Measurements and mapping of 282,420 nerve fibers in the S1–5 nerve roots

Object The study aims to analyze nerve fiber types in the sacral nerve roots as a prerequisite for stimulation. Methods One-micrometer cross-sections of human ventral and dorsal S1–5 roots were stained with osmium and toluidine blue. The total fiber diameter and myelin sheath were measured in 282,420 nerve fibers. Results The analysis revealed the following 3 main nerve fiber types: Aα fibers (diameter 6–14 μm), Aγ fibers (diameter 2–4 μm), and B fibers (diameter < 2 μm). The B fibers were absent in S-1, present in some S-2 fascicles, and abundant from S-3 to S-5. The Aα fibers dominated the S-1 roots and most fascicles of S-2 roots. In the S3–5 roots, only a few Aα fibers were present. The relative occurrence of Aγ fibers increased from S-1 to S-5. In dorsal roots, Aγ fibers represented ~ 70% of all nerve fibers in every root and fascicle. Conclusions The B fibers represented efferent parasympathetic fibers. These fibers were concentrated in certain areas of the nerve roots, not randomly distributed. The Aα fibers innervate lower-extremity muscles and sphincters. The inverse correlation of Aα and Aγ fibers in the ventral roots from S-1 to S-5 is surprising. In dorsal roots, Aγ fibers may conduct pain, touch, and temperature signals. Highly selective fiber stimulation specific for type, location, and direction may improve sacral nerve stimulation for a spastic bladder in paraplegic individuals.