Differences in the size of the somatic action potential overshoot between nociceptive and non-nociceptive dorsal root ganglion neurones in the guinea-pig

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Intracellular action potentials evoked by dorsal root stimulation were intracellularly recorded from L6 and S1 dorsal root ganglion neurones in deeply anaesthetised guinea-pigs in vivo. Units were classed as C, Aδ or Aα/β units and as nociceptive, low-threshold mechanoreceptive or unresponsive. Units with membrane potentials of at least −40 mV and action potentials with an amplitude of >20 mV were included.Nociceptive neurones had significantly larger somatic action potential overshoots than low-threshold mechanoreceptors in C, Aδ and Aα/β units. A higher proportion of low-threshold mechanoreceptors than of nociceptors had action potentials that failed to overshoot in all conduction velocity groups. 60% of muscle spindle afferents failed to overshoot. The size of the overshoot was correlated positively with log10 action potential duration, log10 action potential rise time, log10 afterhyperpolarisation duration, action potential amplitude and membrane potential and negatively (weakly) with log10 conduction velocity.We conclude that nociceptive neurones are more likely to have somatic action potential overshoots than low-threshold mechanoreceptors in any conduction velocity group. This effect was not due to electrode properties or conduction failure at site(s) of failure of action potential regeneration. Differences in overshoot may affect the influence of neuronal firing on cellular processes. If an overshooting action potential is used as a selection criterion, a bias towards nociceptive neurones is likely to occur. An overshooting action potential coupled with a long afterhyperpolarisation or broad action potential may help in identifying sensory neurones as nociceptive.