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

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.