Quantitative studies of sequential peripheral nerve fiber diameter histograms and biophysical implications

Abstract

Peripheral nerve cross sections were analyzed for the variation in peripheral nerve histograms obtained along a single short segment of cat saphenous nerve. Individual myelinated fibers in one fascicle were traced through nine sequential sections. Two mean fiber diameter histograms were obtained from the sequential cross sections: a "mean fiber diameter histogram" constructed from the arithmetic average of the number of fibers in each bin of each section, and a "histogram of mean fiber diameters" constructed from the mean diameter of each fiber traced through multiple secrtions. Compared to the individual sectional histograms, the number of fibers at the two extremes of the diameter range was generally less and the mean histograms were smoother. The small-diameter peak of the "histogram of mean fiber diameters" was enhanced 22 to 65% over the peak of the individual sectional histograms and 40% over the "mean fiber diameter histogram". These findings indicate that any single-cross section fiber diameter histogram may be a relatively poor quantitative approximation of the histogram of mean fiber diameters; particularly at the extreme diameters and at the peaks and valleys of the distribution. A computer-based model was developed to adjust single-section fiber diameter histograms to compensate for sampling error associated with single-section histograms. The adjustment procedure enhanced the narrow small diameter peak and broadened the valleys in accordance with the model theory and the experimental results. The biophysical implications of the results were examined in terms of (i) the relationship between fiber diameter and conduction velocity, and (ii) the relationship between fiber diameter histograms and the neural compound action potential.