Intracellular Slow Potential of Dorsal Root Fibers

¹ From the Neuropsychiatric Institute, Psychiatric Division, University of Illinois, Chicago, Illinois

Intracellular potential changes associated with excitation were recorded from a dorsal root fiber in the funiculus of an isolated spinal cord of a bullfrog or toad. The spike potential of a dorsal root fiber was observed to follow the slow positive (inside) potential. This slow potential was recorded without a spike potential by applying stimuli sub-threshold for the impaled nerve fiber. The size of the slow potential increased gradually with increased strength of dorsal root stimuli, but with no accompanying prolongation of its declining phase. Dorsal root reflex discharges were occasionally superimposed on this slow potential. No summation of the slow potential resulted from two successive supramaximal stimuli. The slow potential induced by a test stimulus showed considerable occlusion even half a second after the conditioning stimulus. The slow positive (inside) potential was also recorded intracellularly by applying antidromic ventral root stimuli. The slow potential recorded here proved to be generated intrinsically by the depolarization of the surface membrane of the impaled nerve fiber. Active depolarization of the membrane of the terminal endings of the dorsal root fiber may spread out electrotonically along the membrane of the nerve fiber and provide this slow potential. The membrane of terminal endings of a single dorsal root fiber may be depolarized secondarily by the activity of other dorsal root fibers or secondary neurons. The relationship between positive deflection of the cord dorsum potential, the dorsal root potential, and the slow potential recorded intracellularly in this experiment has been analyzed.