Effects of hypercapnia and hypoxia on abdominal expiratory nerve activity

Electromyographic activity of the stylopharyngeus muscle in exercising horses

REASONS FOR PERFORMING STUDY

There is a need to understand the process which leads to failure of recruitment of the stylopharyngeus muscle in clinical cases of nasopharygeal collapse. We therefore studied the timing and intensity of stylopharyngeus muscle activity during exercise in horses.

OBJECTIVE

To measure the electromyographic (EMG) activity of the stylopharyngeus muscle in exercising horses and correlate it with the breathing pattern.

METHODS

Five horses were equipped with a bipolar fine wire electrode placed on the stylopharyngeus muscle and a pharyngeal catheter. The horses exercised on a treadmill at speeds corresponding to 50 (HRmax50), 75 and 100% of maximum heart rate, and EMG activity of the stylopharyngeus muscle and upper airway pressures were recorded. The EMG activity of the stylopharyngeus muscle was then correlated to the breathing pattern and the activity quantified and reported as a percentage of the baseline activity measured at HRmax50.

RESULTS

There was ongoing activity of the stylopharyngeus muscle throughout the breathing cycle; however, activity increased towards the end of expiration and peaked early during inspiration. Tonic activity was present during expiration. Peak, mean electrical and tonic EMG activity increased significantly (P<0.05) with exercise intensity.

CONCLUSION

The stylopharyngeus muscle has inspiratory-related activity and tonic activity that increases with speed.

POTENTIAL RELEVANCE

The stylopharyngeus muscle is one of a group of upper airway muscles that function to support and maintain the patency of the nasopharynx during inspiration. Failure of recruitment of the stylopharyngeus muscle during exercise is a potential explanation for clinical cases of dorsal pharyngeal collapse, but further work investigating the activity of the stylopharyngeus muscle in horses affected by this disease is needed.

Influence of hypercapnic acidosis and hypoxia on abdominal expiratory nerve activity in the rat

We studied the influence of hypercapnic acidosis and hypoxia on the neural drive to abdominal muscles in anesthetized and decerebrate rats; this information is unavailable despite widespread use of the rat as an experimental model in respiratory physiology and neurobiology. To minimize confounding influences from receptors in the lungs and chest wall, the animals were vagotomized, paralyzed and mechanically ventilated, and electrical activity was recorded from abdominal muscle nerves. In anesthetized and decerebrate rats, both stimuli evoked steady, low amplitude expiratory discharge that persisted throughout the expiratory phase (E-all activity), but was inhibited during inspiration. We also observed late expiratory, high-amplitude bursts (E2 activity) superimposed on this steady activity, but only at the highest levels of respiratory drive. Hypoxia enhanced abdominal motor activity transiently, whereas hypercapnic acidosis caused a sustained increase in activity. Thus, both hypercapnic acidosis and hypoxia activate abdominal muscle motoneurons in the absence of phasic afferent inputs.

Respiratory drive in hindlimb motoneurones of the anaesthetized female cat

Anatomical studies have shown a monosynaptic projection from nucleus retroambiguus (NRA) to semimembranosus (Sm) motor nucleus in female cats, which is stronger in oestrus. Expiratory bulbospinal neurones are the best documented functional cell type in the NRA. If these cells participate in this projection, an expiratory drive would be expected in Sm motoneurones and this drive would be expected to be stronger in oestrus. In anaesthetized, paralyzed, ovariohysterectomized female cats, artificially ventilated to produce a strong respiratory drive (as monitored by phrenic nerve discharges), intracellular recordings were made from Sm motoneurones and from motoneurones in the surrounding hindlimb motor nuclei that are outside the focus of the NRA projection. The animals comprised two groups: either treated for 7 days with oestradiol benzoate (oestrous) or untreated (non-oestrous). Central respiratory drive potentials (CRDPs) were observed in most motoneurones of both groups, with amplitudes larger for the oestrous than for the non-oestrous group (1.58+/-1.34 mV versus 0.89+/-0.79 mV, mean+/-S.D.). However, the CRDPs most often consisted of a maximum depolarization in early expiration, which declined in late expiration and into inspiration. This pattern is different from the incrementing firing pattern of most expiratory bulbospinal neurones. The CRDPs in Sm and semitendinosus motoneurones (located in the same motor column) were of similar size and frequency to CRDPs in motoneurones outside that column. The hypothesis that expiratory bulbospinal neurones are significantly involved in the projection was rejected. Alternative sources and possible functional roles for the CRDPs are discussed.

