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  • I'm going to be totally honest with you:

  • I don't really spend a lot of time thinking about my bodily functions.

  • For the most part. Maybe sometimes.

  • But in the next few episodes, I'm going to be talking

  • about all of the organ systems that make our lives possible,

  • even occasionally pleasant!

  • And to start it all off, I'm going straight to mission control:

  • the Nervous System!

  • Pretty much every animal, except for some really simple ones,

  • have nervous systems, which is great,

  • because it's what lets things do things like: have behaviors.

  • It makes you the sentient, living thing that you are.

  • The whole set-up here: your brain, your nerves,

  • your spinal cord, everything

  • is made up of specialized cells that you don't find

  • anywhere else in the body.

  • Most of those are neurons, which, you've seen them before,

  • they look kind of like a tree with roots, a trunk and branches.

  • Neurons bundle together to form nerves,

  • pathways that transmit electrochemical signals

  • from one part of your body to another.

  • So, when you bite into a piece of pizza-

  • I love it when there's pizza in the video...

  • The receptor neurons in my taste buds recognize

  • I'm eating something salty and fatty and awesome.

  • And they carry that information along a nerve pathway to my brain.

  • And then my brain can be like "Yeah! Pizza!"

  • and then it can respond by sending back information

  • through different nerve pathways that say:

  • "You should eat more of that pizza!"

  • And despite what my brain is telling me,

  • I'm going to try to not eat any more of that pizza.

  • You wouldn't think that it's terribly complicated

  • to know that pizza tastes good and to tell someone to eat more pizza.

  • But it turns out that our brains

  • and our nervous systems are crazy complicated.

  • Your nervous system basically has a big old bureaucracy of neurons,

  • and it's divided into two main departments:

  • the central nervous system and the peripheral nervous system.

  • Central and peripheral.

  • The central nervous system,

  • basically your brain and your spinal cord,

  • is responsible for analyzing and interpreting

  • all those data that your peripheral nervous system,

  • all of the nerves outside of your brain and spine,

  • collects and sends its way.

  • Once the central nervous system makes a decision about data,

  • it sends a signal back through to the peripheral nervous system

  • saying "Do THIS thing!"

  • Which the peripheral nervous system then does.

  • Both of these systems contain two different types of neurons:

  • afferent and efferent.

  • Afferent and efferent are biological terms,

  • and they're horribly confusing, and I apologize

  • on behalf of the entire institution of biology for them.

  • Afferent systems carry things to a central point,

  • and efferent systems carry things away from a central point.

  • So afferent neurons carry information to the brain

  • and spinal cord for analysis.

  • In the peripheral nervous system,

  • afferent neurons are called sensory neurons,

  • and they're activated by external stimuli

  • like the complex and glorious flavor of pizza

  • and then they convert those data into a signal

  • for the central system to process.

  • The central nervous system has afferent neurons too,

  • and there they bring information into special parts of the brain,

  • like the part of the brain that goes, "Mmmmmm, salty!"

  • Efferent neurons carry information out of the center.

  • In the peripheral nervous system,

  • they're called motor neurons because many of them

  • carry information from the brain or spinal cord

  • to muscles to make us move,

  • but they also go to pretty much every other organ in your body,

  • thus making them, like, work and do stuff to keep you alive.

  • In the central system, efferent neurons carry information

  • from special parts of the brain to other parts

  • of the brain or spinal cord.

  • Of course if it ended there, it would be way too simple

  • and no good bureaucracy just has two departments.

  • So the peripheral nervous system is actually made up of

  • two different systems with two very different jobs:

  • the somatic nervous system and the autonomic nervous system.

  • The somatic system controls all the stuff you think about doing

  • like all the information coming through your senses,

  • and the movement of your body when you want it to make movements.

  • But here's something interesting:

  • Since we're totally in love with our brains

  • as sort of the center of all being, of ourselves,

  • we think that all the information about

  • everything going on in our bodies goes to our brains

  • for some kind of decision.

  • Not so!

  • Sometimes, like when we touch a hot stove,

  • the afferent neurons carry the signal "HOT!"

  • to the central nervous system, but that information

  • doesn't even ever get to the brain

  • the spinal cord actually makes that decision

  • before it gets to the brain,

  • sends a message directly back to the muscle saying,

  • "Get your hand off the freakin stove, *******!"

  • This bit of fancy nerve-work lets the spinal cord

  • make decisions rather than the brain, it's called the reflex loop.

  • So, the other branch of the peripheral nervous system,

  • the autonomic system, carries signals

  • from the central nervous system that drive all of the things

  • your body does without thinking about them:

  • your heartbeat, your digestion, breathing,

  • saliva production, all your organ functions.

  • But we're not done yet here.

  • We need to go deeper.

  • The autonomic nervous system has two divisions of its own:

  • the sympathetic and parasympathetic.

  • And the jobs that these two perform aren't just different

  • they're completely opposite, and frankly,

  • they're always vying for control of the body

  • in some kind of nervous system cage match.

  • The sympathetic division is responsible for, like,

  • freaking out.

  • You've probably heard this talked about

  • as the fight-or-flight response.

  • In other words, stress.

  • But stress isn't all bad:

  • it's what saves our lives when we're being chased

  • by saber toothed tigers, right?

  • The sympathetic system prepares our body for action

  • by increasing the heart rate and blood pressure,

  • enhancing our sense of smell, dilating the pupils,

  • activating our adrenal cortex to make adrenaline,

  • shutting down blood supply to our digestive

  • and reproductive systems so there will be

  • more blood available for our lungs

  • and muscles when we have to, like, RUN!

