Action Potentials and Synapses: Nervous System Physiology | Corporis
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Action Potentials and Synapses: Nervous System Physiology | Corporis


Today, we’re gonna keep the nervous system
train rolling and getting into some physiology. We’re talking about action potentials, and
synapses, and neurotransmitters and we’ll finally answer that counterintuitive question, just how electric is our body’s electrical system [intro] Okay, big picture: the nervous system gets
its “electric” label because it transmits something called action potentials, and whether
you want to call that a signal, or an impulse, action potentials are the electric part. Action potentials aren’t exclusive to the
nervous system. Like the pacemaker cells of the heart also
make action potentials. It’s a similar electrical impulse that’s
responsible for that classic ECG waveform. Either way, these things don’t start as
like a lightning bolt in your brain. They start out as /chemicals/. So let’s get real zoomed in here. There are plenty of individual chemicals in
and around the neuron, but we only care about three right now: sodium and potassium which
are positively charged ions, and chloride which is a negatively charged ion. /At rest/ before any action potential starts,
there are more sodium ions outside the neuron than inside the neuron, vice versa for potassium. And the membrane isn’t like a super exclusive
gatekeeper. Potassium actually seeps through really easily,
while /some/ sodium gets through too, but not nearly as much as potassium. So there are some of each ion on /both/ sides
of the membrane, but at rest, this is the general pattern we see. This is what’s called a concentration gradient,
there’s a difference in concentrations for each of these ions across the membrane. And because that cell membrane isn’t super
strict on what it keeps in or out, at rest, these ions will tend to even out the concentration
gradient. Now, this is where electricity and this graph
come in. Look familiar? It’s because every teacher loves to quiz
on it, so we’ll use it too you’re welcome ahead of time. /BUT/ what are you actually looking at? The Y axis is the membrane potential. It’s a voltage, which means the /difference
in charge/ between two points. At any given point on this graph, we’re
looking at how much more positive or how much more negative each side of the cell membrane
is from the other. So if the outside of the membrane is very
positive and the inside is very negative, that’s a /big/ voltage. Meanwhile, the x axis is time, and while there’s
/some/ variation depending on which source you read, we’re talking units of /milliseconds/
here, super fast. So if we apply this to resting membrane potential,
the graph starts out super negative since there were all those positive ions outside
the cell and very few inside the cell. On the other hand, the inside of /this/ cell
would be more positive. Look at all those positively charged ions
inside the cell. Ooookay. I think we got enough background that we’re
gonna understand action potential. So let’s say some kind of neurotransmitter
— it’s an excitatory neurotransmitter and it’s like “hey. WAKE UP!”
it allows loads of positive ions into the neuron. So now the inside of our cell is getting more
positive, and our graph ticks up. And if it gets up to -55 millivolts, we cross
the “threshold of excitation” where the axon hillock throws its sodium channels open. This is because those particular channels
are voltage gated channels, they literally open with voltage. But as we’ll see in /other/ videos, not
/every/ channel opens with voltage. Now all of a sudden, these positive sodium
ions rush in, and more positive ions inside the cell means our graph goes up as well. This usually tops out around +30 or +40 millivolts. This part of the action potential, that crossing
of the threshold and positive shoot up, or depolarization, is all or nothing. If the voltage wiggles between negative seventy
and negative 56 millivolts, no depolarization. So after that action potential kicks off,
a bunch of voltage gated channels upon up all along the axon and in the period of less
than a millisecond, an action potential went from soma to terminals. At the peak of that action potential, the
sodium channels close and potassium channels open. This is repolarization, and on our graph,
this looks like a sharp drop in membrane potential since that bulk of positive ions is exiting
the cell, and the inside of our cell is starting to become more negative. Quick aside, students get depolarization and
repolarization mixed up all the time. So I like to think of the negative part of
this graph as like the South Pole, and when you’re going away from it, you’re depolarizing,
