Control of Diaphragm Muscle in Health and Disease

Unique to mammals, the diaphragm muscle (DIAm) is the major pump muscle for breathing. The DIAm is a skeletal muscle composed of different fiber types with different mechanical and fatigue properties. Neural control of the DIAm is mediated by recruitment of motor units (MUs), each comprising a phrenic motor neuron in the cervical spinal cord and the set of muscle fibers (of a single type) it innervates. Thus, by recruiting different MU types, a range of motor behaviors can be accomplished. To support breathing, which is a sustained behavior, fatigue-resistant slow (type I fibers) and fast (type IIa fibers) MUs are recruited. However, the DIAm also subserves short duration, higher-force airway clearance, and voiding behaviors that require recruitment of more fatigable fast (type IIx/IIb) fibers. Importantly, in disease conditions, these fatigable DIAm MUs are more vulnerable, while fatigue-resistant MUs are more resilient.