Proprioceptive control of human wrist extensor motor units during an attention-demanding task.
The responsiveness of the tonically firing single motor units (SMU) to Ia afferent volleys elicited by either mechanical (T-reflex) or electrical nerve stimulation (H-reflex) was tested in the extensor carpi radialis muscle (ECR) while the subjects were maintaining a steady wrist extension force using visual feedback set either at low or high gain. The aim was to determine whether the proprioceptive control of tonic motoneuronal activity depends on the level of attentiveness required by the behavioural context. The response probability of the SMUs to tendon taps was significantly higher (p<0.0001) and that to electrical nerve stimulation was lower (p<0.001) during the more demanding task. Since these changes in SMU responsiveness were not accompanied by any differences in either the motor unit firing patterns or the mean levels of EMG muscle activity, it can be concluded that there were no attention-related changes in the net excitatory drive to the ECR motoneurons. These results are consistent with the idea that fusimotor sensitization of the muscle spindle may have occurred in the more demanding task: an increase in the mechanical sensitivity of the muscle spindles would certainly account for both the T-reflex facilitation and the H-reflex depression observed. The attention-demanding task therefore seemed to involve an independent fusimotor drive activation process. The results of this study suggest that an adaptation of the fusimotor system occurs in humans, depending on the levels of attention and accuracy required to perform the ongoing motor task, as previously reported to occur in animals.
Laboratoire de Plasticité et PhysioPathologie de la Motricité (P3M), UMR 6196, CNRS/Université de la Méditerranée, 31, chemin Joseph-Aiguier, 13402 Marseille Cedex 20, France.
Gilel
G
Nafati
Christiane
C
Rossi-Durand
Annie
A
Schmied
D000200:Action Potentials / Q000502:physiology; D000328:Adult; D001288:Attention / Q000502:physiology*; D016009:Chi-Square Distribution; D004576:Electromyography; D030141:Feedback, Psychological / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009048:Motor Skills / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology; D010470:Perceptual Masking / Q000502:physiology; D011434:Proprioception / Q000502:physiology*; D012016:Reference Values; D014953:Wrist / Q000502:physiology*