[Evaluation of motor neuron excitability in lumbosacral spinal cord: Transcutaneous spinal cord stimulation as...

Multisegmental muscle responses (MMR) are reflexes in the leg muscles evoked by transcutaneous electrical spinal cord stimulation over the Th11–Th12 vertebrae. We have used MMR to evaluate the excitability of lumbosacral motor neurons in individuals suffering lower paraplegia. Ten individuals were tested using H-reflexes and MMR bilaterally before (n 0 = 20) and during 4-weeks course of rehabilitation (n=76). The H-reflex and MMR of m. gastrocnemius lateralis were obtained in: 15 and 13 cases out of 20, respectively. Both reflexes were recorded in 11 and were absent in 3 cases, matched up to 70% of recordings. In dynamic, the both methods were 100% reproducible and the responses’ amplitude varied in similar directions in 67% of records. The data confirm the validity and reproducibility of the MMR for evaluation of the motor neurons excitability in lumbosacral cord. The H-reflex magnitude shows moderate correlation with MMR in m. gastrocnemius lateralis (r = 0.59, p < 0.001), and low correlation with MMR in mm. rectus femoris, biceps femoris, and tibialis anterior (r < 0.40, p < 0.001). These findings do not allow extrapolate the results from the H-reflex measurement on the state of lumbosacral cord. At the same time, measurements of the MMR allow estimate simultaneously the excitability of motor pools innervating several muscle groups. This makes advantageous to assess the functional state of the motor neurons in the lumbosacral spinal cord for clinical and experimental studies, including the spinal cord damage.