Phrenic motor unit recruitment during ventilatory and non-ventilatory behaviors

Abstract

Phrenic motoneurons are located in the cervical spinal cord and innervate the diaphragm muscle, the main inspiratory muscle in mammals. Similar to other skeletal muscles, phrenic motoneurons and diaphragm muscle fibers form motor units which are the final element of neuromotor control. In addition to their role in sustaining ventilation, phrenic motor units are active in other non-ventilatory behaviors important for airway clearance such as coughing or sneezing. Diaphragm muscle fibers comprise all fiber types and are commonly classified based on expression of contractile proteins including myosin heavy chain isoforms. Although there are differences in contractile and fatigue properties across motor units, there is a matching of properties for the motor neuron and muscle fibers within a motor unit. Motor units are generally recruited in order such that fatigue-resistant motor units are recruited earlier and more often than more fatigable motor units. Thus, in sustaining ventilation, fatigue-resistant motor units are likely required. Based on a series of studies in cats, hamsters and rats, an orderly model of motor unit recruitment was proposed that takes into consideration the maximum forces generated by single type-identified diaphragm muscle fibers as well as the proportion of the different motor unit types. Using this model, eupnea can be accomplished by activation of only slow-twitch diaphragm motor units and only a subset of fast-twitch, fatigue-resistant units. Activation of fast-twitch fatigable motor units only becomes necessary when accomplishing tasks that require greater force generation by the diaphragm muscle, e.g., sneezing and coughing.

Respiratory mechanics of eleven avian species resident at high and low altitude

“…An increase in the proportion of oxidative fibers in these muscles might allow them to be thinner and thus contribute to increased compliance, but this would come at the expense of having fewer glycolytic fibers to support non-respiratory high-flow breathing maneuvers involved in airway clearance (e.g. coughing, sneezing; Mantilla and Sieck, 2011).…”

Impact of unilateral denervation on transdiaphragmatic pressure

“…The properties of both phrenic motor neurons and muscle fibers are matched (Enad et al, 1989; Sieck, 1994) and critically determine the efficacy of the central nervous system in accomplishing specific motor tasks. During neural activation, motor units are recruited in an orderly fashion, based on the intrinsic electrophysiological properties of motoneurons such that, for a given synaptic input, smaller more excitable motor neurons innervating more fatigue resistant muscle fibers (type I and IIa fibers comprising type S and type FR motor units) are recruited before larger motor neurons innervating more fatigable muscle fibers (type IIx and IIb fibers comprising type FInt and type FF motor units) (Butler et al, 1999; Henneman et al, 1965; Mantilla and Sieck, 2011; Sieck and Fournier, 1989). The recruitment order of motor units is not likely to change after acute DIAm hemiparalysis.…”

“…In all animals, DIAm EMG was measured using methods similar to those previously described (Dow et al, 2006; Mantilla et al, 2011; Mantilla et al, 2010; Sieck and Fournier, 1990; Trelease et al, 1982). Briefly, pairs of multistranded fine wire, insulated (stripped to expose ~2 mm segment) stainless steel electrodes (0.28 mm diameter – model AS631, Cooner Wire Inc., Chatsworth, CA) were implanted (~3 mm apart) into the mid-costal regions of both right and left sides of the DIAm following laparotomy.…”

Localized Delivery of Brain-Derived Neurotrophic Factor-Expressing Mesenchymal Stem Cells Enhances Functional Recovery following Cervical Spinal Cord Injury

“…14,21,46 It is important to note that ventilation during eupnea can be sustained across a range of species by generation of transdiaphragmatic pressure that is *20% of the total force generating capacity of the diaphragm. 47,[78][79][80] It is unknown if injection of BDNF-MSCs after SH also increased EMG activity during other ventilatory (hypoxiahypercapnia) and nonventilatory behaviors (i.e., airway occlusion, sneezing) that would require generation of larger pressures and force by the diaphragm muscle. Future studies could explore treatments to enhance recovery of expulsive behaviors, as maintenance of airway clearance is an important predictor of survival following spinal cord injury.…”