The Nervous System - Control & Coordination Video Lecture - Class 10

hi it's mr. Andersen and welcome to
biology essentials video 41 this is on
the animal nervous system and most of
this podcast I'm going to be talking
about these things over here these are
called neurons how they send messages
called action potentials and how they
can eventually jump off across these
synapses but I want to start with big
picture and nothing gets more big
picture in the nervous system than the
brain and so this is what your brain
looks like looked from above it has two
different hemispheres so you have a left
hemisphere and a right hemisphere and
we're starting to learn more about the
two different hemispheres there are
certain things like speech is clearly
centered over here on the left side your
handedness has to do with a lot of which
of these is dominant but vision is
pretty interesting and so you have eyes
that will say or write up a year and so
when you look at a sign let's say you're
looking ahead this side of the sign
actually is being picked up by this side
of your eye and that information is
eventually going to the right side of
your brain and so if you look straight
ahead everything in your right side of
your field of view actually goes to your
left brain or left hemisphere and
everything on your left goes to your
right hemisphere now luckily it is
connected in other words we have a
corpus callosum that connects the two
sides of our brain and so we can share
information back and forth but
occasionally our brain goes haywire and
we get what's called a seizure or an
electrical storm that goes all the way
across your brain and if you have
epilepsy this gets to be a huge problem
and so a really radical procedure that
scientists will do is they'll
occasionally sever this corpus callosum
and when you sever that corpus callosum
the two hemispheres can't communicate
and if you look at people who have this
split brain yeah they look really normal
but occasionally you can trick them in
and you start to discover how the how
the brain is really set up so let's do
an example of that so what I have right
here is in a plus sign so if what I want
you to do is stare at the plus sign and
I'm going to flash an image to the right
that's going to go to your left brain
but always stare right at the plus don't
stare off to the
right and so we had an image that showed
up and you would say that was an image
of the earth and so you can say that
because you're using the left portion of
your brain let's try it again that was
pretty big flash that's a flower and so
you can say that it's a flower and if
you do this to one of those split brain
people they'll do the same thing they'll
say that was the earth and then they'll
say that was a flower because they're
seeing that on the left side there are
speeches on the left side and so they're
good to go
but now let's put one on the left side
of your field of view so that's going to
go to your right brain so let me flush
one there and you can say was a Sun or a
cartoon of the Sun you would show that
to a split brain person and they would
say I didn't I didn't see anything and
so that seems a little weird and the
reason why is it's going over to the
right side of their brain where they
can't actually make speech to explain
what that was now what's interesting in
these is you can actually give them a
piece of paper and say I know you didn't
see anything but could you draw what you
might have seen and they'll start to
draw like a picture of it because they
did see it and then they might get to
this point it's a flower I don't know
what it is or it's oh it's like a Sun
and so once they see it then they could
say it so it's like playing pictionary
with yourself so that's weird but that
shows you how there are different
portions of our brain and those
different portions of our pain our brain
do different things and so in this
podcast I'm going to talk about nervous
system we've already talked about the
brain and has different portions like
hemispheres but we're going to spend
most of the time talking about the base
unit of the nervous system which is the
neuron neurons can send messages those
are called action potentials action
potentials work by polarizing a cell in
this case it's the neuron they set up
this polarization through sodium
potassium pump if you remember that's a
form of active transport and then when
they depolarize it by opening up these
ion channels then we can send a message
down the neuron now when that message
gets to the end of the neuron jumps to
another neuron and those neurons aren't
usually connected there's a gap between
the two and that's called a synapse and
so
is not a chemical-electrical message
going through it it's actually
neurotransmitters which are chemicals an
example I'll give you is GABA and what
those do is send a message to the next
neuron those messages can either be
excitatory so yes you need to keep that
keep that message going or inhibitory no
you need to stop the message right here
and so the synapse gives us a lot of
control over that message and what
happens when it gets to the next neuron
and so this is a neuron this is a basic
neuron that actually has two different
parts this part up here is going to be
the dendrites dendrites are going to be
I always use my hand so if a neuron is a
your arm these things up here are going
to be the dendrites it's a regular got a
cell body but then this portion right
here is called the axon and so most of
what we think of as a neuron is is
called the axon I don't think I added
that here so that's a X o n okay so
we've got the dendrites and the axons if
we label some of these other things this
right here is going to be the nucleus
it's a regular cell so it's going to
have all the parts of a cell this is
going to be the cell body or we
sometimes refer to that as the soma then
this whole thing I would say from here
to the end is going to be called the
axon a Xon 2-speed that we have it
wrapped in this fat material called
myelin and those are actually made of
something called a Schwann cell that
wraps its way around the axon and then
