The neural control of human inspiratory muscles

The neural control of inspiratory muscles can be assessed in human subjects by measurement of the behavior of populations of single motor unit from the various inspiratory muscles. The discharge frequencies and patterns of firing of the motor units directly reflect the output of the motoneurons that innervate them. With the use of these methods, our work has revealed several features of the way the output of different inspiratory motoneuron pools are controlled. The output of inspiratory motoneurons is nonuniform across pools during quiet breathing and this coordinates the contraction of all the different muscles. This output is geared to the mechanical advantage of the muscles that they innervate. For the intercostal muscles, there is recruitment of the motor units by a principle of neuromechanical matching in which neural drive is higher in the muscles with the greatest mechanical advantage for inspiration, presumably to minimize the metabolic cost of ventilation. We summarize some evidence that this principle is likely to be organized at the spinal cord, although the exact underlying mechanisms are not known. The specific differences in the output from motoneurones innervating parasternal intercostal and diaphragm muscles during trunk rotation suggest that the output of inspiratory motoneurones engaged in a nonrespiratory voluntary task involve integration of corticospinal and bulbospinal drives at the spinal cord. An evolutionary argument is presented to support the importance of a role for spinal integration in ventilatory control.