Types of Mechanoreceptors in the Skin ➔ -1-Merkel's organ: We spoke earlier about Merkel cells and indeed the organ of Merkel it is in fact at the level of the basal layer of the epidermis of the cells which associate with a nerve ending so the axon has lost its Schwann sheath when passing through the basal lamina, can become arborized and form a kind of nerve plate on which rests the Merkel cells and this plate is also called the tactile disc and it is a sort of support for Merkel’s cell. A disk is associated with a cell but again an axon canarborize significantly and come into contact through various arborizations with several Merkel cells. The fibers, before they pass through the basal lamina, are large myelinated fibers of around 7/10 micrometers in diameter and therefore these fibers are excited by the pressure on this Merkel cell and therefore the pressure is transmitted to the level of the touch disk. It is a mechanoreceptor that is quite slow to react and has rapid adaptation, that is to say that it responds to the start of stimulation. ➔ -2- Messner corpuscle. It is a perfectly well defined structure, surrounded by a capsule (which is fibrous connective tissue) and this capsule will be in continuity with the envelope of the afferent nerve and will delimit a space in which the cell mass is located, therefore effective inside of this Messner corpuscle, there are Schwann cells, flattened, stacked on top of each other, these are slightly modified Schwann cells and the nerve fiber which enters this capsule, which was a myelinated fiber,loses its myelin envelope just before entering in the corpuscle gives a certain number of branches and these branches will ramify and carry out a more or less tortuous path between the Schwann cells. Usually they terminate at the part opposite the afferent fiber and end with branches parallel to the surface of the skin. This structure which is ovoid which is quite voluminous with 100 microns long, which is generally perpendicular to the surface of the skin and then which is located in the papillary dermis just under the dermo-epidermal junction therefore at the upper part of the dermis. We see a lot of them on the palmar surface of the fingers, on the palms of the hands, on the soles of the feet and a lot on the lips of the external genitalia. This Messner corpuscle is a mechanoreceptor that responds quickly to pressure stimuli, so we say that he has a slow adaptation. The stimulus is the deformation of the epidermis, deformation which is transmitted at the level of this corpuscle and this corpuscle is relatively fixed and therefore any pressure exerted on it is transmitted to its basal part and therefore exits the nerve. Inside, with these cells of Schwann and the nerve fibers that pass around the cells of Schwann, there are also collagen fibers which will be inserted into the capsule in different points and which pass in bridges around these Schwann cells. In another more demonstrative image, we see this Messner corpuscle with the flattened Schwann cells, stacked on top of each other and the arborizations of the nerve fibers. ➔ -3-Pacini corpuscle. This one is the biggest that exists, is 1mm long and located in the hypodermis of the skin but it is also found in other territories of the organism, and for example in the muscle. It is also an ovoid structure made of rings of flattened cells, lamellae separated by a liquid, a fluid and there is a capsule which surrounds the corpuscle which as usual is in continuity with the envelope of the nerve and then we see a fiber
myelinated which penetrates into this capsule through one of the ends, which loses its sheath myelin and a fiber which remains straight, which does not arborize except at the end where there are some final connections. This unmyelinated area is surrounded by lamellae, debris of cells and this whole, it is axis surrounded by the cells, it is the CORE. This Paccini corpuscle is sensitive to movement and therefore it will respond to stimuli vibratory by a very brief discharge of influx and this discharge has a frequency which decreases very quickly even if stimulation persists.