Effects from fine muscle and cutaneous afferents on spinal locomotion in cats

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Abstract
The effects of chemically activated fine muscle afferents (groups III and IV) and electrically activated cutaneous afferents on motoneuronal discharges were studied before and during fictive locomotion induced pharmacologically by i.v. administration of nialamide and L-dopa in high spinal cats. Efferent activity was recorded simultaneously from nerve filaments to ipsi- and contralateral extensor and flexor muscles. Intracellular recordings were made from lumbar .alpha.-motoneurons. After nialamide but before treatment with L-dopa, in some cases, transient locomotor-like discharges were induced by an increased activity in fine muscle afferents. The response pattern in nerves to both hind limbs could be different showing, e.g., only transient alternating activity between knee flexor and extensor of 1 limb but not of the other one. Treatment with L-dopa did not always cause fictive locomotion. Often not all motoneuron pools showed rhythmic activity. On these cases, stimulation of group III and IV muscle afferents usually caused transient periodic activity. In cases with apparent rhythmic activity, algesic stimulation of the gastrocnemius-soleus muscle caused an accentuation of the rhythm by a more abrupt transition from the active phase to the non-active interval. The response patterns on both sides were not uniform in all cases. A 2nd type of response to activation of fine muscle afferents had a quite different character; the rhythmic activity was more or less completely overridden by a strong transient tonic hyperactivity or the rhythm was transiently blocked. These phenomena did not occur in the same way in all nerves. Electrical stimulation of cutaneous nerves of the hind limb generally induced the same response pattern as chemical stimulation of the group III and IV muscle afferents. The effects varied depending on the stimulus strength and the nerve. Cutaneous and fine muscle afferents not only have similar functions in the reflex control of a limb but also in evocation and modulation of locomotion. It is assumed that both types of afferents may serve together as a peripheral feedback to the spinal locomotor center.