Chapter 22 Recent developments in our understanding of motor control, cortico-motoneuronal connections and the activation of motoneurons and motoneuronal pools

Abstract

This chapter describes the investigations carried out to study the response of the first dorsal interosseous (1DI) muscle to transcranial magnetic stimulation (TMS) during performance of different manual tasks. Electromyography (EMG) activity across tasks was kept constant by providing subjects with visual feedback of their muscle activity. TMS produced larger motor evoked potentials (MEPs) during performance of complex tasks than during the simple index abduction task. The results suggested that even with a constant level of muscle activity during performance of different tasks, the corticospinal volleys evoked by TMS vary in amplitude. The role of a brain structure in a motor action can be assessed by attempting to interfere with its function using repetitive TMS (rTMS). A number of studies have shown an increase in MS evoked responses with motor imagery. On an average, MEPs are larger in the flexor muscle during the phase of imagined flexion than during extension, while the opposite is true for the extensor muscle. The EMG remains relaxed in all muscles and does not show any significant temporal changes during the test. Volitional unit can be described as consisting of the cortical mono-neurons (CMNs), the spinal motoneurons (SMNs), and the anatomical and interneuronal connections between them and identified by its volitional control signal—the AFR. Analysis of the motoneuronal firing rates versus the force data reveals that major features in the weighted sum of the firing rates are highly correlated with similar major features in the force. The interaction between firing rate and force depends on the muscle fiber type and the length of the contraction.