Journal of Neurophysiology, Vol 55, Issue 5 1104-1113, Copyright © 1986 by APS
Dorsal horn potentials and current source densities evoked by single action potentials in single slowly adapting type I axons
P. B. Brown and L. A. Ritz
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.
