  there's a whole bunch of  little bumps and things on it.

And of course, it's three-dimensional, which is hard to draw on paper. So you do your best. And I suggest starting with just the very basic shape, don't even worry about all the bumps the first time you draw it.

Look at the books, study it, get an idea for the shape and then draw it. And this is where, if you're a horrible artist like me, my dad's an artist, I didn't get those genes, and if you're a horrible artist like me, it's actually good because you don't worry about getting all the little details and the shading, just get the basic shape. Draw that and label whatever you can then go back to your book or go back to whatever kind of reference you're working on and see where you could improve or see if you got everything right.

See if you could add one more detail or add one more label. And then close the resource, draw it again only looking at your previous drawing. So make it a little bit better, do it all again, label what you can and then compare it to the resource and just kind of go back  and forth and slowly build  up your knowledge that way.

If your teacher, like I do to my students,  I'll hand them a list of  300 terms to know in a lab. And that's totally overwhelming. Don't study the whole thing all at once.

One thing at a time, or  maybe two things at a time. And so drawing is really good for that. We have to sort of understand the whole cascade of heart cells and what they're doing? <Yeah.

Well, we actually  already know some of that. So there's really two parts to understanding heart function. One is electrical, and we mostly just talked about that.

We can talk a little bit more about that. And the other is really like physical and this is when we talk about like- >What happens in what order? <Yeah, and pumping the blood that the pressure and  stuff that is involved in moving  the blood through the body. So here's a rule and this  is again, mostly accurate.

Some physicists may not think that I'm phrasing this properly, but for the purposes of Anatomy &  Physiology, this is what you need to know. Fluids move from high pressure to low pressure. I mean, that's pretty simple, right?

And this is fluids including air and liquids like blood. So actually this tells us how we breathe, how we move air in and out of our lungs. It's high pressure and low pressure.

Okay, but back to the heart. So what is the heart? The heart is a muscle, right?

That's kind of where we started. And so muscles contract, and when the heart  contracts, it produces  pressure inside of the heart. And so this is how the blood is going to get moved around but it's important that the heart is not all contracting at all at once like your gluteus maximus might contract when you're running, right?

The heart actually contracts in kind of two parts. So the top part of the heart, they're called  the atria so you have a left  atrium and a right atrium. And then in the bottom half of the heart, you have the ventricles, a left ventricle and the right ventricle.

And the blood goes from atria on one side to ventricles on the same side. So what we want to have happen is the atria to contract on top to send the of the blood down to the ventricles. And then once the ventricles are fully filled up, then we want them to contract.

We don't want them contracting at the same time as the atrium. So there's this little delay in there. That delay is actually part  of the electrical system.

So, again, we'll come back to that electrical  system so just kind of ignore  the delay for right now. So the atria, they're going to squeeze and create higher pressure, higher fluid pressure or hydrostatic pressure than what we find in the ventricles. And therefore we have a pressure gradient  and the blood will flow from  atria down to ventricles.

When the atria are done squeezing, then the big ventricles are going to squeeze at the bottom and they can produce a lot of pressure. And so they start squeezing. As the pressure in the ventricles gets above the pressure and the atria, then the blood will want to flow to that  low pressure in the atrium,   and it will actually start to flow that direction.

But then it gets stuck on those valves that we were talking about earlier, that kind of leathery, tough connective tissue. And we'll shut those valves, the back flow  will actually close those  valves and they slam shut. And that kind of slamming shut and this pressure wave that happens is the first heartbeat sound that you hear, right?

So we talk about the lub-dub of  heartbeat sounds, the two sounds, this is the lub, this is the first one. Then the ventricle keeps contracting and keeps building up pressure. I mean, this all happens in a fraction of the second so I'm kind of slowing this way down.

So as the pressure builds in the ventricle, it eventually gets high enough that it's higher than the pressure out in the big arteries, like the aorta. So the aorta at rest, when the heart is at rest, is about 80 millimeters of mercury, mercury abbreviated, Hg, there's your other favorite one? And that's your resting blood pressure, what we call your diastolic blood pressure.

So if you have 120 over 80 for your blood pressure, that's that bottom number. So the ventricle's going to eventually get higher pressure than the pressure in the aorta. At that point, now we have a pressure gradient again, and the blood is going to want to flow from the high pressure in the ventricle to the lower pressure in the aorta.

So then it'll actually open a valve called  the semilunar valve and will  push out into the aorta. But at some point, the ventricle has squeezed out almost all of its blood and so it can't keep up with that pressure anymore and the pressure in the ventricle will start to drop, but there's still a lot of  pressure up in the aorta. And so once we get that reverse pressure gradient, again, the blood will try to flow from the higher pressure in the ventricle or in the aorta back into the ventricle.

