5 SOURCES OF A DORSAL-ROOT POTENTIAL - THEIR INTERACTIONS AND ORIGINSIN THE SUPERFICIAL DORSAL HORN

The dorsal root potential (DRP) was measured on the lumbar dorsal root s of urethan anesthetized rats and evoked by stimulation of five separ ate inputs. In some experiments, the dorsal cord potential was recorde d simultaneously. Stimulation of the L-3 dorsal root produced a DRP on the L-2 dorsal root containing the six components observed in the cat including the prolonged negative wave (DRP V of Lloyd 1952). A single shock to the myelinated fibers in the sural nerve produced a DRP on t he L-6 dorsal root after the arrival in the cord of the afferent volle y. The shape of this DRP was similar to that produced by dorsal root s timulation. Repetitive stimulation of the myelinated fibers in the gas trocnemius nerve also produced a prolonged negative DRP on the L-6 dor sal root. When a single stimulus (<5 mu A; 200 mu s) was applied throu gh a microelectrode to the superficial Lissauer Tract (LT) at the bord er of the L-2 and L-3 spinal segments, a characteristic prolonged nega tive DRP (LT-DRP) began on the L-2 dorsal root after some 15 ms. Stimu lation of the LT evoked DRPs bilaterally. Recordings on nearby dorsal roots showed this DRP to be unaccompanied by stimulation of afferent f ibers in those roots. The LT-DRP was unaffected by neonatal capsaicin treatment that destroyed most unmyelinated fibers. Measurements of mye linated fiber terminal excitability to microstimulation showed that th e LT-DRP was accompanied by primary afferent depolarization. Repetitiv e stimulation through a microelectrode in sensorimotor cortex provoked a prolonged and delayed negative DRP (recorded L-2-L-4) Stimulation i n the cortical arm area and recording on cervical dorsal roots showed that the DRP was evoked more from motor areas than sensory areas of co rtex. Interactions were observed between the LT-DRP and that evoked fr om the sural or gastrocnemius nerves or motor cortex. The LT-DRP was i nhibited by preceding stimulation of the other three sources but LT st imulation did not inhibit DRPs evoked from sural or gastrocnemius nerv es on the L-6 dorsal root or from motor cortex on the L-3 root. Howeve r, LT stimulation did inhibit the DRP evoked by a subsequent Lissaeur tract stimulus. Recordings were made from superficial dorsal horn neur ons. Covergence of input from LT sural, and gastrocnemius nerves and c ortex was observed. Spike-triggered averaging was used to examine the relationship between the ongoing discharge of superficial dorsal horn neurons and the spontaneous DRP. The discharge of 81% of LT responsive cells was correlated with the DRP.