PRECISION AND VARIABILITY OF HINDLIMB REPRESENTATION IN CAT DORSAL HORN AND IMPLICATIONS FOR TACTILE LOCALIZATION

1. One hundred fifty-eight cells were recorded extracellularly in rows of tracks spanning both left and right dorsal horns, at segmental bou ndaries and midsegment in segments L5-S1, in six anesthetized cats. Fo r each cell the low-threshold cutaneous mechanoreceptive field was det ermined with the use of hand-held probes, and the recording site was m arked with a microlesion. Recording sites were reconstructed, and the mediolateral (ML) and rostrocaudal (RC) locations of each cell were re corded along with the location of the cell's receptive field, expresse d as distance from tips of toes (D). 2. Ninety-five percent of pairs o f cells recorded from bilaterally symmetric locations (+/-10%) in the same animal had receptive fields on opposite legs that had components that were mirror symmetric. Only 42% of cell pairs deviating from bila teral symmetry by approximately +/-240 mum had receptive fields with o verlapping components. This indicated that there was a substantial bil ateral symmetry that was not simply due to large receptive fields. 3. The trajectories of receptive fields of cells in a single row of track s were plotted in order of mediolateral recording site, going from med ial to lateral, combining both sides. These trajectories followed a di stoproximal course on the leg. Of 144 adjacent cells used to plot thes e trajectories, with an average spacing of approximately 120 Am, only 6 reversals of the distoproximal gradient polarity were observed withi n animals. 4. Data from individual animals were shifted rostrally and caudally, to obtain best agreement of mediolateral somatotopic gradien ts with the combined data from the other animals in the sample. Best a greement was obtained with shifts ranging from 0.3 segment rostral to 0.4 segment caudal, with an average absolute value shift of 0.22 segme nt. 5. By comparing cell pairs within the same dorsal horn, on opposit e sides of the same animal, and across animals, variability in cell pl acement given the average map and the receptive field could be calcula ted. Interanimal variability and bilateral asymmetry were approximatel y +/-60 mum, and within-dorsal horn variability was approximately +/-3 5 mum. The interanimal variability is equivalent to a variability of d istoproximal receptive-field location on the leg of +/-13 mm, with a s maller variability in areas of high magnification (e.g., the toes), an d a larger variability in areas with small magnification (e.g., the th igh). This degree of variability is consistent with the ability of ani mals with transected dorsal columns to localize tactile stimuli with a normal degree of precision.