Conditioning of posterior root reflexes by single-joint voluntary motor tasks

Abstract

Electrical stimulation of the human lumbosacral posterior roots is used to evoke spinal reflexes and to neuromodulate altered sensory-motor function following spinal cord injury. The posterior roots can be depolarized by epidural as well as transcutaneous electrical stimulation. Here we present preliminary results of conditioning of the reflex pathway using volitional motor tasks. The posterior root reflexes (PRR) were elicited by transcutaneous stimulation at the T11-T12 vertebral level. The conditioning input was generated by unilateral dorsi- and plantar flexion in supine position. First tSCS was demonstrated to evoke effectively short-latency reflexes, in all recorded leg muscle groups. The reflex nature was proved by attenuation of reflex responses when stimuli were applied in close succession (inter-stimuli interval 35 ms). Test responses, elicited at a stimulation rate of two pulses per second were inhibited in ipsilateral triceps surae during dorsiflexion, compared to rest. When conditioned by plantar flexion, responses in hamstrings, tibialis anterior, and triceps surae showed facilitation accompanied by muscular contraction. Due to functional equivalence, observed PRR modifications can be interpreted by previous studies of H reflex modification, particularly by the mechanisms of reciprocal Ia-inhibition and monosynaptic Ia excitation. Overall, the presented new methodology is a first step to assess spinal networks in healthy and disabled subjects. In further studies, the supraspinal influence to the lumbosacral network can be analyzed in more detail by using this work as a starting point.