Motor neurons | Muscular-skeletal system physiology | NCLEX-RN | Khan Academy

Motor neurons | Muscular-skeletal system physiology | NCLEX-RN | Khan Academy


When we’re at a dance
party, and we’ve determined that this is an
appropriate time to dougie, how does our brain
tell our body to do so? Well, in this video,
we’re going to be talking about motor neurons. Motor neurons. These are the nerve cells
that come from our brain, go to our bodies, and tell
our muscles that it’s time to contract . So let’s start from the top. In the brain, we have what’s
called an upper motor neuron. I’ll just write “upper
motor neuron” right here. An upper motor neuron
sends a signal over to a lower motor
neuron, that I’ll abbreviate right here–
lower motor neuron. And I promise I’m
going to draw this out in greater detail in a moment,
but just to kind of get an idea of what they do,
the lower motor neuron is the direct
messenger to muscle to tell it that it’s time
to start contracting. The upper motor
neuron, because it’s sending a signal to the lower
motor neuron, has two jobs. One, it’ll also have the job
to tell muscle eventually that it’s time to
start contracting. The other job it
has is that it has to tell lower motor neurons
when it’s time to stop. It’ll tell them that there’s
no more signal coming in so you should stop telling
muscles to start contracting. So that’s why upper motor
neurons have two functions. Let’s take a look
at a lower motor neuron in better detail now. The first thing I like to
draw here is called the soma. The soma is just the body
of the lower motor neuron. The part of the
lower motor neuron that receives the signal
from the upper motor neuron is kind of branched like this. And there are
multiple projections that come off right here. These guys are called
dendrites, and they’re going to receive the signal. It’s where a signal is going
to dock on our motor neuron. And after it docks
the dendrite, passes through this station,
the soma, the cell body, it’s cast away along an axon
that I’m drawing right here. And I’ve drawn this extra
long because there’s going to be a lot going on
here before this slab of meat here, or muscle,
receives a message. So this is our muscle,
the motor end plate, or the muscle that’s going
to receive information from our axon right here. So one more time, when
we’re at a dance party and we decide that it’s
time to shake a leg, how do we tell our leg
it’s time to shake? Well, we have this
signal that’s generated from the brain, travels
down the upper motor neuron, and then it goes to this
lower motor neuron right here. And the way I like to
think about a signal, it’s kind of like how I think
about ships in the Navy. So if this little ship
right here is our signal, and it comes from our
upper motor neuron, it needs to have somewhere it
can dock on this lower motor neuron. That’s what dendrites are for. This is where a signal
is going to dock. Once it docks there,
it has to pass through some type of station,
this naval station, which is what the soma is. And after it passes
through the station, it’s going to move down the
axon where it’s cast away. The axon is where a signal
is cast away from this motor neuron before it ends
up going to muscle. But what could happen here? What problem could
probably arise, considering how long
this axon actually is? Well, the signal could die off. It may not be able to make
it all the way to the end. And that’s kind of what would
happen if you had a lower motor neuron injury or a lesion. If something were to happen
to our lower motor neuron, we wouldn’t be able to
tell a muscle it’s time to start contracting. Instead, you would have weakness
because your muscle would not be able to contract. The same thing happens when
you have an upper motor neuron injury. You’re also going
to see weakness because you’re not able to tell
the lower motor neuron it’s time to tell the muscle it’s
supposed to start contracting. But that’s not the
key characteristic we see with upper
motor neuron injuries. The key thing we look
for is that we’re not able to tell lower motor neurons
to stop what they’re doing. And so because of that,
the lower motor neuron just keeps on telling this muscle,
hey, go ahead and contract. I’m not receiving any signal
from here to tell me to stop. And so because of
that, this muscle is just going to continuously,
spastically contract. And so a key symptom
or a key characteristic of an upper motor
neuron injury that you don’t see with lower motor
neuron injuries is spasticity. And so that’s what could
happen if you have dissipation of a signal at the
upper motor neuron. But nature has planned for this. What does she do to
make sure that we don’t have our
signals dissipated? Well, she insulates our neurons. So I’m going to draw
three insulating cells right here that wrap the round
our axonal fiber right here, or this axon. And these three cells–
I’ll label one right here. This is a single cell. It’s known as a myelin sheath. That’s an I. Myelin sheath. What does that mean? Well, myelin just means “fatty.” So, yo mama’s so myelin. They mean the same thing. Myelin sheath. And so we cover up the axon
fiber to help insulate it, so instead of a
signal dying off, it’s able to go all the
way to the end over here. And depending on where you
have this myelin sheath cell in the body,
there’s a different name. In our central nervous
system, which is strictly just the brain and spinal cord,
we call them oligodendrocytes. And that’s only in the
brain and the spinal cord. In the peripheral
nervous system, which is literally everything
else, any other nerve in our body that’s not in
the brain or the spinal cord, we call these myelin
sheath cells Schwann cells. So I’ve drawn three up here,
and what’s going to happen is that you’re not going to
have dissipation of your signal. No. Instead it’s going to continue
on to this myelin cell and then jump and land
on this node right here, and it’s going to jump again. And it keeps doing
this from node to node, or this empty space
to empty space, until it finally makes it
to the end of this axon, or what we call the axon
terminal right here. And so this space right
here, this open, empty area that has nothing in it,
has a specific name. We call this the
node of Ranvier, named after probably
the smartest scientist ever because he got his name
on a scientific structure that literally has nothing. This is just empty
space right here. And so as our signal is going
to propagate down this axon here and jump from node to
node, eventually it’ll finally make it to
our axon terminal right here, where we can then
relay a message to our muscle that it’s time to contract. And that’s how our
motor neurons work.

44 Replies to “Motor neurons | Muscular-skeletal system physiology | NCLEX-RN | Khan Academy”

  1. It's a little confusion to say there is nothing at the Nodes of Ranvier because there's a high density of voltage gated ion channels there allowing the signal to continue through the myelin insulation.ย 

  2. That's a great video. What program and device did you use to make this video? Your handwriting is so neat. Your drawings are fine. Please tell. Thank you.

  3. Raja is solely responsible for me passing my integrative neuroscience and neuroscience & neuropharmacology exams.

  4. I thought myelin sheaths were used to make action potentials move faster, but action potentials dissipate as they move through myelinated areas. The nodes of Ranvier boost action potentials as they have voltage-gated sodium channels that open… Also, action potentials don't actually jump from node to node. They just appear to jump because they move slower at non-insulated areas…

  5. Yo mama is so myelin, when she went to KFC, she ordered a bucket of chicken, and when they asked what size, she said the one on the roof

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