Papers of the Week

Spinal cord stimulation (SCS) is used clinically to limit chronic pain but questions remain on the identity of axonal populations recruited. We developed an adult mouse spinal cord preparation to assess recruitment following delivery of clinically-analogous stimuli determined by downscaling a finite element model of clinical SCS. Analogous electric field distributions were generated with 300 µm x 300 µm electrodes positioned 200 µm above the dorsal column (DC) with stimulation between 50-200 µA. We compared axonal recruitment using electrodes of comparable size and stimulus amplitudes – when contacting the caudal thoracic DC, and at 200 or 600 mm above. Antidromic responses recorded from the DC, the adjacent Lissauer tract (LT), and in dorsal roots (DRs) were found to be amplitude and site-dependent. Responses in the DC included a unique component not seen in DRs, having the lowest SCS recruitment amplitude and fastest conduction velocity. At 200 mm above, mean cathodic SCS recruitment threshold for axons in DRs and LT were 2.6 and 4.4 times higher, respectively, than DC threshold. SCS recruited primary afferents in all (up to 9) caudal segments sampled. While A and C fibers could be recruited at nearby segments, only A fiber recruitment and synaptically-mediated dorsal root reflexes were observed in more distant segments. In sum, clinically-analogous SCS led to multisegmental recruitment of several somatosensory-encoding axonal populations. Most striking is the possibility that the lowest threshold recruitment of a non-primary afferent population in the DC are postsynaptic dorsal column tract cells (PSDCs) projecting to gracile nuclei.