MODULATION, PROBABLY PRESYNAPTIC IN ORIGIN, OF MONOSYNAPTIC IA EXCITATION DURING HUMAN GAIT

Modulation of presynaptic inhibition of Ia afferents projecting monosy naptically to soleus motoneurones was investigated during human gait, Changes in presynaptic inhibition of Ia afferents were deduced from al terations in the amount of heteronymous soleus H-reflex facilitation e voked by a constant femoral nerve stimulation. It has been shown that this facilitation is mediated through a monosynaptic Ia pathway and th at during its first 0.5 ms it is still uncontaminated by any polysynap tic effect and can be used to assess ongoing presynaptic inhibition of Ia terminals to soleus motoneurones. During gait, heteronymous facili tation was reduced with respect to its control value (rest during sitt ing) and modulated during the step cycle: it reached its maximum at mi d-stance and decreased to near zero by the end of stance. At the same time the H-reflex amplitude was to some extent similarly modulated. It is argued that this decrease in heteronymous Ia facilitation and in H -reflex amplitude reflects an increased, ongoing presynaptic inhibitio n of Ia terminals projecting onto soleus motoneurones, which could be from central and/or peripheral origin. D1 inhibition, i.e. the late an d long-lasting inhibition of the soleus H-reflex evoked by a train of stimuli to the common peroneal nerve, was used as another method to as sess presynaptic inhibition. This D1 inhibition was decreased during g ait, and it is argued that this decrease might reflect an occlusion in presynaptic pathways or increased presynaptic inhibition of pathways mediating the conditioning volley.