Changes in presynaptic inhibition of Ia fibres in man while standing.

Presynaptic inhibition of homonymous Ia afferent terminals to soleus, quadriceps and tibialis anterior motoneurons and of heteronymous Ia fibres from quadriceps to soleus was compared in the same subjects when standing without support and during a control situation (sitting or standing with back support). Changes in presynaptic inhibition of Ia fibres were indirectly deduced from alterations in the amount of monosynaptic Ia facilitation elicited in motoneurons by a constant conditioning stimulation. Facilitation was measured during the first 0.5 ms when the monosynaptic Ia excitatory postsynaptic potential (EPSP) was not yet contaminated by polysynaptic effects evoked by the conditioning stimulation. Two indirect methods were used to provide an estimate of the size of the conditioning Ia EPSP: (1) the resulting H reflex facilitation; and (2) the peak of increased firing probability elicited in voluntarily activated motoneurons by stimulation of homonymous and heteronymous Ia fibres (poststimulus time histogram PSTH method). Only those PSTH experiments in which the 'spontaneous' firing rate of the motor unit was identical in the different positions, thus ensuring an identical net synaptic drive to the motoneuron, were considered. Under these conditions it is assumed that changes in the size of the peak of facilitation elicited by the monosynaptic Ia volley are likely to be caused by changes in presynaptic inhibition of Ia fibres. It is argued that the substantial changes in monosynaptic Ia excitation observed when standing without support probably reflect changes in presynaptic inhibition of Ia fibres. Under this interpretation, presynaptic inhibition of Ia fibres to soleus motoneurons is increased while standing without support, whereas presynaptic inhibition of homonymous Ia fibres to quadriceps motoneurons is decreased. There is no evidence for a change in presynaptic inhibition of Ia fibres to tibialis anterior motoneurons. The resulting alterations in the gain of the monosynaptic reflex of these muscles are discussed in relation to the possible role of the monosynaptic stretch reflex in human gait.