Breathing: Motor Control of Diaphragm Muscle

Abstract

Breathing occurs without thought but is controlled by a complex neural network with a final output of phrenic motor neurons activating diaphragm muscle fibers (i.e., motor units). This review considers diaphragm motor unit organization and how they are controlled during breathing as well as during expulsive behaviors.

FIGURE 1.

Different DIAm motor unit types are distinguished by their intrinsic, mechanical, and fatigue properties A: different DIAm motor unit types are distinguished by their intrinsic, mechanical, and fatigue properties, and are classified as type S, FR, FInt, and FF. Within a particular motor unit, all muscle fibers are homogeneous, as evidenced by myosin heavy chain (MyHC) expression. B: force (normalized to percent maximum tetanic force) generated by cat diaphragm motor units at different frequencies of stimulation. Results are for individual motor units classified by their contractile and fatigue properties. The steepest portion of the force-frequency curve occurs between 10 and 30 Hz for all types of motor units in the diaphragm muscle (52), consistent with onset and peak discharge frequencies of ~8 and ~25 Hz, respectively, for type S and FR units (top arrows) and ~15 and ~60 Hz, respectively, for type FInt and FF units (bottom arrows), reported in Ref. . C: diaphragm motor units are recruited to accomplish a range of motor behaviors. Ventilation (eupnea, hypercapnia, and hypoxia) is accomplished by recruitment type S and FR motor units, whereas higher-force airway clearance behaviors require recruitment of more fatigable DIAm motor units.

FIGURE 2.

Technique for estimating maximum specific force of different diaphragm muscle fibers A: technique for estimating maximum specific force of different diaphragm muscle fibers, distinguished by MyHC expression. Single permeabilized diaphragm skeletal muscle fibers are mounted between a force transducer and a servo motor (length control). The chamber is superfused with Ca²⁺-activating solution, and the rate and magnitude of force produced by a single fiber assessed. Differences in specific force between different fiber types is related to the different MyHC content per half sarcomere and differing unitary forces produced by different MyHC isoforms. B: in the diaphragm muscle of most species, type I and IIa diaphragm muscle fibers have smaller cross-sectional areas than those of type IIx and IIb fibers. There are also differences in maximum specific force produced across diaphragm muscle fiber types, with type IIb producing greater force than type IIx > type IIa > type I (59).

FIGURE 3.

Example of single motor unit recordings with compound EMG activity during eupnea A: example of single motor unit recordings with compound EMG activity during eupnea. The motor unit action potentials, represented with their averaged waveforms, are recruited at the initial 150 ms of DIAm EMG activity (165). B: recruitment order was maintained with increasing neural drive (Isyn), whereas recruitment delay decreases and discharge rate increases. Mean recruitment delay (ms) is reduced compared with eupnea for deep breaths and airway occlusions. The peak discharge rate for motor units is increased compared with onset discharge rate for all behaviors assessed. Peak discharge rates for hypoxia-hypercapnia are greater than those of eupnea, with deep breaths and airway occlusion greater than both eupnea and hypoxia-hypercapnia behaviors (163).