Fig. 1. A, Sketch of the experimental set-up used to determine that the origin of spontaneous firing was within the DRG. Single fiber activity was recorded from centrally connected microfilaments teased from the gastrocnemius soleus (GS) nerve (rec 1). At first, electrical stimuli were delivered through the dorsal root (DR) electrode (stim 2), and the evoked single fiber activity was recorded at rec 1, so that the response latency (t = 3.0 msec) could be measured. After determination of propagation distancesa1 = 11 mm anda2 = 47 mm, conduction velocity (v) was calculated as v = (2a1 +a2)/t = 23 m/sec. Next, the DR electrode was used as recording electrode (rec 2). Amplitude window discrimination of spontaneous action potentials, recorded at rec 1, triggered averaging of dorsal root activity recorded at rec 2, from −4 to 4 msec with the trigger point at 0 msec. Onset of the averaged potential was measured 2.5 msec before the trigger. The time between onset of action potentials recorded by electrode 1 and the point at the falling phase of these action potentials that served as trigger (marked with dot) was 0.7 msec. Thus, onset of single action potentials at rec 1 followed 2.5–0.7 = 1.8 msec after onset of the averaged potential at rec 2. This time difference between onset of the potentials recorded at rec 1 and 2 is termed t2 (sot2 = 1.8 msec). The propagation timet1 of action potentials from their site of origin to rec 2 could now be calculated: because t= 2t1 +t2, we gett1 = 0.6 msec. Therefore, the estimated site of ectopic spike generation wast1 × v = 13.8 mm distal to rec 2. B, Schematic drawings illustrate the lesions used in the different experimental groups. TIB, Tibial nerve; SU, sural nerve; PC, common peroneal nerve. Experimental groups: E1, GS and SU nerve cut; E2, TIB, PC, SU, and GS nerve cut;E3, GS nerve intact, TIB, PC, and SU nerve cut.