Response to an itch-producing substance in cat. II. Cutaneous receptor populations with unmyelinated axons

Within the sampled population of cutaneous unmyelinated afferent neurons (n = 94), only the C-polymodal nociceptor population was reactive to the pruritogen cowhage. Of 62 C-polymodal neurons tested, 11 were unresponsive to cowhage. No C-polymodal neurons were more responsive to inactive, than to active, cowhage (n = 17) and all were responsive to mechanical (n = 62) stimuli and noxious heat (n = 24). The range of conduction velocities obtained by single-unit recording techniques was similar to that found by signal averaging the activity from larger strands of nerve. Hence, it is concluded that our recording technique was capable of recording from the smallest afferent fibers in a cutaneous nerve and it was unlikely that we would have missed finding a slowly conducting, pruritus-signaling neuron due to sampling bias. A search of slowly conducting afferents (n = 314) using electrocutaneous stimulation gave no evidence to suggest the existence of an unknown population of unmyelinated fibers that might signal pruritus. A number of alternative mechanisms by which the sensation of itch might be encoded were discussed, the most favored being the activation of a subset of the C-polymodal nociceptive population.