- [Voiceover] And then down to
the next mechanoreceptor.
This is another corpuscle right here.
We're familiar with corpuscles by now,
and it's named after another scientist.
This is Ruffini's corpuscle.
You may have actually seen this by another term.
Ruffini's Ending is another phrase they use
for this mechanoreceptor.
The idea is if we have some external stimulus again,
this guy poking us all over again, generating
this force that goes deep within our skin,
Ruffini's corpuscle, which looks like this guy
right here, will perceive it.
It's very interesting because there are no disks
or rings like we had with the other corpuscles.
Instead, what you can kind of clearly see right here,
and this is so beautiful, there's an actual
nerve fiber right here, and I'll outline it.
So that thing right here that's kind of being
cast this way, that's the nerve fiber.
This is our afferent nerve fiber right here.
Do you remember what type it is?
I think I heard you say a beta,
so you're absolutely right.
So this is our afferent nerve fiber, and actually
this guy sort of branches into the corpuscle.
So Ruffini's corpuscle right here has a whole bunch
of afferent nerve fiber branches in it.
But I didn't highlight the whole thing here,
what else do we have hanging out up here?
Well the other thing that we've got sort of
coating this is collagen.
Collagen you might remember is a structural protein.
So I'll draw it just all around there
and I'll label it right here.
So there's collagen right here, collagen like
what you might remember from the dermis.
So what will happen is that if we stretch
this skin up here with this external stimulus
and we stretch it hard and we generate this force
that goes deep into our skin and kind of hits
Ruffini's Ending right here, it'll cause the collagen
here to shift and be perturbed.
As you might notice, the collagen is very intimately
connected to the nerve fiber branches here
and as the collagen shifts it opens up ion channels
on these nerve branches that allow sodium
that's kind of hanging out in the extra
cellular matrix to sneak in.
It will sneak in and then kind of go along
this afferent nerve fiber to generate
an action potential and move on
to the central nervous system.
It's really interesting because as long as this
stretcher, this stimulus, is being applied
to our skin, we're going to have the collagen
being stretched and pushed out of the way
to allow sodium or other ions to enter
into our afferent nerve fiber.
So you might have guessed this is going to be
one of those receptors that respond
to sustained touch, but the trick here is because
this depends heavily on collagen you've got to think,
what part of our skin has
the most amount of collagen?
Well we're going to have to go pretty deep for that
because the part of our skin that has collagen
is the dermis.
So it'll be the dermis, but the dermis has two layers
and the part of the dermis that has the most collagen
is the reticular dermis.
So that's how you can reason that out.
It's sustained touch that causes the collagen
to move and allow sodium to keep on entering.
So where do we have the most collagen?
The reticular dermis, and that's pretty deep.
Awesome.
Now let's move on to the last mechanoreceptor.
The last mechanoreceptor we're going to talk about
is called a hair follicle receptor.
I'm hoping there are some neurons firing
in your brain right now about this because we
talked a little earlier about a non-hairy receptor,
and now we're talking specifically about hair.
So this should be a callback
to the hairy mechanoreceptor that'll close
that story that we talked about at the beginning.
Likewise, I'm going to draw this stimulus
a little differently.
This perturbation right here will be something
like this and I'll draw the nail if this is going
to be like a finger, and it's kind of initiating
a force in that direction.
If this is over a part of skin that's relatively
hairy and there was hair here,
the hair will be deflected.
So that is deflected hair, very beautiful
shaft of hair right here that goes deep
into the skin because you remember
that hair protrudes from a follicle.
So there's the hair shaft that goes at the bottom here.
I'll draw a little follicle that kind of
nests it right here.
So it kind of goes like this right here,
and that goes in the back,
and that comes out at the front.
The interesting thing about the hair follicle
is that there is a nerve fiber that is actually
wrapped around the part of the hair that sits
in the follicle, and sure enough this is
an afferent nerve fiber, and a beta nerve fiber.
What you'll notice is that when the hair
is deflected that allows for ions to leak
into this afferent nerve kind of where the hair
and the nerve are interfacing, and when it leaks in
it kind of enters and goes along the length
of this nerve and goes down here.
That generates an action potential which can then
convey a signal to our central nervous system.
This all started from hair deflection.
Hair deflection was the impetus
for this signal to occur.
So this is going to be used to perceive light touch
because we're not even touching the skin right here.
We're actually just touching the hair,
so it's light touch on hairy skin.
So light touch on hairy skin because our hairs
are being deflected as we perceive the stimulus.
While the hair kind of runs many layers
of skin, remember that the follicle itself,
this guy sits anchored in the reticular dermis.
It sits in the reticular dermis and so that's
the location we subscribe the hair follicle receptor to.
Lastly, as long as we're kind of pushing
along right here, what'll happen is that the sodium
will leak in so long as there is an active space
that's present, but because the reticular dermis
is a place of such thick, dense connective tissue
if we push the hair shaft and make an opening
right here, remember that there's a ton
of collagen kind of sitting around that can come
and fill the gap and just block it
so sodium could not enter.
So what does that mean?
This implies then that we need constantly changing
stimuli in order to have a signal be generated.
So this is the third of three mechanoreceptors
that requires constantly changing stimuli
so that the hair shaft can constantly be opening
up a gap for sodium to enter.
It has to do so around so much collagen
that will plug the opening if we just
have the hair held, deflected.
That's the whole point of why you don't notice
a hairband that you might be wearing over
a long period of time because that's definitely
deflecting your hair, but over time it's as noticeable
as a smooth cotton t-shirt.
It's there, but you're not constantly detecting it.
It's important to emphasize that this is a very
essential distinction from the other type of
light touch we mentioned earlier with Merkel's disk.