Electrophysiology Of Posterior Root-Muscle Reflexes

Abstract

Posterior root-muscle (PRM) reflexes can be elicited by epidural as well as recently also transcutaneous electrical stimulation of the lumbar spinal cord (1). Epidural spinal cord stimulation can be used for neuromodulation of spinal spasticity and generation of rhythmicity (2, 3). For these purposes it was evident that the stimulation site was causative for the effect (2, 3). Thus, here we tested the selectivity of transcutaneous stimulation of the lumbar spinal cord by eliciting PRM reflexes and recording the compound muscle action potentials. Ten subjects (5 f.) with intact nervous system were studied in supine, standing and prone position. The stimulation set-up consisted of an electrode array centered at the spinal level of T12 and large electrodes over the abdomen. The array had 7 levels, spanning 12 cm in rostrocaudal direction and each consisting of 3 x 1 cm electrodes. Every level was separately used for stimulation. Stimulation was delivered up to 125 mA. Electromyographic activities were recorded in thigh and shank muscle groups bilaterally. With the variation of the stimulation level a definite change in the recruitment of PRM reflexes and M waves was observed. PRM reflexes of the quadriceps muscle group had the lowest thresholds and highest peak-to-peak (PTP) amplitudes at the rostral and triceps surae muscle at the caudal electrode levels. PRM reflexes in quadriceps were markedly reduced and the probability of eliciting M waves increased at the caudal stimulation sites. Whereas, in triceps surae M waves were rarely recorded. Changes of the body position clearly influenced the stimulation effect. In the supine position fewest M waves and lowest reflex thresholds were recorded. The PTP amplitudes of the responses in standing position were in mean 80% of the corresponding responses in supine position. Whereas, in prone position M waves were the predominate finding. Even though the distribution of the generated electrical field with transcutaneous electrodes is unfocused in relation to the target structures (4), a selective stimulation is possible. The anatomical structure and electrical properties of the spine, i.e. relatively high conductivities of the intervertebral discs, might cause a segmental shift of the field, by changing the stimulation level. Changes of the PRM reflex elicitation with body position changes were most probably related to changes of the volume conductor as well as due to additional physiological influences.

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