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.