And that little back flow will slam shut the semilunar valves. And that's the second sound that we hear. So it's all about pressure differentials.

And this actually brings us to one of my favorite  Anatomy facts of all of  Anatomy & Physiology, right? So think about the word circulatory system. It means circle, right?

So the blood is traveling in a circle from the heart back to the heart. But if the heart is both the start and the end and fluid flows from high pressure to pressure, it means the heart is both the highest  pressure and the lowest pressure  just at different times. >Yeah. And not just that, but a big differential, because it has to push it  through all those tissues- like, tight spaces.

Yeah, so that ventricle  can develop 120 millimeters  of mercury of pressure up in the aorta and it carries down your arm. So when you get your blood pressure cuff put on your arm, that's where it's measuring, that's kind of basically getting that same pressure from the heart. And then the atrium and the ventricle, they have to drop all the way back down to essentially a pressure of zero in order to receive the blood all the way back around the other side. >Well, a physicist will argue about pressure of zero. <True.

And this is all relative pressures kind of too, so yeah, yeah. We're all under, yeah.
With blood. <Yeah, but it's actually less pressure. >Not filling the lung with blood, filling all of the alveoli and stuff with blood. <Yeah, the capillaries with blood. Yeah, it's actually far less pressure than the other side. So the left ventricle  develops about 120 millimeters  of mercury, by the population average 120 millimeters, the right ventricle is more like 30 or 40 millimeters. >And that's what's pumping into the- <that's what's pumping into the lungs.

Part of that is the lungs have a very thin membrane between the blood capillaries and the air because we want the air to be able to pass through that membrane. >You don't want to pop those? Wildly delicate system and it works all of the time and never stops working ever I promise. Oh God.
It's actually not that bad. The signal is exactly what we talked about before. It's these waves of voltage-gated channels, sodium channels, opening and carrying the signal around the heart.

It mostly starts in what we call the sin atrial  node, which is on the upper  right corner of the heart. And it's a bunch of cells that they have actually leakage channels, we mentioned before they have some leakage sodium channels. And so sodium is leaking in constantly and causing that voltage to creep up.

And when the voltage hits the threshold voltage, the massive signal goes all the way around all the atrium and they contract and then reset and then the sodium starts leaking in and the voltage creeps up again. And so the SA node has that automatic timer, that's why we call it the internal pacemaker. >Right, so there isn't a part of your brain, some subconscious part of your brain that's like, "Okay, make sure you  keep beating the heart."  The heart beats itself. For when you need more oxygen-- because your big, big butt  muscles are pushing you along. alright. <Well, yeah, so we have this electrical signal around the heart.

There's a little pause, it can't get through those valves to the bottom, to the ventricle so there's a little delay as it goes through the atrioventricular node and then the electrical signal gets dispersed from that and causes the ventricles to contract. >Okay, so it's the same signal that's causing the ventricles to contract too? And it's a delicate system and if anything goes wrong with it, that's why you have all kinds of different heartbeat problems. Yeah. That's pretty cool.

And I'm glad that it works. All right, Brandon, I want to ask you about some   tips and tricks for learning  about Anatomy & Physiology. First of all, with regards  to learning these words. >Yes.

Lots of words. Like I said before, memorize  as little as possible. And one way to do that is to learn the root words of things.

There's a lot of Latin and Greek, it doesn't matter which one it is, but learn things like epi, E-P-I, that means upon or on top of, or you can phrase it in slightly different ways, but really it's that idea of on or around. So learn that word epi and then go find in all of the systems, or all the systems you're studying at that time, all  the words that start with epi. So you have epidermis is on top of the dermis, you have epicardium is the epithelial layer upon the heart or around the heart.

You have epinephrine, which epi is on top of or upon, and nephrine means kidneys. So you'll see words like nephron and stuff like that with kidneys. Well, epinephrine means on top of the kidneys, that's where the adrenal glands are that actually make epinephrine or we also call it adrenaline  depending on which side of  the Atlantic Ocean you're on. Right, they're epi of nephros. <Yeah, which never had occurred to me that epinephrine was at all related to even Anatomy.

I thought it was just a chemical name. >Right, right. So there you go, now you'll never forget where it is and now you know exactly what epi means and you can figure out a lot of other words, that's kind of the fun thing if you know the words, instead of memorizing, you get to figure out other things. And then back to the bone question, right?

So how do you learn all of the landmarks? Well, a lot of the landmarks have these repeating names. So you have fossas and foramen and  trochanters and grooves and a whole bunch of   names like that repeat over and over.

