MENTAL REHEARSAL OF MOTOR-TASKS RECRUITS ALPHA-MOTONEURONS BUT FAILS TO RECRUIT HUMAN FUSIMOTOR NEURONS SELECTIVELY

1. As mental rehearsal of movements activates multiple cortical areas associated with movement, we assessed whether this increases fusimotor drive and whether enhanced muscle spindle activity could contribute t o the improvement in skill that accompanies mental rehearsal. 2. Micro neurographic recordings were made from six muscle spindle afferents in nervating extensor muscles in the forearm or tibialis anterior, which were selected because their discharge increased during very weak contr actions. Activity was monitored while subjects imagined performing a r ange of activities including simple and complex movements involving th e relevant muscles. 3. No activation of muscle spindle afferents occur red during imagined motor tasks without EMG. When the relevant muscles contracted during mental rehearsal, spindle discharge increased, much as in weak contractions. 4. Mental rehearsal increased background EMG in the involved muscles and also increased H reflex amplitude indepen dently of EMG changes. 5. Although there was no evidence for selective fusimotor activation during imagined movement, skeletomotor activity and reflex excitability increased. Similar changes occur with preparat ion for movement following a cue. It is likely that mental rehearsal u sually involves unintentional performance of the planned motor task.