Abstract
The vertebrate skin contains sensory corpuscles that are receptors for different qualities of mechanosensitivity like light brush, touch, pressure, stretch or vibration. These specialized sensory organs are linked anatomically and functionally to mechanosensory neurons, which function as low-threshold mechanoreceptors connected to peripheral skin through Aβ nerve fibers. Furthermore, low-threshold mechanoreceptors associated with Aδ and C nerve fibers have been identified in hairy skin. The process of mechanotransduction requires the conversion of a mechanical stimulus into electrical signals (action potentials) through the activation of mechanosensible ion channels present both in the axon and the periaxonal cells of sensory corpuscles (i.e., Schwann-, endoneurial- and perineurial-related cells). Most of those putative ion channels belong to the degenerin/epithelial sodium channel (especially the family of acid-sensing ion channels), the transient receptor potential channel superfamilies, and the Piezo family. This review updates the current data about the occurrence and distribution of putative mechanosensitive ion channels in cutaneous mechanoreceptors including primary sensory neurons and sensory corpuscles.
Highlights
Tactile sensation is one of the most important components of mechanosensation, and originates in nerve fibers that can be distinguished based on the morphology of their skin terminals, as well as on the conduction speed of their action potentials
low-threshold mechanoreceptors (LTMRs) sensory neurons are pseudo-unipolar, and the axonal processes that extend to the skin are associated with specialized cells: Merkel cells, Schwann-like cells that form part of the sensory corpuscles (Meissner corpuscles, Ruffini’s corpuscles, Pacinian corpuscles), or cells of hair follicles [3,4,5,6]
Mechanotransduction is defined as the conversion of mechanical stimuli into electrical signals, and this process occurs at the periphery of LTMRs, inside the sensory corpuscles [11,12,13]; in this context, the sense of touch is a prime example of mechanotransduction in biology [14,15,16]
Summary
Tactile sensation is one of the most important components of mechanosensation, and originates in nerve fibers that can be distinguished based on the morphology of their skin terminals (i.e., free nerve endings and sensory corpuscles), as well as on the conduction speed of their action potentials. The sensory corpuscles are the receptors responsible for tactile modalities including light brush, touch, pressure sensation, stretch, and vibration [1,2,3]. These mechanosensitivity modalities depend on Aβ, Aδ and C nerve fibers (distinguished according to axon diameter, degree of myelination, and axonal conduction velocity) connected to low-threshold mechanoreceptors (LTMRs). Understanding mechanotransduction in free nerve endings and sensory corpuscles requires the identification of the various molecular mechanisms that translate cell-tissue deformation into action potential firing in the corresponding LTMR. The present review is a compilation of the current knowledge regarding the occurrence of putative mechanosensitive ion channels in sensory corpuscles that are functionally mechanoreceptors. It was focused on ion channels belonging to the degenerin/epithelial sodium, acid-sensing, transient receptor potential, mechanosensitive potassium, and Piezo families
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