but the when you go back down, you’re /re/polarizing, you’re going back to the pole. Now during repolarization, those sodium channels
/will not open/. And if you can’t get sodium into the cell,
you won’t get that concentration gradient, so no action potential. This is called the refractory period where
you could not stoke another action potential no matter how big of a stimulus you had. And while sodium is slipping out, a few more
potassium ions slip out as well. This creates something called hyperpolarization
since the voltage ends up more negative than when we started. This makes for that dip below baseline we
see on the graph. Okay, deep breath in, deep breath out. We’re almost done. Now, all that’s to say we took a message
from the dendrites down the axon and now we’ve gotta do something with it. Luckily at the end of the axon are axon terminals,
and they butt up against other cells in units called synapses. And there are two types of synapses: electrical
synapses which are what we call gap junctions and chemical synapses. Electrical synapses are easy. Really tiny gaps between each synaptic membrane
let ions through that make for super fast signaling. On the other hand, chemical junctions are
trickier so we’re gonna go more in depth with those. Okay, quick lay of the land here. Within the synapse we’ve got the axon terminal
releasing those neurotransmitters to a dendrite, or gland or muscle, and in between them is
the synaptic cleft — that dead space there. Because that receiver can be so a few things,
we end up labeling those surfaces the presynaptic membrane and postsynaptic membrane. For this video, we’re gonna both assume
they’re both neurons. So when the action potential gets to the end
of the line, it triggers these little containers called synaptic vesicles, little pockets wrapped
in membranes that hold chemicals called neurotransmitters — each of which can signal for something
different. These things are made from kinds of different
chemicals and amino acids, and as of 2019 we know of more than 200 of them. You’ve heard of the mainstream ones like
serotonin or dopamine, but everyone knows you’re not a true anatomy student until
you’ve got an underground favorite, like GABA or glutamate. You gotta be a neurotransmitter hipster. Point is, there’s a bunch to pick from. When the action potential gets to the terminal,
it shoves those vesicles to the edge of the membrane and pushes that pocket of neurotransmitters
out. And because each of these neurotransmitters
has a unique shape, they fit into unique receptors on the postsynaptic membrane, like a lock
and key. So neurotransmitters get released, float through
a super tiny space, and land on the postsynaptic membrane. It can excite the neuron and create another
action potential and get it to do something, or inhibit the neuron and get it to chill
for a second. From there, the process of action potentials
and synaptic transmission can happen alllll over again. And this is happening ALL OVER YOUR BODY. Right, if we’re talking about the brain
we’ve got something on the order of a hundred trillion synapses to work with, but they’re
way less dense at places like the synapse between neurons and muscles. And that interaction between muscles and nerves
ends up being a whole beast in itself, so I made a video about it that I’ll link right
here. And if you need help with other nervous system
topics, check out this playlist and I’ll hopefully be able to help you. Otherwise, thank you to my Patrons Jessica
and Diana. Super appreciate you two! Have fun, be good. Thanks for watching.

8 thoughts on “Action Potentials and Synapses: Nervous System Physiology | Corporis

  1. There are a TON of asterisks in regards to action potentials. When we learn cardiac APs for instance, we're gonna have to introduce some new chemicals and waveforms, but for your general nervous system prep, this video ought to get you through. What questions do you still have?

  2. We couldn't have planned the topic of our nearly simultaneously-released videos better! Awesome. I'll link yours in my description.

  3. Dude, the graphics are looking superb. This must've taken you ages. Excited to see the catalogue of videos you're going to build, will end up being a superb body of work for many students to use.

  4. Love the video! 😋 Love how you concised such a huge topic into a short video without missing any key terminology! 😋 Gonna save the video as it will come in handy for my revision. 😋😋

  5. Oxytocin is mine. Cuddly chemicals are da best!

    Your going to metion Chloride and then just drop it. What you gotta against Chloride? Didn't even mention calcium the gate keeper.

  6. Wow, this one is much more complicated comparing to previous videos.
    Insane engineering.
    Well-done, nature, well-done!

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