Task dependent gain regulation of spinal circuits projecting to the human flexor carpi radialis

Abstract

In humans, the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles act as antagonists during wrist flexion-extension and as functional synergists during radial deviation. In contrast to the situation in most antagonist muscle pairs, Renshaw cells innervated by the motor neurons of each muscle inhibit the motoneurons, but not Ia inhibitory interneurons, of the opposite motor pool. Here we compared gain regulation of spinal circuits projecting to FCR motoneurons during two tasks: flexion and radial deviation of the wrist. We also investigated the functional consequences of this organisation for maximal voluntary contractions (MVCs). Electromyographic (EMG) recordings were taken from FCR, ECR longus and ECR brevis using fine-wire electrodes and electrical stimulation was delivered to the median and radial nerves. Ten volunteers participated in three experiments. 1. To study the regulation of the Renshaw cell-mediated, inhibitory pathway from ECR to FCR motoneurons, forty stimuli were delivered to the radial nerve at 50% of the maximal M-wave amplitude for ECR brevis. Stimuli were delivered during both isometric wrist flexions and radial deviation actions with an equivalent EMG amplitude in FCR (approximately 5% wrist flexion MVC). 2. To explore the homonymous Ia afferent pathway to FCR motoneurons, 50 stimuli were delivered to the median nerve at intensities ranging from below motor threshold to at least two times that which evoked a maximal M-wave during wrist flexion and radial deviation (matched FCR EMG at approximately 5% wrist flexion MVC). 3. EMG amplitude was measured during MVCs in wrist flexion, extension and radial deviation. There was no significant difference in the inhibition of FCR EMG induced via ECR-coupled Renshaw cells between radial deviation and wrist flexion. However, the mean FCR H-reflex amplitude was significantly (P<0.05) greater during wrist flexion than radial deviation. Furthermore, EMG amplitude in FCR and ECR brevis was significantly (P<0.05) greater during MVCs in wrist flexion and extension (respectively) than radial deviation. ECR longus EMG was significantly greater during MVCs in radial deviation than extension. These results indicate that the gain of the Renshaw-mediated inhibitory pathway between ECR and FCR motoneurons is similar for weak flexion and radial deviation actions. However, the gain of the H-reflex pathway to FCR is greater during wrist flexion than radial deviation. Transmission through both of these pathways probably contributes to the inability of individuals to maximally activate FCR during radial deviation MVCs.

Robust Control in Human Reaching Movements: A Model-Free Strategy to Compensate for Unpredictable Disturbances

“…On the other hand, an increase in baseline muscle activity, evoked by a background load or by co-contraction, is associated with "gainscaling" (Stein et al, 1995;Pruszynski et al, 2009;Crevecoeur and Scott, 2014;Nashed et al, 2015), which may increase feedback gains by ␣-gamma coactivation (Vallbo, 1974). Indeed a modulation of spindles sensitivity is likely involved, because studies using galvanic stimulations (H-reflex) did not always observe gain-scaling with co-contraction (Nielsen and Kagamihara, 1993;Carroll et al, 2005). The tradeoff between efficiency and robustness is also apparent in this scenario, as it is clear that there is a metabolic cost incurred with the increased activity in neural circuits and in muscles used to maintain higher levels of activity.…”

Changes in wrist muscle activity with forearm posture: implications for the study of sensorimotor transformations

“…Although neurons in M1 were found whose activity parallel changes in muscle activity with limb postures Kalaska 1997, 2003), these changes are likely to be ultimately tuned by feedback integration by lower motor networks (de Rugy et al 2012;Graziano 2006;Loeb et al 1999;Yanai et al 2008). The wrist system appears particularly interesting to investigate these lower-level components, because wrist muscles switch their functional relationship, depending on the direction of action, and the spinal cord circuitry is known to be intimately related to this adjustable functional relationship (Baldissera et al 2000;Carroll et al 2005;Pierrot-Deseilligny and Burke 2005;Raphael et al 2010). For instance, FCR and FCU are synergists for wrist flexion, but antagonists for radial and ulnar deviations.…”

Wrist strength is dependent on simultaneous power grip intensity

“…Activating two muscle groups that create an opposite wrist moment might have been particularly difficult. The ECR and finger flexor muscles (FDS, FDP) may have inhibited each other by a central nervous system mechanism (Aymard et al, 1997; Carroll et al, 2005; Sherrington, 1906). In fact, the grip force measured during maximum wrist extension with maximum grip effort was only 54% of the subjects’ grip strength, and FDS and FDP EMG levels were only 30% and 28%, respectively.…”