Tapering of human nerve fibres.

To determine the tapering of human nerve fibres, rostral and caudal root pieces of cauda equina nerve roots were removed and nerve fibre diameter distributions were constructed for 4 myelin sheath thickness ranges for the two sites, and compared with each other. The reduction of the group diameter in the different α-motoneuron groups was 0.2 % per 13 cm. Accounting for systematic errors, there may be even less tapering. An identified single nerve fibre showed no tapering. Further, there is indication that γ-motoneurons, preganglionic sympa- thetic and parasympathetic fibres and skin afferents also reduce their fibre diameter by 0.2 % per 13 cm or less. Consequently, a nerve fibre with a diameter of 10 µm would be reduced to approximately 9.8 µm at 1m from the cell soma. Preganglionic parasympathetic fibres were found to be represented in roots S1 to S5. At similar distances from the spinal cord, the mean diameter of ventral root α1-motoneuron (FF) axons increased from the thoracic towards the lumbo-sacral region before de- creasing again in the lower sacral region. Usually no α1-motoneuron axons were found in S5 roots. The diameter distribution of unmyelinated nerve fibres of a ven- tral S5 root showed three peaks at 0.25, 0.95 and 1.2 µm. The unmyelinated fibres with diameters around 0.25 µm may represent parasympathetic fibres. In six se- lected areas of the ventral S5 root, 6.6 times more unmyelinated nerve fibres than myelinated fibres were found on the average.