Modulation of expiratory motor output evoked by chemical activation of pre-Bötzinger complex in vivo

We have previously demonstrated that chemical stimulation of the pre-Bötzinger complex (pre-BötC) in the anesthetized cat produces either phasic or tonic excitation of phrenic nerve discharge. This region is characterized by a mixture of inspiratory-modulated, expiratory-modulated, and phase-spanning (including pre-inspiratory (pre-I)) neurons; however, its influence on expiratory motor output is unknown. We, therefore, examined the effects of chemical stimulation of the pre-BötC on expiratory motor output recorded from the caudal iliohypogastric (lumbar, L(2)) nerve. We found that unilateral microinjection of DL-homocysteic acid (DLH; 10 mM; 10-20 nl) into 16 sites in the pre-BötC enhanced lumbar nerve discharge, including changes in timing and patterning similar to those previously reported for phrenic motor output. Both increased peak amplitude and frequency of phasic lumbar bursts as well as tonic excitation of lumbar motor activity were observed. In some cases, evoked phasic lumbar nerve activity was synchronized in phase with phrenic nerve discharge. These findings demonstrate that chemical stimulation of the pre-BötC not only excites inspiratory motor activity but also excites expiratory motor output, suggesting a role for the pre-BötC in generation and modulation of inspiratory and expiratory rhythm and pattern.

Excitatory lung reflex may stress inspiratory muscle by suppressing expiratory muscle activity

Recently, a vagally mediated excitatory lung reflex (ELR) causing neural hyperpnea and tachypnea was identified. Because ventilation is regulated through both inspiratory and expiratory processes, we investigated the effects of the ELR on these two processes simultaneously. In anesthetized, open-chest, and artificially ventilated rabbits, we recorded phrenic nerve activity and abdominal muscle activity to assess the breathing pattern when the ELR was evoked by directly injecting hypertonic saline (8.1%, 0.1 ml) into lung parenchyma. Activation of the ELR stimulated inspiratory activity, which was exhibited by increasing amplitude, burst rate, and duty cycle of the phrenic activity (by 22 +/- 4, 33 +/- 9, and 57 +/- 11%, respectively; n = 13; P < 0.001), but suppressed expiratory muscle activity. The expiratory muscle became silent in most cases. On average, the amplitude of expiratory muscle activity decreased by 88 +/- 5% (P < 0.002). The suppression reached the peak at 6.9 +/- 1 s and lasted for 200 s (median). Injection of H(2)O(2) into the lung parenchyma produced similar responses. By suppressing expiration, the ELR produces a shift in the workload from expiratory muscle to inspiratory muscle. Therefore, we conclude that the ELR may contribute to inspiratory muscle fatigue, not only by directly increasing the inspiratory activity but also by suppressing expiratory activity.

Differential respiratory muscle recruitment induced by clonidine in awake goats

The purpose of this study was to test the hypothesis that dysrhythmic breathing induced by the alpha2-agonist clonidine is accompanied by differential recruitment of respiratory muscles. In adult goats (n = 14) electromyographic (EMG) measurements were made from inspiratory muscles (diaphragm and parasternal intercostal) and expiratory muscles [triangularis sterni (TS) and transversus abdominis (Abd)]. EMG of the thyroarytenoid (TA) muscle was used as an index of upper airway (glottal) patency. Peak EMG activities of all spinal inspiratory and expiratory muscles were augmented by central and peripheral chemoreceptor stimuli. Phasic TA was apparent in the postinspiratory phase of the breathing cycle under normoxic conditions. During dysrhythmic breathing episodes induced by clonidine, TS and Abd activities were attenuated or abolished, whereas diaphragm and parasternal intercostal activities were unchanged. There was no tonic activation of TS or Abd EMG during apneas; however, TA activity became tonic throughout the apnea. We conclude that 1) alpha2-adrenoceptor stimulation results in differential recruitment of respiratory muscles during respiratory dysrhythmias and 2) apneas are accompanied by active glottic closure in the awake goat.

