DIMENSIONS OF INDIVIDUAL ALPHA-MOTOR AND GAMMA-MOTOR FIBERS IN THE VENTRAL FUNICULUS OF THE CAT SPINAL-CORD

Using light and electron microscopy, axon diameter, myelin sheath thic kness (measured as number of myelin lamellae) and internodal length of alpha and gamma motor axons of the L7 ventral root and spinal cord se gment were investigated in serial cross sections. The CNS internodes o f the alpha motor fibres had, on average, an axon diameter of 8.6 mu m , 105 myelin lamellae and a length of about 560 mu m The CNS internode s of the gamma motor fibres had, on average, an axon diameter of 3.4 m u m, 66 myelin lamellae and a length of about 440 mu m. Axon diameter at the nodes of Ranvier was 30-40% of the internodal axon diameter. Ax on diameter, number of myelin lamellae and internodal length varied co nsiderably between consecutive internodes. Statistical analysis showed no systematic increases or decreases. Regression analyses of the scat ter plots of the number of myelin lamellae and internodal length again st axon diameter showed large variations and correlation coefficients of r < 0.50. In conformity with Ventral root (PNS) internodes (Nilsson and Berthold, 1988) the plotting of intrafunicular (CNS) internodal m yelin volume against internodal axon mantle area showed linear correla tions with correlation coefficients of r > 0.90. The mean axon diamete r of the investigated CNS internodes was similar to, the mean number o f myelin lamellae somewhat lower than, and the mean internodal length considerably shorter than that of internodes of axons of the L7 ventra l root (Nilsson and Berthold, 1988); In contrast to the ventral root, the intrafunicular alpha motor fibres had higher g values (axon diamet er/fibre diameter value) and lower il/d ratios (internodal length/axon diameter ratio) than is considered optimal for conduction. We conside r that these deviations from the theoretical optimum are not large eno ugh to impair the conduction properties of the CNS parts of the motor axons in a significant way.