C. H. M. Brunia Tilburg University The Netherlands
For a voluntary plantar flexion of the foot, motoneurons of the calf muscles have to be depolarized by a central command. Achilles tendon (T) reflexes evoked during the foreperiod of a reaction time (RT) experiment can be used to estimate the changes in excitability of these motoneurons, since the voluntary command and the reflex circuit share the same final common path. Bilateral T reflex recordings preceding a unilateral plantar flexion permit a comparison between changes in the excitability of motor structures which are involved or not involved in the movement. Similar recordings preceding a unilateral finger movement provide data about motor structures uninvolved in the response. These are localized at a different spinal level than that at which the response execution takes place. Bilateral T reflex recordings preceding a unilateral dorsiflexion of the foot provide data concerning motor structures not involved in the response, being localized at the same spinal level as the one at which the response execution takes place. Four experiments are discussed. They indicate that between the warning signal (WS) and the movement the three following processes take place. First, the WS causes a nonselective facilitation of the motor structures, irrespective of the kind of movement to be made. Second, in the remainder of the foreperiod a differential pattern in the changes in reflex amplitudes is found. Motor structures not involved in the response show an increase in excitability, whereas the agonist reflex arc shows a selective lack of increase. Third, after the response signal the differential pattern is reversed. Motor structures show a selective increase in excitability when they are agonist and an aselective increase to a lesser extent when they are not involved in the response.
For a voluntary movement to be executed, the motoneurons innervating the agonist have to be depolarized to a critical value by a central command. A way of