  • Even though you're not in a constant state of panic

  • at least, I hope not, I kind of am

  • that system is running all the time, every day.

  • But right next to it is the parasympathetic division,

  • working hard to make sure we take it nice and easy.

  • It dials down heart rate and blood pressure, constricts our lungs,

  • makes our nose run, increases blood flow

  • to our reproductive junk, our mouths produce saliva,

  • encourage us to poop and pee.

  • It's basically what we have to thank for taking a nap,

  • sitting in front of the TV, going to the bathroom and getting it on.

  • So, consider yourself lucky you've got both the stress response

  • and the chill-the-heck out response, working side-by-side

  • because together they create a balance, or a homeostasis.

  • Now, that's what the nervous system does.

  • Next we have to talk about how it does it.

  • The neurons that make up our nervous systems make it possible

  • for our bodies to have their very own little electric systems.

  • So to understand how they work you have to understand their anatomy.

  • Like I said before, a typical neuron has branches like a tree.

  • These are called dendrites, and they receive information

  • from other neurons.

  • Neurons also have a single axon the trunk of the tree

  • which is branched at the end and transmits signals to other neurons.

  • The axon is also covered in fatty material called myelin,

  • which acts as insulation.

  • But the myelin sheath isn't continuous,

  • there are these little bits of exposed neuron along the axon,

  • which have the sweetest names in this whole episode

  • they're called the Nodes of Ranvier.

  • Which seems like an excellent working title for the

  • 8th Harry Potter novel.

  • Harry Potter and the Nodes of Ranvier.

  • Anyway, these nodes allow signals to hop from node to node,

  • which lets the signal travel down a nerve faster.

  • This node-hopping, by the way, has a name.

  • It's called saltatory conduction.

  • Conduction because it's electrical conduction

  • and saltatory because saltatory means leaping.

  • Finally, the place where an axon's branches come in contact

  • with the next cell's dendrite is called a synapse,

  • and that's where neurotransmitters pass information from one neuron

  • to the next.

  • Now, think back to, or just go watch the episode

  • we did on cell membranes, where we talked about

  • how materials travel down concentration gradients.

  • Well, in much the same way, all neurons in your body

  • have a membrane potential, a difference in voltage,

  • or electrical charge, between the inside

  • and the outside of the membrane.

  • You might also remember that this buildup of voltage

  • is handled in part by a sexy little protein

  • called the sodium-potassium pump.

  • Basically, the pump creates a voltage differential,

  • like charging a battery, by moving 3 positively charged

  • sodium ions out for every 2 potassium ions it lets in,

  • creating a net negative charge inside the cell

  • relative to the outside.

  • When a neuron is inactive, this is called its resting potential,

  • and its voltage is about -70 millivolts.

  • But in addition to the pumps, neurons also have ion channels.

  • These are proteins that straddle the membrane,

  • but they're a lot simpler and don't need ATP to power them.

  • Each cell can have more than 300 different kinds of ion channels,

  • each tailored to accept a specific ion.

  • Now, don't zone out here, because all of this stuff

  • has got to come into play when a neuron becomes active.

  • This happens when an input or stimulus creates a change

  • in the neuron that eventually reaches the axon,

  • creating what's called an action potential

  • a brief event where the electrical potential of a cell

  • rapidly rises and falls.

  • When action potential begins, like when a molecule of sugar

  • touches one of my sweet tastebuds, some ion channels open

  • and let those positive sodium ions rush in,

  • so that the inside starts to become less negative.

  • With enough stimulus, the internal charge of the neuron

  • reaches a certain threshold, which triggers more sodium channels

  • to respond and open the flood gates to let even more ions in.

  • That's happening on one tiny little area of the neuron.

  • But this change in voltage creeps over to the next bunch

  • of sodium channels, which are also sensitive to voltage,

  • and so they open.

  • That exchange triggers the next batch, and the next batch,

  • and so on down the line.

  • So this signal of changing voltage travels down

  • the neuron's membrane like a wave.

  • But remember, the myelin sheath insulates most of the neuron,

  • and just leaves those little nodes exposed,

  • so instead of being a steady wave, the wave jumps from node to node,

  • speeding up the travel time of action potential down a neuron:

  • That's your saltatory conduction at work!

  • When the wave reaches the end of the neuron,

  • it triggers the release of neurotransmitters from the neuron

  • through exocytosis, and those neurotransmitters then float

  • across the synapse to the next neuron

  • where they trigger another action potential over there.

  • Now, by this time, so many sodium ions have gotten inside

  • the first neuron that the difference between the outside

  • and the inside is actually reversed:

  • The inside is positive and the outside negative.

  • And it seems like neurons hate that more than

  • pretty much anything else, so it fixes itself.

  • The sodium channels close and potassium channels open up.

  • The positive potassium ions rush down both the concentration

  • and electrochemical gradients to get the heck out of the cell.

  • That brings the charge inside the cell back down

  • to negative on the inside, and positive on the outside.

  • Notice, though, that now the sodium is on the inside

  • of the cell and the potassium is on the outside

  • they're in the opposite places of where they started.

  • So, the sodium-potassium pumps get back to work

  • and burn some ATP to pump the sodium back out

  • and the potassium back in, and phew!

  • Things are now back at the resting potential again.

  • So, that, my friends, is how action potential

  • allows neurons to communicate signals down a whole chain

  • of neurons from the outer reaches of the peripheral nervous system,