Phrenic motoneuron firing rates before, during, and after prolonged inspiratory resistive loading

Abstract

Phrenic motoneuron firing rates during brief inspiratory resistive loading (IRL) are high, and nearly all the motoneurons are recruited. Diaphragmatic fatigue has been difficult to demonstrate during IRL. Furthermore, evidence from studies in limb muscles has shown variable motoneuron responses to prolonged high-intensity loads. We studied phrenic motoneuron firing rates before, during, and after prolonged IRL in anesthetized rabbits. Of 117 phrenic axons, only 2 axons were not recruited; 41 axons were silent during unloaded breathing but were recruited at higher loads. Silent axons showed a more rapid increase in firing rate as the load increased. Phrenic motoneuron firing rates increased throughout the period of loading, whereas airway pressure swings did not. After prolonged IRL, higher motoneuron firing rates were needed during brief loads to produce the same airway pressure. No evidence of a decline in motoneuron firing rates was seen at any point. We conclude that the respiratory muscles can be shown to demonstrate physiological responses consistent with fatigue during prolonged IRL, and activation rates are high and remain so throughout this prolonged loading.

Respiratory muscle injury, fatigue and serum skeletal troponin I in rat

“…increased drive necessary to maintain the same pressure generation) is not. Our results are similar to those of previous work in cats on IRL (Iscoe, 1996) and in rabbits subjected to brief loads after prolonged IRL (Road & Cairns, 1997). They are also consistent with an earlier report in man of rapid recovery of pressuregenerating capacity following progressive inspiratory threshold loading to failure coexisting with reduced twitch amplitudes in response to phrenic nerve stimulation (Eastwood et al 1994).…”

Neural control of phrenic motoneuron discharge

“…However, the relative impact of loading on Early-I vs. Late-I PMN bursting was not described. Silent PMNs are also recruited during loading, and upon activation these cells show a much steeper increase in discharge frequency as compared to previously active PMNs (Road et al, 1995; Road and Cairns, 1997). During prolonged exposure to inspiratory resistive loads (e.g.…”

“…A few studies have investigated the impact of lung inflation and/or airway pressure on PMN discharge patterns (Bishop et al, 1981; Hwang and St John, 1993; Hwang et al, 1987; Road et al, 1995; Road and Cairns, 1997). Pulmonary stretch receptor activation has complex effects on PMN bursting (Hwang and St John, 1993).…”

“…In some cases, negative pressure resulted in PMN bursting throughout the expiratory phase (Bishop et al, 1981). Road and colleagues demonstrated that inspiratory resistive loading increases both the burst frequency and duration of active PMNs (Road et al, 1995; Road and Cairns, 1997). However, the relative impact of loading on Early-I vs. Late-I PMN bursting was not described.…”

“…During prolonged exposure to inspiratory resistive loads (e.g. 30 min), PMNs show a progressive increase in burst frequency, and this may serve to offset the onset of diaphragm muscle fatigue (Road and Cairns, 1997). …”