Structural and Functional Changes in Nerve Roots Due to Tension at Various Strains and Strain Rates: An In-Vivo Study

This study investigates the functional and structural responses of spinal nerve roots in vivo to various strains and strain rates. Seventy-two L5 dorsal nerve roots from male Sprague-Dawley rats were each subjected to a predetermined strain (<10%, 10-20%, and >20%; n = 8) and rate (0.01 mm/sec, 1 mm/sec, or 15 mm/sec; n = 24). Neurophysiologic recordings were performed before and after stretch to determine changes in conduction velocity (CV), amplitude, and area of the compound action potential (CAP). Morphological injury as evident by primary and secondary axotomy as well as impaired axoplasmic transport (IAT) was determined using the palmgren silver impregnation technique and beta APP immunostaining, respectively. The results from neurophysiologic recordings indicate that as strain and rate increased, there was a decrease in CV, amplitude, and area of the CAP. Further, high strains led to a complete conduction block that appeared to be rate dependent. Strains of 16%, 10%, and 9%, at 0.01 mm/sec, 1 mm/sec, and 15 mm/sec, respectively, led to 50% probability of complete conduction block in the nerve roots. Results from histological assessment indicate an increase in periaxonal spacing (secondary axotomy) and torn fibers (primary axotomy), as well as impaired IAT, with increasing strain and rate. Overall, the results from the current study indicate that (1) functional nerve root injuries as evident by changes in the CV, amplitude, and area of the CAP are strain- and rate-dependent; (2) high strains at low rates cause complete conduction block in the roots, while a similar block was observed at lower strains at the high rate; (3) the extent of IAT and primary and secondary axotomy occurred concomitant with functional injury and were strain- and rate-dependent.