Ventilatory effects of specific carotid body hypocapnia and hypoxia in awake dogs

Specific carotid body (CB) hypocapnia in the-10-Torr (less than eupneic) range reduced ventilation in the awake and sleeping dog to the same degree as did CB hyperoxia [CB PO2 (PCBO2); > 500 Torr; C.A. Smith, K.W. Saupe, K. S. Henderson, and J. A. Dempsey. J. Appl. Physiol. 79:689-699, 1995], suggesting a powerful inhibitory effect of hypocapnia at the carotid chemosensor over a range of PCO2 encountered commonly in physiological hyperpneas. The primary purpose of this study was to assess the ventilatory effect of CB hypocapnia on the ventilatory response to concomitant CB hypoxia. The secondary purpose was to assess the relative gains of the CB and central chemoreceptors to hypocapnia. In eight awake female dogs the vascularly isolated CB was perfused with hypoxic blood (mild, PCBO2 approximately equal to 50 Torr or severe, PCBO2 approximately equal to 36 Torr) in a background of normocapnia or hypocapnia (10 Torr less than eupneic arterial PCO2) in the perfusate. The systemic (and brain) circulation was normoxic throughout, and arterial PCO2 was not controlled (poikilocapnia). With CB hypocapnia, the peak ventilation (range 19-27 s) in response to hypoxic CB perfusion increased 48% (mild) and 77% (severe) due to increased tidal volume. When CB hypocapnia was present, these increases in ventilation were reduced to 21 and 27%, respectively. With systemic hypocapnia, with the isolated CB maintained normocapnic and hypoxic for > 70 s, the steady-state poikilocapnic ventilatory response (i.e., to systemic hypocapnia alone) decreased 15% (mild CB hypoxia) and 27% (severe CB hypoxia) from the peak response, respectively. We conclude that carotid body hypocapnia can be a major source of inhibitory feedback to respiratory motor output during the hyperventilatory response to hypoxic carotid body stimulation.

Effect of hypoxia on abdominal motor unit activities in spontaneously breathing cats

These experiments were designed to examine the behavior of external oblique motor units in spontaneously breathing cats during hypoxia and to estimate the contribution of recruitment and rate coding to changes in the integrated external oblique electromyogram (iEMG). Motor unit activities in the external oblique muscle were identified while the cats expired against a positive end-expiratory pressure (PEEP) of 1-2.5 cmH2O. After localization of unit activity, PEEP was removed, and recordings were made continuously for 3-4 min during hyperoxia, normoxia, and hypoxia. A total of 35 single motor unit activities were recorded from 10 cats. At each level of fractional concentration of end-tidal O2, the motor unit activity was characterized by an abrupt increase in mean discharge frequency, at approximately 30% of expiratory time, which then continued to increase gradually or remained constant before declining abruptly at the end of expiration. The transition from hyperoxia to normoxia and hypoxia was accompanied by an increase in the number of active motor units (16 of 35, 20 of 35, and 29 of 35, respectively) and by an increase in the mean discharge frequency of those units active during hyperoxia. The changes in motor unit activity recorded during hypoxia were accompanied by a significant increase in the average peak amplitude of the abdominal iEMG. Linear regression analysis revealed that motor unit rate coding was responsible for close to 60% of the increase in peak iEMG amplitude. The changes in abdominal motor unit activity and the external oblique iEMG that occurred during hypoxia were abolished if the arterial PCO2 was allowed to fall. We conclude that external oblique motor units are activated during the latter two-thirds of expiration and that rate coding and recruitment contribute almost equally to the increase in expiratory muscle activity that occurs with hypoxia. In addition, the excitation of abdominal motor units during hypoxia is critically dependent on changes in CO2 and/or tidal volume.

The effect of exercise on diaphragmatic activation in horses

Horses chronically-instrumented with costal diaphragmatic electromyographic electrodes were studied during exercise while unencumbered by a breathing mask. Exercise-associated changes in esophageal (Pes), gastric (Pga) and transdiaphragmatic (Pdi) pressures were measured and related to diaphragmatic electromyographic activity (CS EMG) and to left forelimb impact. In all breaths examined, CS EMG always coincided with decrements in Pes. For all exercise trials, linear increases in CS EMG, Pga and Pdi and linear decreases in Pes, as a function of exercise intensity, always occurred. During all gaits, breathing frequency (fR) was entrained with stride frequency (fS) one for one. However, a constant phase-coupling relationship between fR and fS, observed when horses cantered and galloped, was absent when horses walked or trotted. We conclude that biomechanical forces contribute minimally to ventilation in exercising horses, that the diaphragm is always phasically active during each breath and its total electrical activity and mechanical output are proportional to the exercise hyperpnea.

