Brain, Vol. 122, No. 9, 1757-1764,
September 1999
© 1999 Oxford University Press
Changes in transmission in the pathway of heteronymous spinal recurrent inhibition from soleus to quadriceps motor neurons during movement in man
.Department of Zoology, University of Oxford, Oxford, UK
Correspondence to:
Dr J. F. Iles, Department of Zoology, South Parks Road, Oxford OX1 3PS, UK E-mail: john.iles{at}zoo.ox.ac.uk
H reflexes were induced in the human quadriceps muscle by electrical stimulation of the femoral nerve. The reflexes were conditioned by prior stimulation of the inferior soleus nerve. The conditioning stimulus produced an inhibition of long duration (>20 ms). The threshold of this inhibition was at zero soleus motor discharge and the inhibition scaled with soleus motor discharge. It was concluded that the inhibition was a heteronymous recurrent inhibition of quadriceps motor neurons mediated by Renshaw cells which had been activated by soleus motor neuron discharge. This recurrent inhibition declined during voluntary tonic contraction of the quadriceps, falling to zero at around one-third of maximum voluntary contraction. Antagonist contraction and weak co-contraction of the quadriceps and its antagonists did not lead to any significant change in recurrent inhibition. It is concluded that motor commands descending from the brain reduce heteronymous recurrent inhibition during isolated quadriceps muscle contraction, but to a much lesser extent during co-contraction. No evidence was obtained for any descending facilitation of heteronymous recurrent inhibition.
recurrent inhibition; spinal cord; motor control; human
C/T% = conditioned reflex amplitude divided by test reflex amplitude; XMT = electrical stimulation relative to motor axon threshold; %Mmax = percentage of the maximum motor discharge; %MVC = percentage of maximum voluntary contraction
* Present address: Department of Physiology, School of Medical Sciences, Bristol, UK
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