Activity-dependent changes in control of human motoneurones

The corticospinal tract is the major pathway for controlling voluntary movements in humans. To understand motor behaviour in humans it is essential to identify the characteristics of cortico-motoneuronal activity and its effect on voluntary motor output. This thesis addresses changes in the responses of motoneurones to corticospinal input after strong voluntary contractions and following an episode of motor training. Chapter II investigates corticospinal responses in a lower leg muscle, tibialis anterior, by using electrical stimulation over the thoracic spine after short and prolonged maximal isometric voluntary efforts. Chapter III examines changes after maximal efforts in an upper limb muscle, first dorsal interosseous, which has a high level of monosynaptic input from corticospinal neurones. Here, responses were evoked by magnetic stimulation over the cervicomedullary junction. Responses in both muscles were altered after maximal efforts. These changes differed between the muscles and were also different from those previously reported for the elbow flexor muscles. Thus, short-term activity-dependent changes in the response of motoneurones to corticospinal input appear to be specific for the muscle involved. Chapter IV examined the effects of two types of motor training (ballistic movement and visuomotor tracking) with first dorsal interosseous. Responses to corticospinal tract stimulation were increased after training in ballistic movement. The findings in this set of experiments provide some evidence that acute plastic changes at a spinal level might contribute to improved motor performance. Together, the studies in this thesis show that the responses of motoneurones to corticospinal input are subject to various activity-dependent changes. They suggest that cortico-motoneuronal synapses may exhibit plasticity with physiological activity and this may alter performance of motor tasks and in the long term, contribute to motor learning