Okay. Now, let’s talk about how the
nervous system is actually organized.
So we know that the nervous system is going
to receive a lot of different information.
So that’s sensory information.
That information needs to then go
on and get passed on to the brain
or the central nervous system to
figure out what to do with it.
And then we need
to have an action.
So the brain is going to say, “I
need to do something about this.”
So send a signal to muscles or
glands to act upon the decision.
So the analogy is you put
your hand on a hot stove,
information coming in.
Sorry, information going into the
nervous system, saying, “Ouch” burning.
And so that signal of activating the
thermal receptors on your hand,
in the dermal layer, sends a signal
up to your brain, and the brain says,
“Oh my God, this is hot, move
your hand, start yelling.”
And then it’s going to send a signal
back to your hand to do this.
And I keep kind of coming back
to do this example because
I think we’ve all done this
before, grab a hot pot,
and how quickly do
you move your hand?
And thank God we do because it
would be terrible if you’re like,
“Hmm, I think this pot
is burning my hand.”
And your hand would be
But instead we have this action.
So think how quickly that happens and
I hope you kind of appreciate that.
Now, let’s take a look at
the different components.
So, all that sensory
information that’s coming in
is going to be down through the
peripheral nervous system.
Figuring out what we need to do,
the process of understanding
perceiving the response and having
a response to the information
coming in would be the
central nervous system
and then we go back to the peripheral
nervous system in order to have an action.
But this time it’s
done through the motor
function side as opposed
to the sensory side.
let’s take a look at how
this sort of process
works live when it’s
all pieced together.
So, motor neurons carry
information from the nervous
system towards the muscles
or glands or organs.
Another -- glands is another
way to talk about an organ.
So, the motor signal is
telling it what to do.
So these organs are known as the effectors
because they’re having an effect
on our response to the sensory
information that came in.
Motor neurons carry the signal from the
central nervous system, so from the brain
to the effectors via
So you should know the difference
between efferent and afferent.
Afferent means information coming in
from the receptor, so say your skin.
And efferent would be information going
out to the muscles to do something.
Now, the simplest sort of --
the simplest example
of this motor --
sorry, sensory to motor,
it would be a reflex.
And reflex is a directive motor response to
sensory input without conscious thought.
So, in English we’re saying, a reflex
is you moving in response to some
type of sensory input without you
even happening to think about it.
And that’s why you hear in the -- when
people just talk in layman’s terms, we say,
“Well, that’s just a
Meaning I wasn’t
thinking about it.
And that’s what they’re
referring to is that
it was something you do
without even thinking.
So a reflex is exactly that.
It’s your muscles responding to some type of
input without actually thinking about it.
So, the muscle stretch reflex
is a great example of that.
So there’s a sensory neuron
that detects stretching of a
muscle and it transmits this
signal via motor neuron.
And the reflex involving
is only two neurons
and one synapse and this is known
as a monosynaptic reflex arc.
An example of that is the quad and thigh
and the knee reflex of your tendon.
And you know, we’ve all
been to the doctor
and he pulls out that magical
little rubber hammer
and he does the little hit
on your knee and you’re
wondering what does this mean
and why is he doing this.
What he’s actually
looking for here.
She’s looking for is that
your reflex is working;
now you have good conduction between
your sensory and motor neurons
and that your muscles
are working okay.
So they just want to
make sure that the whole
process of signal transmission
is working smoothly.
And you know, if all goes well at
the MCAT and you become a doctor,
you might get one of those
magical little rubber hammers.
here’s a blowup looking
at the specific steps.
And you know, we’re not going to
go through every little box here,
but the point is you can see the hammer,
you can see the tendon that we’re hitting.
And that is, once you hit it,
it actually causes a stretching
of that neuron which
activates that neuron
and that’s how we
get the contraction
of the quadriceps
muscle or your thigh.
And then, we have also another
concurrent activation of the hamstring
muscle which is at the back of your leg.
So it’s actually a really great example
of sort of two things happening at once.
we’ll walk through
the steps here.
So, sensory neuron synapses
with the inhibitory interneuron
which goes toward the hamstring.
it’s doing two things
which is really cool.
So this is an example of an
the integrative role
of the nervous system.
So the interneuron is, like the name implies,
something that’s kind of in the middle.
And it’s a neuron
that’s saying, “Hey,
the quadriceps muscle just contracted.
And so, for it to be able to do that,
I’m going to have to relax the
opposing hamstring muscle.”
And so, by activating one, you’re actually
inhibiting or relaxing the other and
that’s what allows you to sort of kick your
leg when you get that little hammer hit.
So, concurrent relaxation and contraction
is an example of reciprocal inhibition.
Okay? So you should understand
that that reflex arc.
You should understand the
fact that you’re activating
one and you’re inhibiting
the other and this
process is quite integrative
which is done by the
interneurons and we call
this reciprocal inhibition.