Neural control of human inspiratory muscles. What have we learnt from the study of single motor units?

This review covers the knowledge gains made about human respiratory neural drive resulting from ∼30 years of single motor unit recordings from human inspiratory muscles. Section 2 illustrates the non-uniformity of output across the various inspiratory motoneurone pools innervating diaphragm, scalene, intercostal, and genioglossus muscle activity during quiet breathing and during voluntary breathing. Section 3 describes the rostrocaudal graded timing and magnitude of respiratory neural drive across the parasternal intercostal and external intercostal muscles, which identified a principle of motoneurone recruitment termed the principle of neuromechanical matching that has been since identified as a principle of motor control more generally. Section 4 focuses on the changes in diaphragm motor unit discharge and morphology in ageing, chronic obstructive pulmonary disease and cervical spinal cord injury, linking increased drive to breathe to the changes in respiratory mechanics, with the intriguing findings that this is not always associated with a cortical contribution to breathing (i.e. change in the central control of breathing). Our studies have revealed an elaborate organisation of respiratory neural drive to the motoneurones to match the anatomical and functional complexity of the muscles themselves.