Intracellular responses of dorsal horn cells to cutaneous and sural nerve A and C fiber stimuli

Data point to the existence of long-term postsynaptic facilitatory responses activated by small-fiber inputs in spinal cats, related in most cases to a recruitment of additional EPSP per stimulus and was often related to a progressive membrane depolarization.

Abstract

Intracellular recordings were made in 69 L-7 dorsal horn cells in spinal cats. Cells in Rexed's layers IV–VI responding to single shocks to the sural nerve were studied. Three types of short latency (5–10 msec) PSP and spike responses were evoked by low-intensity nerve stimulation: Initial EPSP followed by a long IPSP (31 cells); EPSP only (21 cells); and spikes from baseline only (17 cells). Increasing stimulus intensity to excite both A and C nerve fibers elicited both short-latency and long-latency (> 200 msec) PSP sequences and spikes in 31 cells. Long-latency PSP and spike responses were similar to the shorterlatency responses, but were more frequently followed by spike discharges lasting for several hundred milliseconds. Recruitment of additional spikes per response was sometimes produced by stimulation of C fibers at rates greater than 0.3/sec ("windup"). This phenomenon was related in most cases to a recruitment of additional EPSP per stimulus and was often related to a progressive membrane depolarization. Responses to constant intensity heat were positively accelerating functions of time. These data point to the existence of long-term postsynaptic facilitatory responses activated by small-fiber inputs.