Dimensions of individual alpha and gamma motor fibres in the ventral funiculus of the cat spinal cord.

Using light and electron microscopy, axon diameter, myelin sheath thickness (measured as number of myelin lamellae) and internodal length of alpha and gamma motor axons of the L7 ventral root and spinal cord segment were investigated in serial cross-sections. The CNS internodes of the alpha motor fibres had, on average, an axon diameter of 8.6 microns, 105 myelin lamellae and a length of about 560 microns. The CNS internodes of the gamma motor fibres had, on average, an axon diameter of 3.4 microns, 66 myelin lamellae and a length of about 440 microns. Axon diameter at the nodes of Ranvier was 30-40% of the internodal axon diameter. Axon diameter, number of myelin lamellae and internodal length varied considerably between consecutive internodes. Statistical analysis showed no systematic increases or decreases. Regression analyses of the scatter plots of the number of myelin lamellae and internodal length against axon diameter showed large variations and correlation coefficients of r < 0.50. In conformity with ventral root (PNS) internodes (Nilsson & Berthold, 1988) the plotting of intrafunicular (CNS) internodal myelin volume against internodal axon mantle area showed linear correlations with correlation coefficients of r > 0.90. The mean axon diameter of the investigated CNS internodes was similar to, the mean number of myelin lamellae somewhat lower than, and the mean internodal length considerably shorter than that of internodes of axons of the L7 ventral root (Nilsson & Berthold, 1988). In contrast to the ventral root, the intrafunicular alpha motor fibres had higher g values (axon diameter/fibre diameter value) and lower il/d ratios (internodal length/axon diameter ratio) than is considered optimal for conduction. We consider that these deviations from the theoretical optimum are not large enough to impair the conduction properties of the CNS parts of the motor axons in a significant way.