Mechanoreceptors afferent in the skin. There are quite a few.
We will start with Pacinian corpuscle.
This Pacinian corpuscle 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 vibrations are sensed very easily through this type of a mechanism.
So what vibrations?
They're pretty fast about 40 to 500 Hz.
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 ones and you're not still compressing it,
they’ve adapted 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’s 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 really a deep press or vibration,
this is more light touch or kind of fluttering sensations.
They also adapt quite rapidly but they have a much narrower receptive field
meaning that there’s less skin surface area per receptor in comparison to something like the pacini corpuscles.
Ruffini endings, 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 an increasing firing frequencies
and then a continued firing frequency once the skin is in a new stretched position.
They are fairly slowly adapting mean that overtime they don’t reduce their firing frequency
at 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.
Continuing on with some more mechanoreceptors that we have in the skin,
there are few that are encapsulated.
This means that they don’t have like a circle around them that can be compressed.
Good examples of these are things like hair cells -
yes, we have little sensory nerves around your hairs.
In this case as your hair deflects 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,
this is not something that we talked about quite in the same level
but if you definitely, if you have a four legged companion of some sort
whether that’s a dog or a cat, their whiskers
are definitely things that they’re utilizing these hair cells to a greater degree than we are.
We do though still sense based upon hair deflections
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 but will eventually adapt and they have a very narrow receptive field
and one reason is they are located very superficially in the skin surface itself.
Really, just below the epidermis.
Here we’re just gonna take a second to review which particular receptor is associated
with which cutaneous sensation, so remember that pacini corpuscles sense vibration,
meissner’s corpuscles sense touch and light flutter,
Ruffini ending sense skin stretch, root hair plexus nerves are sensing hair deflection
and finally, Merkel disks sense light pressure.
How do we sense mechanoreceptors 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 fields.
So you have a calipers that has two distinct points and you know there’s two distinct points
because you can see the gap between either the protractor
or the other device that looks more 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 what we have to do is not look at it
so you close your eyes, you put out your hand or whatever body part 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 sensed it as one point,
even though there were two points this perception of one point
means you are in the same receptive field versus in two separate receptive fields.
These simple test can be done all across the body and you have different densities
and mechanoreceptors in different locales.
For example, if we tested meissner’s corpuscle
you can see there’s a close clustering of these in the hand.
If we look at the pacinian or pacinical 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 looked at across the different areas of the hands, the arms,
up the way to the shoulder and you can see as you start to get more proximal to the hand,
there’s longer and longer spaces in which you can determine two points,
that means that the receptive fields are larger, so places like the shoulder and 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 of mechanoreceptors.
And as you go up to the fingers,
you can see you have a greater number or 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 locales.