Non-myelinated axons are rare in the medullary pyramids of the macaque monkey.
Previous electron microscopic studies of the medullary pyramids have concluded that non-myelinated axons constitute about 30-60% of all axons in the pyramid of the rat, and about 8-15% in the cat and monkey. Physiological studies of pyramidal tract axons have not found fibers conducting in the range predicted for non-myelinated axons, less than 1 m/s. This present study of the primate pyramid demonstrates that most of the profiles which could be interpreted as being non-myelinated axons when viewed in cross-section, are actually astroglial processes when examined in longitudinal section. We conclude that non-myelinated axons constitute less than 1% of the pyramidal tract axons in the old world adult primate.
D000818:Animals; D001369:Axons / Q000648:ultrastructure*; D008252:Macaca fascicularis; D008253:Macaca mulatta; D008854:Microscopy, Electron; D009413:Nerve Fibers, Myelinated / Q000648:ultrastructure*; D011712:Pyramidal Tracts / Q000648:ultrastructure*; D013045:Species Specificity