Abdominal muscle activation by expiratory threshold loading in awake dogs

Abdominal muscle activation produced by expiratory threshold loading (ETL) helps prevent an increase in FRC thus, optimizing diaphragm length and defending VT. However, anesthesia may affect abdominal muscle activation, and the pattern of recruitment and level of activation of individual abdominal muscles may well be dependent on body position. Therefore, individual abdominal muscle response to ETL was assessed in awake dogs, lying in the lateral decubitus position. Eight, tracheotomized dogs were chronically instrumented with sonomicrometer transducers and bipolar, fine wire EMG electrodes, in each of the four abdominal muscles. ETL produced increases in active, expiratory shortening of the transversus abdominis (TA), internal oblique (IO) and external oblique (EO). In addition, tonic activity, assessed from a decrease in baseline length, increased in the IO. There was a significant increase in FRC during ETL but it was less than would be expected without tonic and phasic abdominal muscle activation. Although FRC increased, VT and breathing frequency were maintained. As was found previously in supine, anesthetized dogs, the internal abdominal muscle layer (TA and IO) was recruited preferentially; substantiating its greater role in the defence of lung volume.

Upper airway dilating muscle hyperactivity during non-rapid eye movement sleep in English bulldogs

Sleep-disordered breathing (SDB) is seen during rapid eye movement (REM) sleep in English bulldogs, but it is absent during non-REM sleep. The SDB during REM sleep is associated with changes in neural drive to the diaphragm (DIA) and to an upper airway dilator, the sternohyoid (SH). In the present study, the EMG activity of the DIA was recorded in unrestrained, naturally sleeping, English bulldogs (n = 6) and in control dogs (n = 5). The EMG of the SH was recorded in five of these bulldogs and in four of the control dogs. The activity of the DIA was similar in the two groups of dogs throughout sleep, with the normal increased variability and altered recruitment patterns during REM sleep in all dogs. However, in the presence of the narrowed upper airway of bulldogs, the pattern of the upper airway dilator was dramatically different. In bulldogs, SH activity was virtually always related to inspiration (96 to 100% of breaths during both waking and non-REM sleep). In contrast, SH activity showed inspiratory-related increases in only a minority of breaths during non-REM sleep (32%) in control dogs (p < 0.05). Furthermore, SH drive, as measured by the plateau amplitude, fell during REM sleep in bulldogs, whereas it increased in control dogs (p < 0.05). In control dogs without SDB, we found that central respiratory drive to the SH was highest but variable during waking and minimal during non-REM sleep and that it fluctuated with phasic events during REM sleep. In bulldogs, however, high levels of SH activity occurred during waking and throughout non-REM sleep, apparently preventing SDB in these states. Episodic decreases in SH drive were observed during REM, and they were associated with SDB. These data support the proposition that compensatory pharyngeal dilator hyperactivity is necessary to maintain airway patency and normal breathing in bulldogs, a canine breed with an anatomically compromised upper airway.

Effects of specific carotid body and brain hypoxia on respiratory muscle control in the awake goat

1. We assessed the effects of specific brain hypoxia on the control of inspiratory and expiratory muscle electromyographic (EMG) activities in response to specific carotid body hypoxia in seven awake goats. We used an isolated carotid body perfusion technique that permitted specific, physiological, steady-state stimulation of the carotid bodies or maintenance of normoxia and normocapnia at the carotid bodies while varying the level of systemic, and therefore, brain oxygenation. 2. Isolated brain normocapnic hypoxia of up to 1.5 h duration increased inspired minute ventilation (VI) by means of increases in both tidal volume (VT) and respiratory frequency (fR). Electromyographic activities of both inspiratory and expiratory muscles were augmented as well. These responses were similar to those produced by low levels of whole-body normoxic hypercapnia. We conclude that moderate levels of brain hypoxia (Pa,O2 approximately 40 mmHg) in awake goats caused a net stimulation of ventilatory motor output. 3. Hypoxic stimulation of the carotid bodies alone caused comparable increases in VT and fR, and EMG augmentation of both inspiratory and expiratory muscles whether the brain was hypoxic or normoxic. These responses were quite similar to those obtained over a wide range of whole-body normoxic hypercapnia. We conclude that the integration of carotid body afferent information is not affected by moderate brain hypoxia in awake goats. 4. We found no evidence for an asymmetrical recruitment pattern of inspiratory vs. expiratory muscles in response to carotid body hypoxia or in response to brain hypoxia alone. 5. Our data support the concept that moderate brain hypoxia results in a net stimulation of respiratory motor output. These findings question the significance of 'central hypoxic depression' to the regulation of breathing under physiological levels of hypoxaemia in the awake animal.

