Aβ-fiber nociceptive primary afferent neurons: a review of incidence and properties in relation to other afferent A-fiber neurons in mammals

Abstract

The existence of nociceptors with Aβ-fibers has often been overlooked, and many textbooks endorse the view that all nociceptors have either C- or Aδ-fibers. Here we review evidence starting from the earliest descriptions of A-fiber nociceptors, which clearly indicates that a substantial proportion of cutaneous/somatic afferent A-fiber nociceptors conduct in the Aβ conduction velocity (CV) range in all species in which CV was carefully examined, including mouse, rat, guinea pig, cat and monkey. Reported proportions of A-fiber nociceptors with Aβ-fibers vary from 18% to 65% in different species, usually >50% in rodents. In rat, about 20% of all somatic afferent neurons with Aα/β-fibers were nociceptive. Distributions of CVs of A-fiber nociceptors usually appear unimodal, with a median/peak in the upper Aδ or lower Aβ CV range. We find no evidence to suggest discontinuous differences in electrophysiological or cytochemical properties of Aδ and Aβ nociceptors, rather there are gradual changes in relation to CV. However, some functional differences have been reported. In cat, A-fiber nociceptors with lower mechanical thresholds (moderate pressure receptors) tend to have faster CVs [P.R. Burgess, D. Petit, R.M. Warren. Receptor types in cat hairy skin supplied by myelinated fibers. J. Neurophysiol. 31 (1968) 833–848]. In primate (monkey) A-fiber nociceptors that responded to heat were divided into type I A mechano-heat (AMH) units (Aδ and Aβ CVs) with lower mechanical and higher heat thresholds and may include moderate pressure receptors, and type II AMH units (Aδ CVs) with higher mechanical/lower heat thresholds. It is important that the existence of Aβ nociceptors is recognised, because assumptions that fast conducting, large diameter afferents are always low threshold mechanoreceptors might lead/have led to misinterpretations of data.