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Intracellular voltage recordings were made from the somata of L6 and S1 dorsal root ganglion (DRG) neurones at 28·5-31 °C in young guinea-pigs (150-300 g) anaesthetized with sodium pentobarbitone. Action potentials (APs) evoked by dorsal root stimulation were used to classify conduction velocities (CVs) as C, A
or A
/
. Units with overshooting APs and membrane potentials (Vm) more negative than -40 mV were analysed: 40 C-, 45 A- and 94 A/-fibre units.
Sensory receptive properties were characterized as: (a) low-threshold mechanoreceptive (LTM) units (5 C-, 10 A- and 57 A/-fibre units); (b) nociceptive units, responding to noxious mechanical stimuli, some also to noxious heat (40 C-, 27 A- and 27 A/-fibre units); (c) unresponsive units that failed to respond to a variety of tests; and (d) C-fibre cooling-sensitive units (n = 4). LTM units made up about 8 % of identified C-fibre units, 36 % of identified A-fibre units and > 73 % of identified A/-fibre units. Compared with LTM units, the nociceptive units had APs that were longer on average by 3 times (C-fibre units), 1·7 times (A-fibre units) and 1·4 times (A/-fibre units). They also had significantly longer rise times (RTs) and fall times (FTs) in all CV ranges. Between A/-nociceptors and A/-LTMs there was a proportionately greater difference in RT than in FT. The duration of the afterhyperpolarization measured to 80 % recovery (AHP80) was also significantly longer in nociceptive than LTM neurones in all CV ranges: by 3 times (C-fibre units), 6·3 times (A-fibre units) and 3·6 times (A/-fibre units). The mean values of these variables in unresponsive units were similar to those of nociceptive units in each CV range; in C- and A-fibre groups their mean AHP duration was even longer than in nociceptive units.
A-fibre LTM neurones were divided into A- (D hair units, n = 8), and A/- (G hair/field units, n = 22; T (tylotrich) hair units, n = 6; rapidly adapting (RA) glabrous units, n = 6; slowly adapting (SA) hairy and glabrous units, n = 2; and muscle spindle (MS) units n = 17). MS and SA units had the shortest duration APs, FTs and AHP80s of all these groups. The mean RT in D hair units was significantly longer than in all A/ LTM units combined. T hair units had the longest mean FT and AHP of all the A-LTM groups. The mean AHP was about 10 times longer in T hair units than in all other A-LTM units combined (significant), and was similar to that of A-fibre nociceptive neurones.
These differences in somatic AP shape may aid in distinguishing between LTM and nociceptive or unresponsive C- and A-fibre units but probably not between nociceptive and unresponsive units. The differences seen may reflect differences in expression or activation of different types of ion channel.
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