we have nodes of ranvier
and nodes of ranvier are going to be
these gaps between the myelin and really
what happens and I'll come back to that
at the end is that the message actually
it doesn't go all the way through the
axon it's able to jump from spot to spot
because we put all the ion channels
right here so it speeds it up so if you
think about a neuron like a wire it's
kind of like that and then this myelin
sheath is going to be like the
insulation that wraps around the outside
and it speeds up that message as it goes
at the end we then have a terminal what
would be next well we'd have the next
neuron that's going to be connected
right there with a synapse or a gap
between the two now if you've ever heard
me talk about neurons before you've
heard this I always like to say that
nerves or neurons a nerve is just a
bunch of neurons together that a neuron
is simply a salty banana
and that allows you to remember where
the ions are and so what do you know
about a salty banana so this visual over
here on the left side is super important
to remember we've got a salt grinder up
here it's going to put salt on the
surface of a banana well what do you
know about salt salts going to be high
in sodium and what do you know about a
banana it's going to be high in
potassium and so if we look at let's get
rid of the salty banana for a second if
we look at a neuron neuron is going to
and we're just I've just drawn right
here a section of it so we're just
taking one portion of a neuron so up
here we're going to have the dendrites
down here we're going to have the axon
terminal so this is just a portion of
the axon you've got sodium out here a
lot of sodium ions out here and you've
got potassium ions in the inside you
have other things in here like this is a
protein that I've drawn in here but the
big ones to remember our sodium and
potassium and then you have these
channels channels are proteins that
allow sodium and potassium to pass if
they're not open you can you shall not
pass and so I kind of color-coded those
and so if you look at it every sodium
ion is going to have a positive charge
that these are positive and every
potassium is going to have a positive
charge as well so if you count the
number that we have on the outside and
the number that we have on the inside we
have a lot more sodium on the outside
and or excuse me positive charge than we
do on the inside and these proteins are
going to have negative charges as well
and so what does that mean it's more
negative on the inside and it's more
positive on the outside and we can
actually measure that in a typical
neuron there a typical nerve cell it's
going to have a voltage of negative 70
millivolts what does that mean it's a
battery and so there's a battery across
our neuron and so it has energy or has
the ability to do work across that
membrane and so it sits at negative 70
millivolts
now what happens let me get some of this
out of the way is that something will
happen to trigger the opening of this
first channel and so right now you're
seeing light and that's actually opening
up sodium channels so if you open up
sodium channels where is the sodium
going to want to flow well there's a
whole bunch of sodium out here
and so it's going to move or it's going
to diffuse along its gradient and so if
we open up that sodium channel what's
going to happen the sodium is going to
flow in now what's that going to do it's
going to actually change the voltage so
it's going to change the voltage
remember it used to be negative 70
millivolts and now it's going to be less
than that so maybe it's going to be
negative 50 millivolts what does that do
well these channels not only allow
sodium and potassium to move through
they're actually activated by changes in
the voltage and so when this goes to
negative 70 what that does is it opens
up the next sodium channel a little bit
down the way now potassium channels
aren't affected by this and so what is
going to happen next well that's going
to open up the sodium channel a little
bit farther down and that's going to
open up the sodium channel a little bit
farther down as well and so we get flow
of sodium we get this cascade it's
almost like dominoes this cascade of
sodium which triggers the next sodium to
go and the next sodium to go and so
neuron isn't passing electricity what
it's doing is is opening up these
channels and it's allowing those
chemicals to influence the next channel
which opens that up and so you have this
cascade almost like dominoes flowing up
so now where is our charge now we have a
positive charge here and a negative
charge down here these sodium channels
then are going to close up so now sodium
can't pass but the potassium channels
are going to open up so when the
potassium channels open up we have all
this positive charge in here so what's
going to happen to the potassium
potassium is going to flow out what's
going to happen to our voltage again our
voltage is going to become more negative
and and we're going to kind of get back
to an equal charge on either side for
just a moment now what happens next well
we have to reestablish that gradient and
to do that we use something called the
sodium potassium pump and so when you
see light you're sending literally
thousands of action potentials down a
neuron between each of those action
potentials the sodium potassium pump is
going to reestablish that gradient and
so let's actually put a graph to that
and so this is what it looks like at
rest remember it's at negative 70
millivolts
and so here's our axon it's at negative
70 millivolts in other words it's it's a
negative charge here and a positive
charge here
what was the first thing that happened
remember that sodium opened up and so we
add sodium channels moving in here so
what happens to this phase well our
neuron which is which is polarized is
going to depolarize and so it's going to
move towards the positive now if it
reaches this point that's called the
threshold it's going to have an action
potential in other words sometimes we'll
get a little bit of flow but each of
these are a failed initiation in other
words until we hit this critical point
