An input-output analysis of the dorsal column nuclei

The input-output (IO) relation of a nucleus is defined here as the total spike activity leaving the nucleus as a function of the total activity entering the nucleus measured over an appropriate time interval. The purpose of this study was to elucidate the mechanisms underlying the shape of this function. Parallel studies were conducted on chloralose-anesthetized cats to provide IO functions based on both evoked potential and single cell measures of output. Two sets of bipolar electrodes applied to the superficial radial nerve were used to stimulate and record input activity of various intensities. A bipolar electrode placed in the contralateral medial lemniscus provided evoked potential measures of output in one series of experiments. Glass micropipet electrodes placed in the cuneate nucleus provided single cell measures of output in the other series. We found good agreement between the average IO curves generated from the evoked potential and pooled single cell measurements. The single cell measurements showed further that the steep rise in the normalized IO relationship is due primarily to the recruitment of additional output cells by the increasing input. The saturation of output, which appears at 50 to 60% of the maximum input magnitude, reflects a limiting of both the number of cells active and the number of impulses evoked per active cell. The input-output relationships are well described by the function y = 1 ? e^?kx, derived from a simple saturation model of the nucleus. This model and our experimental data are consistent with the concept that single afferent fibers are capable of activating several output cells and, conversely, that each output cell receives input from several such afferent fibers.