[Variations of the presynaptic inhibition of Ia fibers during voluntary movements in man].

Presynaptic inhibition of Ia fibres can strongly reduce and even extinguish Ia excitatory postsynaptic potentials (e.p.s.p.). This phenomenon has been described in animal, but so far there was no method for studying presynaptic inhibition of Ia fibres in man. We have therefore firstly set up a method suitable to appreciate the amount of presynaptic inhibition of Ia fibres in man. Presynaptic inhibition is reduced for voluntary contractions, while it is enhanced for Ia fibres directed to the motoneurones innervating synergistic muscles not involved in the contraction. This control increases the motor contrast and very likely plays a role in the selectivity of movement. In the standing position, presynaptic inhibition of Ia fibres directed to soleus motoneurones is enhanced. This prevents development of an excessive soleus stretch reflex which might hinder forward progression during walking.