Dorsal horn potentials and current source densities evoked by single action potentials in single slowly adapting type...

The slow-wave response of cat dorsal horn, elicited by single action potentials in single slowly adapting type I (SAI) axons, was mapped by averaging the slow wave recorded from each locus in a rectangular array of recording loci in the transverse plane. Current source-density (CSD) waveforms were computed from these averages. Evoked potentials always included N-waves, with mean latency = 4.9 ms, rise time (base line to peak) = 2.3 ms, and duration (base line-peak-base line) = 13.1 ms. In some planes, the N-wave was followed by a longer P-wave. The N-wave timing corresponded to previously described excitatory postsynaptic potentials (EPSPs) recorded intracellularly and excitatory discharges recorded extracellularly from single units evoked by single SAI spikes. The P-wave timing corresponded to a previously described postexcitatory suppression of SAI spike-evoked EPSPs and discharges following single conditioning SAI action potentials. The current sink during the N-wave had the following properties: It occurred in a column of tissue, perpendicular to the laminar borders, less than 600 microns wide; this is similar to the terminal domain of a single SAI collateral. It remains stationary during the N-wave, indicating that the excited population of dorsal horn elements does not spread or shift within the transverse plane during the response. It is somatotopically organized in the mediolateral dimension, in a manner similar to the somatotopic organization of primary afferent terminations and of dorsal horn cell receptive fields.