Modulation of the Ia Input- Motoneuron Output Relationship of Human Flexor Carpi Radialis During Muscle Contraction

Abstract

<p> A novel method has been developed to determine the quantitative relationship between the percentage ofla fibres stimulated synchronously, and the percentage of human flexor carpi radialis (FCR) motoneurons (MNs) discharged reflexly. The method assumes a normal distribution of Ia fibre thresholds to electrical stimulation. Among the 11 healthy subjects tested during relaxation, there were considerable differences in the reflex excitability of the FCR MNs to quantitative Ia fibre inputs. The Ia fibre input-FCR MN output curves were either initially steeply-rising, initially slowly-rising, or initially and latterly steeply-rising. When the results were averaged, however, the curve for the 11 subjects in the relaxed state appeared to be fairly linear throughout the entire range of the Ia fibre inputs, and a mean of82% of the Ia fibres discharged approximately 20% of theMNs. </p> <p> Regardless of the variability in the shape of individual input-output curves during relaxation, potentiation of the FCR MN output was observed during weak wrist flexion in 10 of the 11 subjects over the full range of the Ia fibre inputs. In contrast, a depression of the MN output was exhibited in all 8 subjects who weakly contracted the extensors over the full range of the Ia fibre inputs. The changes in the Ia input-MN output relationship in going from rest to voluntary contractions of wrist muscles are thought to reflect modulation by presynpatic inhibition of the Ia terminals. With very large Ia inputs during wrist extension, however, there is a steep rise in the input-output curve, which could indicate a decrease in presynaptic inhibition of Ia terminals in the FCR muscle. The modulation ofthe input-output relationship observed in the present study is consistent with the task-dependent differences of reflex excitability observed by Stein et al. (1988). </p>