The Functional Organization of Cutaneous Low-Threshold Mechanosensory Neurons

Innocuous touch of the skin is detected by distinct populations of neurons, the low-threshold mechano-receptors (LTMRs), which are classified as A beta-, A delta-, and C-LTMRs. Here, we report genetic labeling of LTMR subtypes and visualization of their relative patterns of axonal endings in hairy skin and the spinal cord. We found that each of the three major hair follicle types of trunk hairy skin (guard, awl/auchene, and zigzag hairs) is innervated by a unique and invariant combination of LTMRs; thus, each hair follicle type is a functionally distinct mechanosensory end organ. Moreover, the central projections of A beta-, A delta-, and C-LTMRs that innervate the same or adjacent hair follicles form narrow LTMR columns in the dorsal horn. These findings support a model of mechanosensation in which the activities of A beta-, A delta-, and C-LTMRs are integrated within dorsal horn LTMR columns and processed into outputs that underlie the perception of myriad touch sensations.