Phenotype and Function of Somatic Primary Afferent Nociceptive Neurones with C‐, Aδ‐ or Aα/β‐Fibres

Abstract

Nociceptive dorsal root ganglion (DRG) neurones have fibres that conduct in the C, Aδ and Aα/β conduction velocity range. The properties of nociceptive compared with non‐nociceptive somatic afferent dorsal root ganglion neurones appear to fall into two patterns, A and B. Pattern A properties of nociceptive neurones, the more common type, include longer action potential duration and slower maximum rate of fibre firing, as well as a greater expression of substance P and calcitonin gene‐related peptide immunoreactivity. The values of pattern A properties appear to be graded according to the conduction velocity group (C, Aδ or Aα/β) of the fibres. The most pronounced forms of A‐type properties are expressed by nociceptive neurones with C‐fibres, and these become less pronounced in nociceptive neurones with Aδ‐fibres and least pronounced in those with Aα/β fibres (C > Aδ > Aα/β). Some of these properties are also expressed in a less extreme but similarly graded manner through C, Aδ and Aα/β groups of non‐nociceptive low threshold mechanoreceptive (LTM) neurone. The less common pattern B properties of nociceptive neurones have similar values in C‐, Aδ‐ and Aα/β‐fibre nociceptive neurones but these clearly differ from LTM units with C‐, Aδ‐ and Aα/β‐fibre conduction velocities. These features of nociceptive neurones include consistently larger action potential overshoots and longer after‐hyperpolarisation durations in nociceptive than in LTM neurones.