Sensory afferent impulses originate from dorsal root ganglia as well as from the periphery in normal and nerve injured rats

Abstract

Single units were recorded in dorsal roots or in the sciatic nerve of anesthetized rats. Sections made by stimulation and by collision showed that some ongoing nerve impulses were originating from the dorsal root ganglia and not from the central or peripheral ends of the axons. In a sample of 2731 intact or acutely sectioned myelinated sensory fibers, 4.75 .+-. 3.7% contained impulses generated within the dorsal root ganglia. In 2555 axons sectioned in the periphery 2-109 days before, this percentage rose to 8.6 .+-. 4.8%. Considerable variation existed between animals; 0-14% in intact and acutely sectioned nerves and 1-21% in chronically sectioned nerves. The conduction velocity of the active fibers did not differ significantly from the coduction velocity of unselected fibers. The common pattern of ongoing activity from the ganglion was irregular and with a low frequency (.apprx. 4 Hz) in contrast to the pattern of impulses originating in a neuroma which usually have a highly frequency with regular intervals. Slight mechanical pressure on the dorsal root ganglion increased and frequency of impulses. Unmyelinated fibers contained impulses originating in the dorsal root ganglion. In intact or acutely sectioned unmyelinated axons, the percentage of active fibers 4.4 .+-. 3.5% was approximately the same as in myelinated fibers but there were no signs of an increase following chronic section. Fine filament dissection of dorsal roots and of peripheral nerves and collision experiments showed that impulses originating in dorsal root ganglia were propagated both orthodromically into the root and antidromically into the peripheral nerve. The same axon could contain 2 different alternating sites of origin of nerve impulses: one in the neuroma or sensory ending and one in the ganglion. The dorsal root ganglion with its ongoing activity and mechanical sensitivity could be a source of pain producing impulses and could particularly contribute to pain in those conditions of peripheral nerve damage where pain persists after peripheral anesthesia or where vertebral manipulation is painful.