Ia presynaptic inhibition in human wrist extensor muscles: Effects of motor task and cutaneous afferent activity
Section snippets
The importance of antagonist muscle co-contraction in motor control
Tilney and Pike (1925) have reported that `muscular co-ordination depends primarily on the synchronous co-contraction relation in the antagonist muscle groups' (quoted by [39]). The most obvious functional consequence of the co-contraction of antagonist muscles is that it might enhance the joint stiffness, as first established by Paillard [30] in the case of human leg muscles. Co-contraction results in a greater joint stiffness than that which results from adding together the activation of the
The changes in the monosynaptic control exerted by the muscle spindle Ia afferents on motoneuron activity during the co-contraction of antagonist muscles
In a study on task-dependent changes in the effectiveness of the monosynaptic Ia muscle spindle afferent inputs to the motoneurons of human leg muscles, Paillard [30] established that the reflex response of the soleus muscle to electrical stimulation of the muscle spindle primary afferents (H-reflex) was dramatically enhanced when all the muscles of the leg were co-activated. Nielsen and Kagamihara [29] have observed on the contrary that the soleus H-reflex was lower during the co-contraction
Evidence for the involvement of presynaptic inhibition
The results of the previous studies indicate that various changes in the effectiveness of the monosynaptic Ia muscle spindle afferent inputs to the motoneurons of the human leg and arm muscles occur during the co-activation of antagonist muscles, depending on the muscles. This therefore raises the question as to what mechanisms might be responsible for these differential task-effects.
The first hypothesis is that the efficiency of the gamma drive might be enhanced during the co-activation of
Possible reasons for changes in Ia presynaptic inhibition during co-activation of antagonist muscles
Among the various possible explanations for the co-contraction-dependent changes in Ia presynaptic inhibition, the first is that the muscle spindle afferents originating from the antagonist muscles might play a major role, as originally observed in leg muscles [40]. In the human wrist myotatic unit, Day et al. [10] have reported that the Ia muscle spindle afferents from the wrist flexor muscles projected to the interneurons of the extensor Ia presynaptic inhibitory pathways (figure 1). A
Evidence for the involvement of hand cutaneous receptor inputs
In some recent experiments [1], the question was addressed as to whether changes in Ia presynaptic inhibition might be at the origin of the task-dependent changes in the monosynaptic Ia muscle spindle afferent inputs to the motoneurons of the wrist extensor muscles. Special attention was paid here to the possible contribution of hand cutaneous afferent inputs to these changes in Ia presynaptic inhibition. Seven subjects were asked either to selectively activate their wrist extensor muscles by
Acknowledgements
We are grateful to Dr. J. Blanc for correcting the English manuscript and to P.A. Styss for his logistic assistance. This research was supported by Grants from the Association Française contre les Myopathies (A.F.M.), the Fondation pour la Recherche Médicale (F.R.M.), and the Direction des Recherches, Etudes et Techniques (D.R.E.T.-D.G.A.).
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