and in all animals it's negative 50 55
millivolts once we hit that negative 55
millivolts then this is going to be an
all process and so what's going to
happen is all the sodium is going to
flow in what does that do that makes our
our charge go really positive then what
happens we're going to open up our our
potassium gates that's going to flow in
the other direction and we're going to
have this plunging falling phase and
then we have what's called the
undershoot as it resets itself with the
sodium potassium pump now the reason we
have this undershoot is that we don't
have an action potential going in both
directions we want it to be directional
we want to move in the direction of the
axon and so then there's another action
potential so what does that mean um if I
were to flick myself on the finger like
that that pain that I experienced is a
number of action potentials that make it
that was a really bad one a number of
action potentials going to my brain and
I'm perceiving that as pain if I were to
flick it lightly like that it's still
going to be the same size of action
potentials but they're not going to be
close together and if I were to cut my
finger and that would be a huge amount
of action potentials really close
together and so what goes to your brain
are simply action potentials your brain
then has to decide well where is that
coming from is it coming from my nose is
it a smell is it from my ears is it from
my eyes is it vision all the neurons
transmit that same action potential to
your brain but your brain decides on
where it's coming from and as a result
of that decides on what it is now before
it can get your brain it actually has to
go across a number of gaps and so a
synapse is a gap between two different
neurons and so that action potential is
going to go down this neuron and then
it's going to go down this neuron in
other words we're going to have opening
the sodium opening the sodium opening
the sodium opening the sodium it's
discharging it and then we're opening
the potassium open the potassium open so
the action potential is going to move in
this direction down here but eventually
it gets down here to the synapse and it
doesn't just flow across the side
doesn't just flow across this gap what
happens is that you're going to get an
influx of something called
calcium so calcium ions are super
important for nerves to function
correctly and then you have these things
these neurotransmitters
neurotransmitters are going to be
chemicals and those neurotransmitters
are going to float across the gap and so
once the action potential gets to the
end so this is B the presynaptic side
it's going to get an influx of calcium
that's going to release these
neurotransmitters and these
neurotransmitters are going to float
across the gap so they're going to move
from narrate high concentration to low
concentration they're going to they're
going to match up right here with
another ion channel on the other side
and they're going to change its shape so
it can take ions in now we've got sodium
flowing in potassium flowing out and the
action potential moves in the other
direction a very famous example of that
is G a B a GABA but we can have a number
of different chemicals that form as as
neurotransmitters that move across GABA
is actually a negative neurotransmitter
and inhibitory neurotransmitter and so
if gaba flows across the to the side
it's going to actually hit receptors on
the side that say don't send an action
potential don't send an action potential
but there are likewise going to be a
number of different ones that are
excitatory and they're going to send a
message across that says action
potential go action potential go and so
is your brain like a computer not really
on a better way to do it think about it
is like a vote and so if we have this
action if we have this neuron right here
let's say this one is headed to the
brain for example um it's going to get a
number of different message
is from a number of different neurons so
the connections are super important some
of those messages are going to be
inhibitory let's say inhibitory is is
red and so they're going to say you need
to fire you need to fire you need to
fire you need to excuse me inhibitory so
they're going to say don't fire don't
fire don't fire there's also going to be
a number of those that are excitatory
we'll make those in blue and so they're
saying fire fire fire fire and so all
those connections are saying fire and so
depending on the amount of excitatory
and an inhibitory that we have it's
either going to fire or it's not going
to fire let's go back to that diagram
that we have remember it's it's at
threshold and then an action potential
is going to depolarize and then it's
going to repolarize and then it's going
to go like this again or excuse me let's
if that's bad let's do that again so
it's going to reach up it's going to
have an undershoot and it's going to
come back to here and so what's really
happening this is at negative 70
millivolts every time we get an
inhibitory that's actually going to be
pushing the voltage of this in this
direction every time we get an
excitatory message is going to be
pushing it in the other direction until
we hit that threshold of negative 55 and
so again all these neurons are voting
they're voting should it fire shouldn't
it fire and as a result of this so this
one passes that message on and so what's
super important in nerves is is not only
sending a message for example from my
toe when I step on a tack all the way to
my brain saying move your toe you're on
the tack it's the connections in your
brain it's all these inhibitory and
excitatory messages that are forming
memories and so you're forming memories
right now and those memories are
connections between the neurons in your
brain and the more information that you
can get be it auditory verbal the more
connections you can make the more likely
you are to remember this in the future
and so that's a lot as far as learning a
lot of information to to hold on to but
that's the nervous system it's really
cool it's really important and I hope
that's helpful