Strategies of motor unit recruitment in human inspiratory muscles

Abstract

This thesis addresses the neural control of the human inspiratory muscles. Single motor unit and multiunit electromyographic recordings are made from inspiratory and limb muscles to investigate the organisation of inspiratory motoneurone output and strategies by which motoneurone output is optimised. In the first two chapters, the output of parasternal intercostal motor units in respiratory and non-respiratory ‘tasks’ are described. The results show that across parasternal intercostal spaces, there is a decreasing rostrocaudal gradient of motor unit output in targeted voluntary breaths, such that inspiratory activity is greatest and earliest in cranial interspaces. This topographic recruitment is similar to quiet breaths and the majority of motor units are active in both voluntary and quiet breaths. In a non-respiratory voluntary task, rotation of the trunk (Chapter 3), common motor units are also active, but inspiratory motor unit output on both sides of the chest wall is altered by unilateral rotation. Multiple descending drives to the respiratory muscles may be organised and integrated at the spinal cord such that the same motoneurones are depolarised but their output depends on the task(s) performed. The strategy of motor unit recruitment in two inspiratory muscles is examined in Chapter 4. The profiles of activation of an obligatory inspiratory muscle, scalenes, and an accessory muscle, sternocleidomastoid, are not altered by lung volume or task (isovolumetric ‘static’ versus ‘dynamic’ task). Rather than the activation of these muscles being dependent on afferent signals of lung volume, they are recruited according to their relative mechanical advantage for inspiration. These results suggest that neural drive to these muscles has an element that is ‘preset’. The final study investigates neural and mechanical behaviour in the first dorsal interosseous muscle. The results show that there is coupling between neural drive and mechanical advantage in a limb muscle, as occurs for the intercostal muscles during inspiration. Furthermore, this coupling is adaptable to acute changes in mechanical advantage. There is an increase in motor unit recruitment in first dorsal interosseous when it has a greater mechanical effectiveness in an index finger flexion task. This is consistent with a strategy to produce efficient voluntary movements.