Functional Properties of C-Low Threshold Mechanoreceptors (C-LTMRs) in Nonhuman Mammals

In humans, unmyelinated C-tactile fibers, referred to as C-low threshold mechanoreceptors (C-LTMRs) in nonhuman mammals, are found exclusively in hairy skin and preferentially respond to slow moving gentle touch, such as that produced by lightly stroking the skin. While substantial species differences exist in the proportion of C-LTMRs to the total C-fiber population, C-LTMRs appear to be expressed more densely in proximal regions of the limbs and the trunk. Functionally, C-LTMRs are specifically tuned to relatively low velocity (~0.1 cm/s) cutaneous stimulation, respond with biphasic adaptation to a single sustained stimulus and exhibit prolonged fatigue in response to repeated stimulation. While a molecular marker of the global C-LTMR population is lacking, subtypes expressing MrgprB4, VGLUT3, and TH have been identified. Considering that C-LTMRs terminate in lamina II of the spinal dorsal horn, there is increasing evidence supporting their involvement in the modulation of spinal responses to nociceptive input.