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, all the way up the spinal
cord and to the brain, and then back out agian. So, let's zoom out, and look
at the broad view here. I'm gonna take a
bite of this pizza. All my tastebuds have
neurons in them. Each of my taste buds
contains between 50-100 specialized taste
receptor neurons. Chemicals from this beautiful
pizza dissolve in the saliva and then stimulate the
dendrites on the afferent neurons. This generates a bunch of
action potentials that travel from the afferent neurons in
my tongue all the way to my brain, which is like, "My goodness,
I think that's pizza! Let's have another bite!" The brain then sends
messages through the efferent nerve pathways
to do all sorts of things: 1. Chew, which involves
constricting the muscles in my jaw over and over again. 2. Lower my head down
to catch another bite, which involves moving all
kinds of neck muscles. 3. Swallowing, which involves
constricting the muscles in my throat and esophagus. 4. Opening my mouth again
to receive another bite. That signal is also
going to my jaw. And that's not even to mention
what's going to go on with the digestion
of this bad boy, driven by the autonomic
nervous system. But digestion is still
a couple episodes from now. Hopefully there
will be more pizza. Thank you for watching
this episode of Crash Course, and for giving me an
excuse to eat more pizza. If you want to review what
we learned in this episode, check out the table of contents. Thanks to everyone
who put this together. If you have questions for us: Facebook, Twitter,
or the comments below. We'll see you next time.
