- [Voiceover] So now that we understand
the structure of skin, let's talk about how our skin
helps us perceive our environment.
How does it enable us to sense what's going on around us?
And the basic idea behind it is when we have our skin,
let's say this is just a schematic of skin right here
and there are all the layers that are going on here,
and there's some type of perturbation.
Let's say someone just kinds of pokes us right here.
And of course this poke doesn't penetrate into our skin
but it causes a force that is transmitted
deep into the different layers.
What'll happen then is that there's a mechanoreceptor
that sits about here that will perceive this force
and then from it will generate an Action Potential,
or a signal that it can then transmit to a neuron,
or this nerve fiber that sits here
that'll carry this message on to our Central Nervous System.
And that's the basic layout of how these mechanoreceptors,
and I'll write it out right here,
how these mechanoreceptors
help us perceive our external environment.
And the other thing I guess I should mention at this point
when we're talking about these nerve fibers
is that they're called Afferent Nerve Fibers.
So Afferent Nerve Fibers.
And notice there's an a right here.
What Afferent Nerve Fibers do is that
they'll take some stimulus from our
external environment right here
and then create a signal that will then be transmitted
to our central nervous system.
And so that's what this Afferent Nerve Fiber is it's
taking this stimulus and telling the Central Nervous System
something is going on.
And this is to contrast of what happens on the flip side.
So that would be an Efferent Nerve Fiber.
Notice that there's an e right there.
And that does the exact opposite,
where the Central Nervous System is going to
respond to some stimulus.
So this would be a nerve that goes to a muscle fiber
that says, "Oh there's something that's really itching us
"right here let's move our hand away."
So that's what an Efferent Nerve Fiber would do.
But because in this video we're talking about sensation
we'll focus only on afferent fibers.
And there are two main afferent fibers we talk about.
There are A beta fibers and A delta fibers.
A delta fibers are used to perceive pain and temperature.
Pain and temperature, I'll just write temp right here.
The A beta fibers perceive everything else
so these are going to be the afferent fibers we talk about
for all the mechanoreceptors in this video.
There's a separate video
that talks about the A delta nerve fiber.
And the main thing to keep in mind as go and talk about
the mechanism through which each of these mechanoreceptors
work or their structure is that their
structure is very important to understand
because the structure of the mechanoreceptor
actually determines their function.
So structure determines function.
So if we understand the structure
we'll understand or be able to figure out
what the function is.
But furthermore, once we understand what the function
of the mechanoreceptor is, from there the function
will actually help us determine the location
of the mechanoreceptor.
So the function will help us figure out the location.
So let's test that out below.
The first mechanoreceptor we're going to talk about
is called Meissner's, with two s's, Meissner's Corpuscle.
And you'll see this word often, "corpuscle,"
and all this means is just body.
So let's run our scenario if there's something
that perturbs the skin or applies a force to it,
it could be a finger or anything else of that variety and
it transmits a force deep into the skin.
We'll have Meissner's Corpuscle right here,
that's what it looks like actually under a microscope.
It'll perceive this force, so what happens then?
Well one thing that you can kind of see here is that
there are different layers of disks in Meissner's Corpuscle
and there're even these nuclei that you can see right here.
See there's one nucleus, there's another nucleus,
and there's anther nucleus right there.
There's a whole bunch of nuclei
and they're all within these cells.
Now I'll just draw one right there
and let's say there's another disk right there
and even another right there.
And I'll draw one right here just kind of
stacked right on top of this one right here.
And so let's say this force is transmitted
deep down right here and it kind of nudges this disk.
It's also known as an epithelial
or a laminar disk or a laminar disk.
They're just specialized epithelial cells.
So this initial nudge that we have
from our external environment, our stimulus right here
will then in turn cause this epithelial disk
to nudge over this way.
It actually shifts across from where
this other epithelial disk was.
And so this disk right here will stay put.
So I'm just drawing these dots right here
that kind of show the center.
So these aren't going to be nudging
but this guy up here nudges that way.
And when this disk moves away that actually
allows sodium and other ions to kind of
leak into the disk below it.
And these disks are actually connected
and so when they shift across you'll have ions enter
and they'll kind of propagate through all of the cells
until they get to the bottom of Meissner's Corpuscle
which actually ends in an Afferent Nerve Fiber.
And as you know if a bunch of sodium
is present within a nerve it will fire an Action Potential.
And that's exactly what happens, an Action Potential
is generated and we send a signal on
to the Central Nervous System.
And so notice here, this all started from a nudge.
And this entire mechanism initiated.
And so if this is a nudge, the way I think of it then
is that this Meissner's Corpuscle can't sit
too deep in our skin if it's perceiving a nudge.
And so the function of this Corpuscle then is to
perceive light touch.
Light touch, and specifically in non-hairy skin.
Non hairy skin.
And we'll talk about what works in hairy skin.
But another term for non-hairy skin is Glabrous skin.
Glabrous skin.
So it perceives light touch in Glabrous skin.
And an example of light touch would be something like
putting on a smooth cotton t-shirt.
So putting on some type of smooth cotton t-shirt.
And I want to emphasize here that Meisnner's Corpuscle
only works when you're putting this
smooth cotton t-shirt on.
It's not firing after the t-shirt's already on you.
You don't feel that you're wearing clothes
every second of the day, that would be too much stimuli.
And so a very important thing to keep in mind here
is that Meissner's Corpuscle, in order to fire,
requires constantly, not constant,
but constantly changing stimuli.
Constantly changing stimuli.
And this kind of makes sense when we take a look
at the mechanism involved here.
Because when this disk moves across,
that's when sodium can enter into the disk below.
When it's moved across already and it's just
kind of sitting adjacent here,
the sodium's not going to be able to enter.
It's the movement that allows the sodium to kind of
tumble across or between these two disks
to enter into a gap that will eventually
lead into this Afferent Nerve Fiber.
And so keep that in mind.
We'll mention two other mechanoreceptors
that have this same criterion here.
And then continue with this thing that we were talking about
that a nudge started all of this happening here
and it's light touch that we're perceiving.
The location can't be somewhere too deep in our skin.
And so sure enough Meissner's Corpuscle
sits in the papillary dermis.
So the papillary dermis, this is the
top layer of the dermis, right below the epidermis.
So good you'll notice this trick is going to work
when we talk about all the other mechanoreceptors below.