So pick one, like fossa, a  fossa is a shallow depression   in a bone usually where a muscle attaches. And then go find all the fossas and figure out where they are and what they look like. And then as you put all these words together,  suddenly some words start  to make a lot more sense.

So on the scapula, on your shoulder blade, there's a couple of large fossas one of them is the infraspinus fossa of the scapula. And that might seem like a kind of intimidating word at first or set of words at first. Well, infra means below, spinus is refers to the spine that runs along the scapula, not your vertebra spine, but the spine on the scapula.

And then fossa is a shallow depression. So the infraspinus fossa is the shallow depression that sits below the spine of the scapula. Once again, if you know those parts, that word is a lot easier to remember and then you can picture exactly where it is.

And even more helpful, the muscle that attaches there is called infraspinatus. And yeah, it makes it more fun I think, too. It gives you tools instead of... <Yeah.

Instead of just memorizing. Yeah, alright. You got any  other things, any other ways you  see working? >Yeah.

So a lot of my students tell me that they make flashcards and flashcards are great but I think you have to use them the correct way. And we've learned a lot and there's the Crash Course Study [Skills], the whole course that covers some of this. But one of the keys to using flashcards is to randomize them and also use them to figure out what you know and what you don't know.

And really we should all work  on our weaknesses at first. It's easier to work on our strengths, we need to work on our weaknesses. So if you have flashcards and I've had students come in with a stack of 300 index cards, beautiful flashcards, artwork, all kinds of stuff on them.

And they say, "I'm studying them. I'm not learning anything." And I will show them what to do. I'll take the whole stack, let's say this is all the bones and bone landmarks and on one side they have bones and on the other side  they have landmarks or something like that.

I take their whole stack of flashcards and I throw them up in the air as high as I can in my office. They scatter and they all flip over and then we pick them up together. Now, the order has changed and they're flipped in different directions.

So that's part one. Now that's already pretty good to just study from those, but really you kind of have to put yourself in a testing situation, you have to use what's called recall practice. And the way to do that, one way I suggest doing that is to take maybe just the top 10 flashcards, don't flip them over, don't reorganize them, exactly how you picked them up.

Take the top 10 and lay them out on your desk and then get a piece of paper and put numbers one through 10. And if the first flashcard has a term on it and the back has a definition, then you write out the definition. And if it has a definition, you write out the term.

If it has, however you have your flashcard set up, if it has a muscle name on it, you write out the bone it connects to, or however you cut it, right? You give yourself a quiz using those top 10, and then you go back... Oh, and as you're answering, add a little check mark or a star if you're really confident in your answer, that you know it, then go grade yourself by flipping over the flashcard.

So you haven't flipped them over yet, you haven't cheated on your own test. Now, flip them over, see if you got it right. If you got it right and you were confident in it, put it in a pile far away from you.

You're done. You don't need that. Right.

If you got it right, but you weren't confident put that in another pile, maybe you will get back to that, but you knew it. And unless you know it was a total guess, you don't put that aside. That's not where you really need to spend your time.

Trust yourself. Now you should be confident that you got it. The ones that you got wrong, those stay close to you and that's now your new pile.

And then that's what you study. And then you do this again. And then you study and then you do this again.

And so you're slowly moving cards into that higher confidence or the correct piles and your stack of stuff to study gets smaller and smaller and smaller and you can feel like you're learning stuff that way. And in fact, you can get this in apps and other things, the Crash Course App for Anatomy & Physiology helps you track your confidence and helps you figure out what you know and you don't know in the same way.
And I remember doing this to myself. >Yeah. Yes. That's how I learn now is teaching myself.

That takes some practice, you really do have to know what you don't know before I think you can teach yourself. And so that can be difficult. I actually started, when I first took Comparative Anatomy in graduate school out in Montana, I taught my dog.

It was just someone else to talk to. But she had big rippling muscles in short bursts so when I was learning all the muscles, I could pet her. She enjoyed just being pet, any attention  she could get, but I would  pet her and name the muscles.
Get a dog, but you got to make sure it's not very shaggy, or one of those hairless cats. >Right, right, right. And so you can see the muscles. But teach anyone.

I have students that say, "I don't have anyone to teach. My roommate is an English major." Perfect, teach them. They'll get really bored.

But they understand you know it. <I'll tell you what, my wife hates this about me, but she knows so many things now. >I'm pretty sure my wife would say the same thing. <"I have to tell you about this thing I learned." >Yeah, yeah, yeah. It's definitely the best way because like you said, it helps you process and reformulate your own ideas so that someone else, even if that someone else is you, can understand it.
We have a couple of chat questions. I'm going to ask you a chat question. And I'm curious about this from, from Katrina who asks, "What happens when a muscle cramps?

