Modulation by corticospinal volleys of presynaptic inhibition to Ia afferents in man

New methods have been recently developed to explore selectively presynaptic inhibition of Ia afferents in humans. They have allowed us to describe a highly specialized organisation in these pathways. A differential control has been disclosed during voluntary movements among various motoneuronal pools: at the onset of a selective voluntary contraction presynaptic inhibition of Ia afferents projecting to the 'contracting' motoneurons is strongly decreased whereas presynaptic inhibition of Ia afferents to antagonistic or synergistic motoneuronal pools, not involved in the contraction, is increased. Indirect arguments suggested that these modulations are centrally patterned. A differential control has been also disclosed between upper and lower limb pathways. Using transcranial magnetic stimulation to induce a descending corticospinal volley, we have shown that a corticospinal volley inhibits preferentially 'presynaptic interneurons'at the lumbar spinal level, an effect which is strengthened by a cutaneous input whereas it preferentially activates 'presynaptic interneurons' at the cervical spinal level, an effect which is inverted by a cutaneous input.