Figure 6.

Intermittent failure of spike propagation accounts for integer-multiple-patterned spiking. A, Injection of 1%–2% lidocaine intradermally into the hindpaw increased the mechanical threshold for spiking and reduced the duration and rate of spiking in 3 of 3 units tested, but it did not induce skipping in any units. Sample ISI plots are shown for a typical unit during 7.5 and 12.5 g stimulation before (black) and after (gray) 2% lidocaine. Right, Average ISI calculated from the initial 1 s of spiking for all forces applied. B, Application of 1%–2% lidocaine directly onto the DRG converted regular spiking (top) into semiregular spiking (bottom) in 3 of 3 units tested. Lidocaine initially blocked all spikes, but skipped spikes were evident during recovery. Insets, Continuous recording over interval indicated on ISI plot with skipped spikes indicated by asterisks. In the illustrated unit, adaptation during the response caused progressive lengthening of the ISI, causing peaks on the ISI histogram to “smear.” Smearing was corrected in histograms on the right by detrending the response (i.e., subtracting the slow change estimated by a fitted monoexponential curve). Return plots still show clusters despite smearing. C, Proportion of skipped spikes before and after adding lidocaine.