CORTICAL CONTROL OF PRESYNAPTIC INHIBITION OF IA AFFERENTS IN HUMANS

The effect of transcranial magnetic stimulation was investigated on pr esynaptic inhibition of Ia terminals in the human upper and lower limb . Presynaptic inhibition of Ia afferents was assessed by three differe nt and independent methods: (1) heteronymous Ia facilitation of the H- reflex (assessing ongoing presynaptic inhibition of Ia afferents in th e conditioning volley): (2) long-lasting inhibition of the H-reflex by a group I volley (D1 inhibition, assessing presynaptic inhibition on Ia afferents in the test volley); (3) measurement of the monosynaptic Ia peak evoked in single motor units by a homonymous or heteronymous v olley (post stimulus time histogram method). The first two methods wer e used on the lower limb; the last two on the upper limb. Provided tha t the corticospinal volley and the explored Ia volley were directed to the same target motoneurones, cortical stimulation evoked significant and congruent changes: (1) In the lower limb, transcranial stimulatio n provided increased heteronymous Ia facilitation and decreased D1 inh ibition, both of which suggest a decrease in presynaptic inhibition of Ia afferents; (2) in the upper limb, transcranial stimulation provide d an increase in the radial-induced inhibition of the wrist flexor H-r eflex and a decrease in the peak of monosynaptic Ia excitation in sing le units, both of which suggest an increase in presynaptic inhibition. Selectivity of corticospinal effects was explored by testing presynap tic inhibition of Ia afferents to soleus motoneurones and focusing the transcranial stimulation to excite preferentially different motor nuc lei (soleus, quadriceps and tibialis anterior). A cortical-induced dec rease in presynaptic inhibition of Ia afferents was seen when, and onl y when, cortical and peripheral Ia volleys were directed to the same m otor nucleus.