NUMBERS AND SIZES OF NERVE FIBRES IN MOUSE SPINAL ROOTS

Abstract

The numbers of myelinated and non-myelinated nerve fibers in the spinal roots of 1 strain of mouse were counted from electron micrographs and the accuracy of counting procedures assessed. Non-myelinated nerve fibers are present in the ventral roots at all levels though the largest number are found in the thoracic and L1 [lumbar], L5, L6 and S1 [sacral] ventral roots. The number is very small in L2, L3 and L4 ventral roots which are those supplying the hind limb. Fiber diameter histograms were obtained from measurements of cross sectional area for myelinated and non-myelinated fibers in L1 and L4 ventral roots. The relationship of myelin thickness to axon diameter shows that to a near approximation the myelin thickness is 1/5 to 1/4 the axon diameter. A measure of circularity was calculated, the ratio of the measured cross-sectional area to the area calculated from the measured perimeter assuming the fiber to be circular, and plotted against the axon perimeter for L1 and L4 ventral root myelinated fibers. This showed a tendency for larger fibers to be less circular than smaller fibers.