Unmyelinated Fibres in Normal and Compressed Peripheral Nerves of the Baboon: A Quantitative Electron Microscopic Study

The calibre spectrum of unmyelinated axons in normal medial popliteal nerves of baboons, fixed by perfusion with glutaraldehyde and measured by electron microscopy, is consistently unimodal. Following acute compression with a pneumatic tourniquet sufficient to cause a local conduction block, unmyelinated fibres are usually undamaged. This offers an anatomical basis for the selective sensory sparing (particularly pain) in a neurapraxia. Following local compression severe enough to cause Wallerian degeneration in a proportion of the myelinated fibres, a new population of axonal profiles eventually grows within the unmyelinated fibres. The features of these profiles are those of axonal sprouts and their presence results in a bimodal calibre spectrum of unmyelinated axons. This suggests that in these cases the period of prolonged compression has resulted in damage to the unmyelinated fibres.