Phase-specific recruitment underlying sighs in awake rats

Neuromotor control of sighs consists of two phases: Phase I, which resembles a preceding eupneic breath, and Phase II, an extra inspiration superimposed on Phase I. The diaphragm muscle (DIAm)—the primary inspiratory muscle in mammals—is activated during multiple behaviors, including sighs, and its activity can be quantified using electromyography (EMG). Although the biphasic structure of sighs has been described using airflow and pressure signals, how these phases are represented in DIAm EMG activity—and the underlying dynamics of DIAm motor unit (MU) recruitment—remains unclear. Previously we described that eupnea consists of a period of MU recruitment followed by a period of sustained MU activity before derecruitment. We hypothesize that during sighs Phase I follows a similar trajectory of MU recruitment as eupnea while Phase II reflects superimposed additional MU recruitment. DIAm EMG was recorded using fine wire electrodes in awake Sprague-Dawley rats (n=6) of both sexes. To determine the period of DIAm MU recruitment during both Phase I and Phase II, a stationarity analysis was performed. The stationarity analysis uses a reverse arrangement test applied to sequential, non-overlapping 10-ms bins across a longer analysis window. Because DIAm MU discharge rates rarely exceed 100 Hz, it is unlikely that a single MU fires more than once within each 10-ms window; thus, non-stationarity reflects MU recruitment rather than changes in discharge rate. An average of 17 ± 4 sighs per animal were identified (range: 10-20), with a frequency of 2.8 ± 0.1 min-1. Sighs had a burst duration of 1300 ± 405 ms, much longer than the burst duration of eupneic breaths (~598 ± 111 ms). During sighs there were two distinct periods of MU recruitment reflected by non-stationarity of the EMG signal. The MU recruitment duration of Phase I was significantly shorter than Phase II (100 ± 28 ms vs. 179 ± 53 ms, p = 0.041). The onset-to-peak duration during sighs averaged 489 ± 135 ms, with the duration of Phase I lasting longer than Phase II (298 ± 119 ms vs 179 ± 56 ms). It is important to note that Phase II is characterized by rapid, continuous recruitment throughout its duration. In contrast, Phase I had a longer overall duration, although the period of MU recruitment was relatively brief—similar to eupneic breathing. Using EMG stationarity in awake animals allowed us to characterize these underlying MU patterns without altering ventilation or disrupting the intact of the respiratory system. These results indicate that sighs reflect a distinct pattern generator superimposed on that generating eupneic breathing. It remains unclear whether sighs are triggered (e.g., reflexes) or spontaneously generated. Funding: NIH NHLBI 5R01HL146114 and NIA 5R01AG044615 This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.