Receptors Associated with Skin
One of the functions that skin performs is that of sensory input to the brain, letting us know a lot about the world around us as well as how the skin is doing. In order to perform this function the skin has receptors that detect touch, pressure, pain, and heat.
Here is a list of the known skin receptors and what they detect:
Tactile corpuscles (Meissner’s corpuscles): These receptors detect light touch at the skin surface. Basically when something touches the skin surface these receptors detect the vibrations of that touch and send signals to the brain. Tactile corpuscles are found very close to the surface of the skin, just below the epidermis and usually inside of dermal papillae. Tactile corpuscles are most concentrated in thick skin (especially at the tips of the fingers). These receptors are also quick adapting, meaning that if the stimulus remains constant these receptors stop sending signals within a few moments. You can experience this sensory adaptation by placing a small coin on the back of your hand and letting it sit there, you will feel it touch you when you place it there, but that feeling will go away pretty quickly.
One way to easily recognize tactile corpuscles is to look for an oval structure that has straight lines across it. Take a look at what they look like under a microscope…
Lamellar Corpuscles (Pacinian Corpuscles)
Lamellar corpuscles detect deep pressure and vibration. These are important for detecting the pressures involved when you pick up an object for example, and they may also be important for our sense of an objects surface texture. Tactile corpuscles are found deep in the dermis or in the adipose tissue that underlies the dermis. They are generally round in shape and have multiple layers (kind of like an onion). Here are a few images of lamellar corpuscles under the microscope…
Hair Follicle Receptors (hair plexus -or- root hair plexus)
Hair follicle receptors are nerve endings that wrap around the base of a hair follicle and are stimulated by movement of the hair. If something bends or vibrates the hairs on your forearm for example you can feel it (if an ant is crawling on your arm, or if wind blows over the surface of your arm). These hair follicle receptors contribute to our sense of touch.
I don’t have any histology images of hair follicle receptors, but the image of the model above shows where they are and what they basically look like.
Free Nerve Endings
Free nerve endings are sensitive to changes in temperature (we sense this as warm and cold) and to stimuli that we sense as pain (those are the two stimuli that I have my students remember, but you may also need to know that there are different types of free nerve endings including some that detect touch and pressure). Free nerve endings that sense pain have receptors for chemicals released from damaged tissues/cells and they also send signals if they are damaged themselves. One example of a chemical that can stimulate these receptors is the chemical found in hot peppers called capsaicin; this chemical binds to receptors on pain sensing nerve endings and causes them to send pain/heat signals to the brain. Pain receptors are also called nociceptors and lastly they do not adapt well at all (this is at least part of why pain is hard to ignore).
I don’t have histology images of free nerve endings, but the image of the skin model above should give you an idea of what they look like.
Bulbous Corpuscle (Ruffini)
Bulbous corpuscles are oblong receptors that are stimulated by stretch and pressure. Bulbous corpuscles are also very slow to adapt, so these are important for sensing sustained pressure/stretch. They appear to be important for sensing how firmly to hold objects as well as sensing when our grip is slipping.
I don’t have any histology of bulbous corpuscles, but here is a link to one if the first images of one of these receptors from Wikipedia…
Note that in that image you can see some light colored fibers (those are collagen fibers) and you can see some very dark stained fibers (the stain is selective for nervous tissue, so the dark stained areas are nerve fibers of the bulbous corpuscle receptor)
Bulboid corpuscle (Karause’s end bulb)
These are rounded receptors found in the upper dermis that detect heat loss (cold).
Again I have no histology images of this receptor, so look at the image of the model above to get an idea of what these look like.
Not Just for Skin
Another good thing to know (for later when you cover senses) is that most of these receptors (and some other specialized receptors) are found all over inside the body as part of what we call general senses (sense of touch, stretch, pressure, pain, and hot & cold). Inside joints like the knee for example we have receptors for stretch, pressure, and pain and these let our brain know the position our limbs (proprioception) as well as when a joint gets damaged. Muscles, bones, tendons and ligaments also contain receptors for the same basic reasons. Even our viscera (internal organs like the stomach, intestines, etc.) have general receptors so that we have an idea of what is going on with them.
As always if you have any questions or comments please add them below.