Measurement of nerve fiber diameters in the region of the dorsal spinocerebellar and spinocervical tracts in the cat

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Abstract
Nerve fibers in the region of the dorsal spinocerebellar tract (DSCT) and the spinocervical tract (SCT) in the adult cat were measured at various spinal levels. Fiber measurements were made from photographs of transverse sections of Epon embedded material that had been fixed by vascular perfusion with a paraformaldehyde solution, post‐fixed in osmium tetroxide, and stained with Azur II‐methylene blue.The fiber diameter histograms from the region of the DSCT exhibited a bimodal distribution and between 50 to 75% of the fibers had a diameter of less than 5 μ. The presence of a subpopulation of large sized nerve fibers was identified between the 10 to 15 μ diameter intervals and this subpopulation appeared to decrease in size at the rostral spinal segments. This observation agrees with the electrophysiological studies of Lundberg and Oscarsson ('60) who found that the conduction velocity of the DSCT decreases at rostral levels.The fiber diameter measurements from the region of the SCT appeared to exhibit a unimodal distribution and between 40 to 75% of the fibers in this region had a diameter of less than 5 μ. By omission of fibers less than 5 μ, it could be shown that the mean values of the SCT decreased at successively rostral spinal levels. However, when the cumulative frequency distributions of the SCT data from different spinal levels were compared, a definite trend of statistically significant differences at the 0.05 level could not be established.The average fiber density (number of fibers/mm2) of the DSCT was 16,617 and the average fiber density of the SCT was 15,927.The heterogeneous organization of the dorsolateral fasciculus is discussed.