The volitional unit: a functional concept in cortico-motoneuronal connections in humans

We used the EMG precision decomposition technique to resolve complex EMG signals and derive information about the firing times of a family of motor units (MUs) and the force they produce. The active units shared a common behavior among their firing rates, a concept described by DeLuca et al. and termed the ‘common drive’. This ‘common’ behavior was extracted as the average of the firing rates of MUs and found to track the subject's force trajectory. In this paper, we propose the existence of functional cortico-motoneuronal connections which provide for a large number of combinations between affector cortical motoneurons (CMNs) and effector spinal motoneurons (SMNs) for the generation of a purposeful movement. We argue that these connections provide the essential link between volition and movement and function as a ‘volitional unit’ which consists of the CMNs, the SMNs and the anatomical and interneuronal connections between them.