Elsevier

Journal of Physiology-Paris

Volume 93, Issue 4, September 1999, Pages 395-401
Journal of Physiology-Paris

Ia presynaptic inhibition in human wrist extensor muscles: Effects of motor task and cutaneous afferent activity

https://doi.org/10.1016/S0928-4257(00)80067-4Get rights and content

Abstract

The task-dependence of the presynaptic inhibition of the muscle spindle primary afferents in human forearm muscles was studied, focusing in particular on the modulation associated with the co-contraction of antagonist muscles and the activation of cutaneous afferents. The changes known to affect the motoneuron proprioceptive assistance during antagonist muscle co-activation in human leg and arm muscles were compared. The evidence available so far that these changes might reflect changes in the presynaptic inhibition of the muscle spindle afferent is briefly reviewed. The possible reasons for changes in presynaptic inhibition during the antagonist muscle co-contraction are discussed. Some new experiments on the wrist extensor muscles are briefly described. The results showed that the changes in the Ia presynaptic inhibition occurring during the co-contraction of the wrist flexor and extensor muscles while the hand cutaneous receptors were being activated (the subject's hand was clenched around a manipulandum) could be mimicked by contracting the wrist extensor muscles alone while applying extraneous stimulation to the hand cutaneous receptors. It is concluded that besides the possible contribution of inputs generated by the co-contraction of antagonist muscles and by supraspinal pathways, cutaneous inputs may play a major role in modulating the proprioceptive assistance during manipulatory movements.

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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