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.