While unmyelinated sensory afferent C fibers are classically considered to code for painful stimuli and tissue status, skin C-fiber low-threshold mechanoreceptors (C-LTMRs) respond instead to innocuous tactile stimuli and are uniquely tuned to transmit information about pleasant affiliative touch. They respond well to gentle moving stimuli so that their firing rates correlate significantly with stimulus pleasantness ratings. Unlike the myelinated mechanosensory pathway that project to somatosensory cortex, C-LTMRs in humans – where they are called C-tactile or CT afferents – relay information to the insula, an area associated with allostatic control, motivation, and feelings.
We hypothesize that SCI transforms these pleasant touch-encoding C-LTMRs into allodynia-encoding nociceptors in body regions associated with the segmental site of injury. To test this hypothesis we have developed an ex-vivo skin-nerve preparation to characterize the properties of C-LTMRs, first by their selective optogenetic activation, then in response to mechanical brush stimuli applied at a range of velocities within preferred forces. Our collaborator, Dr. Sandra Garraway, is undertaking important complementary studies in vivo.