Sensing Touch: The Skin’s Mechanoreceptors
Inside the skin’s dermis are four distinctive types of touch receptors, termed mechanoreceptors. The following figure shows the layers of the skin and some of its receptors.
Merkel disk receptors, as their name implies, are disk-shaped receptors located close to the border between the dermis and epidermis; sometimes they extend into the epidermis.
Merkel disks are receptors for pressure, meaning they’re activated in the areas of your skin pressing against the chair in which you are sitting, for example. If someone were to sit in your lap, receptors in the skin over your thigh muscles would be recruited.
These receptors respond to relatively constant pressure over small areas of the skin, giving you the perception of the amount of force being exerted against different areas of your skin.
Meissner corpuscles, like Merkel disks, also respond to pressure, but they can respond to more rapid changes in pressure than Merkel disks can, such as those generated by shearing forces. The sensation evoked when the Meissner corpuscles are stimulated is usually called flutter.
Like Merkel disks, Meissner corpuscles also have small areas of sensitivity, called their receptive fields. A touch receptive field is the area of skin that, when mechanically disturbed, activates a particular somatosensory receptor there. Small receptive fields are usually associated with shallow receptors (those that are near or in the epidermis), whereas receptors (deep in the dermis) typically have large receptive fields. Pressure at any point on the skin compresses the skin at greater distances deep in the skin than more shallowly.
Ruffini corpuscles respond to skin stretch (think of the way your skin pulls when it’s being dragged across a surface). These receptors, which serve an important function in protecting the skin from tearing, have large receptive fields.
Pacinian corpuscles, which also have large receptive fields and tend to be deep within the dermis, are the fastest responding of all the touch mechanoreceptors. These receptors have a myelin wrapping similar to the glial wrapping around axons.
However, in Pacinian corpuscles, the function of the wrapping isn’t to allow the action potential to jump from one node to the next. Instead, it allows the receptor to respond to rapid changes in pressure. Through the Pacinian corpuscles, you are able to perceive vibrations. If you drag your fingertips across a coarse surface like sandpaper, you can assess its level coarseness, thanks to Pacinian corpuscles.