Why am I in pain?" >You know what, that's a good question. That is not in my wheelhouse. So I can't give you a definitive answer. <It's a muscle! >I know, I know.

And I actually am a muscle physiologist, but for birds and they never tell  me when their muscle is cramping. But what I will say is, so I'm not defining what a muscle cramp is, but you can think of all the steps of a muscle contraction and what eventually could go wrong if the muscle is cramping and it's actually contracting. I do teach students about different kinds of toxins and venoms as a way of learning how muscles contract. So you can have things that are kind of going wrong on the nervous system side, either the brain is constantly sending a signal or the neuron is firing on its own too much or the acetylcholine that's floating across and binding to its channel, there's something wrong with that channel and so the muscle cell thinks it's constantly being told to contract.

You can also get problems in the muscle itself where you can have say too much calcium in the muscle and that's the final signal for the actual contraction phase. You can get electrolyte imbalances, right? There's a lot of things that can interfere with that nice clean system of signals that we've talked about that could potentially cause a muscle cramp.

But as far as a cramp during  exercise, I definitely  don't know enough to give a definitive answer. Yes. Muscle contraction in general changes blood flow and can constrict it.

And muscles hurt a lot, like during a heart attack, even cardiac muscle hurts a lot when the oxygen delivery rate  is too slow for the demand. Yeah. Although importantly, this is surprisingly not well known, for women having heart attacks that pain is not usually- Yeah, it's often actually more like fatigue. Yeah, the pain can show up in different places.
Stupid bodies. William had a question, do you have any tricks for remembering the veins and the arteries? >You know, I actually do. It worked for me, I think it works for a lot of my students, and that's to draw a map.

And like I said before with  the bones, start simple. And the best maps, they're not really accurate. They actually are easier to follow.

So think of a subway map or a transit map where you can see the order of things and you can see the connections, but it's not like it's geographically 100% accurate. So if you draw your map and just start by thinking, "I'm giving someone directions to the spleen or to the stomach, how do I get from the heart down there?" And you just learn that part first. And then you say, "Well, what if I also wanted to go down to the leg?" Then you go to the spleen, you draw your map to the spleen so just to refresh your brain, and then you continue, you go past that turn and you go to the leg and you label it.

So again, start with just a few arteries and veins and label them and then build up on that. Every time you redraw it, just add a few more, adding a few more turns. It's like learning your way  around a new city, right?

You learn just one simple path from home to work, and then you start learning  the scenic routes around that. Once you get that pathway down, then say you're dissecting, you're looking at a much more realistic model, it's much easier to find the actual arteries and veins because you can always go back to the aorta and start from there, start from where you know, and then follow the arteries and veins out in the dissection. And if you know your map well enough, then you will be able to follow the actual things. <Right.

And also you know where you lose track, if you're following a map you know and then you reinforce the most common boulevards, the bigger roads. And so every time you're going down, you're reinforcing that, the most important and the most common bits before you get to the branches that are going to be harder to remember because there are so many of them. >Correct, yeah. And that's kind of part of spaced repetition, which is the learning strategy of repeating, but making sure to space it out over days or weeks or even longer- <It's so hard to do because that is not how I am   tested or was tested. >No, it's not, no. Until maybe the final exam. <Yeah, exactly.

Yeah. >But if you're in an anatomy class sure, sure you probably need to get a certain grade to continue on in whatever program, but it's probably not the last time you're going to see this stuff and that's big spaced repetition. You see it maybe first year in college or community college and then you might not see it again until four years later. But if you work hard in that first year, it'll be there. <It's amazing how much stuff is still there.

I recently started learning Spanish again and I hadn't looked at it since my freshman year of college and I was like, "Wow, there's a fair amount of Spanish still in this brain." So yeah, they're amazing organs. Okay. Well.

I feel as if I learned some wonderful things about Anatomy & Physiology. So thank you, everybody, for asking thoughtful questions, and thanks again to Flipgrid for sponsoring the livestream, making it all happen, and you can check them out, there's a link to them in description below. Brandon, thank you very much for all of your expertise and yeah, I just really appreciate seeing you again. >Yeah.

And Hank, thank you  for Crash Course, I know  it's helped a lot of my students  in lots of different classes. I think it's been a great resource. <Well, thanks so much. Thank you for contributing to it and  making Anatomy & Physiology possible.

Thank you all for joining us. I have been Hank Green,  that's been Brandon Jackson. Thank you.

It's been a good old time!