Modulation of reciprocal inhibition at the wrist as a neurophysiological correlate of tremor suppression: A pilot healthy subject study

It has been shown that Ia afferents inhibit muscle activity of the ipsilateral antagonist, a mechanism known as reciprocal inhibition. Stimulation of these afferents may be explored for the therapeutic reduction of pathological tremor (Essential Tremor or due Parkinson's Disease, for example). However, only a few studies have investigated reciprocal inhibition of wrist flexor / extensor motor control. The main goal of this study was to characterize reciprocal inhibition of wrist flexors / extensors by applying surface electrical stimulation to the radial and median nerves, respectively. Firstly, the direct (M) and monosynaptic (H) reflex responses to increasing median and radial nerve stimulation were recorded to characterize the recruitment curve of the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles, respectively. Based on the recruitment curve data, we then stimulated the median and radial nerves below (< MT) and above (> MT) motor threshold (MT) during a submaximal isometric task to assess the amount of inhibition on ECR and FCR antagonist muscles, respectively. The stimulation of both nerves produced a long-duration inhibition of the antagonist motoneuron pool activity. On average, maximum peak of inhibition was 27 ± 6% for ECR and 32 ± 9% for FCR with stimulation < MT; maximum peak of inhibition was 45 ± 7% for ECR and 44 ± 13% for FCR when using stimulation > MT. These results validate this neurophysiological technique that demonstrates a mechanism similar to classical reciprocal Ia inhibition reported for other limb joints and that can be used to benchmark strategies to suppress pathological tremor.