A detailed morphometrical analysis of the pyramidal tract of the rat

The present study is aimed at a quantitative analysis and comparison of the fibers of the pyramidal tract of the rat at two levels: the pyramis medullae and the second cervical segment. For this purpose both levels of the pyramidal tracts of 5 rats have been studied morphometrically at the ultrastructural level. Fiber numbers as well as fiber diameter distributions are presented for both myelinated and unmyelinated axons. At the level of the pyramis medullae the number of unmyelinated fibers (140,000 +/- 7000) exceeds the number of myelinated axons (103,000 +/- 6000). In contrast at the level of the second cervical segment the numbers of fibers of the axon populations studied are not significantly different, viz. 43,000 +/- 2000 myelinated and 35,000 +/- 8000 unmyelinated fibers. However, these numbers mean a significant decrease of myelinated axons (48%) compared with the pyramis medullae level and an even much larger decrease (75%) in the numbers of unmyelinated fibers. Diameter distributions, obtained from the minimal fiber diameter, show a similar, monomodal shape for all axon and myelin profiles. For unmyelinated axons the minimal diameter ranges from 0.05 to 1.21 micron with a mean of 0.18 +/- 0.03 micron. For myelinated axons a mean minimal diameter of 0.72 +/- 0.12 micron has been found (range 0.13-4.92 micron), whereas their diameter measured with myelin sheath measure 1.08 +/- 0.13 micron (range 0.25-6.03 micron). The average thickness of the myelin sheath is 0.2 micron and is strongly correlated to axon diameter. Furthermore, g-ratio has been computed at a mean of 0.65 and is fairly constant throughout the diameter spectrum. Several possibilities concerning the functional and anatomical qualities of the numerically important unmyelinated axon population in the pyramidal tract of the rat are discussed.