Mechanoreceptors afferents in the skin.
There are quite a few.
We will start with pacini corpuscles.
This Pacini corpuscles look like
more of an onion type of a shape.
They are compressible, meaning
that you can press them together.
They have a gelatinous substance within
each of those particular layers.
So as you squeeze them down, you’ll get
an increase in firing frequency.
So you notice that this
response to a change in compression.
Thus, vibration are sensed very easily
through this type of a mechanism.
So what vibrations are pretty
fast about 40 to 500 hertz.
Interesting Pacini corpuscles
adapt quite rapidly.
And what we mean by that, is they only respond
when they are changing the vibration
frequency or compressing.
After you’ve compressed it once and
you’re not still compressing it,
they’ve adapt it and they
won’t send signal back anymore.
They have a really wide receptive field.
Meaning that they capture
a large skin surface area.
If we can trust that with
things like Meissner corpuscles,
they are located in a different plane.
They are usually more vertical
versus horizontal in nature.
They respond more rapidly
to changes in compression.
Now, this compression is
a little bit different.
Instead of being a really
a deep pressed or vibration.
This is more light touch or
kind of fluttering sensations.
They also adapt quite rapidly but
they have a much narrower receptor field.
Meaning that there’s less
skin surface area per receptor
in comparison to something
like the Pacini corpuscle.
These are ones that are now
located in series with the skin.
What do I mean by in series?
In series means as the skin moves,
these will move along with it.
And these are great for sensing stretch.
And here’s an example of
the skin being stretched.
You can see that there is a
increase in firing frequencies
and then a continued firing frequency
once the skin is a new stretched position.
They are fairly slowly adapting,
meaning that overtime they don’t reduce their
firing frequency add a new level of stretch
and they have a fairly
wide receptive field.
Meaning that they are not. They have a
low density per surface area of skin.
Continue it on with some more
mechanoreceptor that we have in the skin,
there are few that are uncapsulated.
These means that they don’t have like
a circle around that can be compressed.
Good examples of these are things like hair cells.
Yes, we have a little sensory
nerves around your hairs.
And these case as your hair
deflexed one way or another,
you sensed that as a change in
that hair follicle or root plexus.
These are free nerve endings
that surround the hair follicle.
And so, by surrounding it as the hair
moves it will enact these particular receptors.
In human since we don’t have a lot of hair.
These is not something that we
talk about quite in the same level.
But if you definitely, if you have
a four legged companion of some sort
whether that a dog or a cat.
Their whiskers are definite things
that are utilizing these hair cells
to a greater degree than we are.
We do those still sense base
upon hair deflexions
and you can do that by lightly rubbing
just over the skin and not touching it.
But if you do touch your hairs, you can feel it.
Merkel cells sense light pressure.
These particular ones fire quickly upon light touch
and then they are sustained over time.
They adapt slowly about will eventually adapt.
And they have a very narrow receptive field.
and one reason is they are located very
superficially in this skin surface itself.
Really just below the epidermis.
How do we sense mechanoreceptor in the skin?
One way to do that is via 2-point
touch discrimination test.
This is a very simple test but I’ll explain it
because it helps us think about receptive field.
So you have a calipers that has two distinct points.
And you know there’s a two distinct points
because you can see the gap
between either the protractor or the
other device that looks like a wrench.
What you do is you place it on the skin and
then have the person not look at that area
because for example if you look at it,
you can see, oh I see a gap
Therefore, I’m gonna feel multiple points.
But we have to do is not look at it so
you close your eyes, you put out your hand
or whatever body parts is being tested
and they touch it with these two points.
If you sense it as two points,
you just respond two points.
If however, you sense it as one point.
even though there were two points,
these perception of one point means
you are in the same receptive field
versus in twos separate receptor fields.
This simple test can be done all across the body,
and you have different densities of
mechanoreceptors in different lowcals.
For example, if we tested Meissner’s corpuscles,
you can see there’s a close
clustering of these in the hand.
If we look at Pacinian corpuscles,
they have a really wide range or receptive field.
And therefore, across most of the hand,
you can still only feel one point.
This is easily look at across the different
areas of the hands, the arm up ways to the shoulder.
And you can see as you start
to get more proximal to the hand,
there’s longer longer spaces in which
you can determine to the points.
That means that the receptive fields are larger.
So places like the shoulder in the upper arm,
you can barely tell there are
two points for a number of millimeters.
If you look at the face like the
upper lip, the nose, the cheek,
Those are much more sensitive.
They have a higher density
And as you go up to the fingers,
you can see you have a greater number
or a higher density of mechanoreceptors
to be able to determine
fine types of sensations.
so you can grasp and do other potential
important items with those lowcals.