Activity of abdominal muscle motoneurons during hypercapnia

Our purpose was to examine the influence of hypercapnia on the activity of motoneurons innervating the transversus abdominis and internal oblique abdominal muscles, and of integrated phrenic and abdominal motor nerve activities. Studies were done in nine adult cats that were decerebrated, vagotomized, thoracotomized, paralyzed and ventilated mechanically. Of 42 motoneurons examined, 24 showed strong respiratory modulation (RM neurons), with the discharge confined primarily to the central expiratory period. The remaining 18 motoneurons discharged tonically, and failed to show respiratory modulation even at increased levels of central respiratory drive. Hyperoxic hypercapnia augmented the activities of the phrenic and abdominal nerves and increased the early expiratory discharge frequency of the RM neurons. The hypercapnia-induced increase in firing frequency during early expiration was accompanied by a corresponding decline in late expiration, and a virtual abolition of the inspiratory activity in the few neurons that discharged in this phase under normocapnic conditions. Finally, hypercapnia induced an increase in the number of spikes generated during each expiratory period in about half of the RM neurons, whereas the remaining cells showed a decrease. Thus, the increased peak activity of the integrated whole abdominal nerve burst with hypercapnia was brought about by a shift in the temporal pattern of motoneuron firing, or by an increase in the number of spikes generated during the expiratory period. The steep rate of rise and the pronounced early expiratory peak observed in the integrated abdominal nerve burst during hypercapnia in this preparation are consistent with the increase in motoneuron firing frequency during the early stages of the expiratory phase.

Respiratory muscle recruitment during selective central and peripheral chemoreceptor stimulation in awake dogs

1. In four awake dogs we measured EMG activity of three inspiratory and four expiratory muscles during sustained central chemoreceptor stimulation (CO2 inhalation), and peripheral chemoreceptor stimulation (intravenous infusion of almitrine bismesylate (almitrine)). By using this selective pharmacological stimulation of the peripheral chemoreceptors and reversibly cold-blocking pulmonary stretch receptors, we were able to determine the effects of each type of stimulation on respiratory muscle recruitment in the absence of such complicating influences as pulmonary stretch receptor feedback, cerebral hypoxia or hypocapnia, and differences in breathing pattern. 2. During 10 min of steady-state hyperpnoea (minute ventilation VI, approximately twice eupnoea) caused by either hypercapnia or isocapnic stimulation of the carotid bodies with almitrine, all three inspiratory and all four expiratory muscles demonstrated significant and sustained elevations in EMG activity. 3. With both types of chemoreceptor stimulation, as tidal volume, VT, increased, so did the mean electrical activities of the crural diaphragm (r = 0.88), costal diaphragm (r = 0.93), parasternals (r = 0.82), triangularis sterni (r = 0.74), transversus abdominis (r = 0.77), external obliques (r = 0.68) and internal intercostals (r = 0.75). 4. In each dog, the response of ventilation and of the diaphragmatic EMG to a given level of central or peripheral chemoreceptor stimulation is highly reproducible from one test day to the next. On the other hand, accessory inspiratory and expiratory abdominal and rib cage muscles in two of the four dogs showed highly significant changes from day to day in the amount of their EMG activity at any given VT. 5. During steady-state ventilatory stimulation, 2 min intervals were chosen during which the two types of chemoreceptor stimulation had caused hyperpnoeas with similar values for VT, total time per breath (TTOT) and inspiratory time divided by the total time (TI/TTOT). Comparison of EMG activities during these matched hyperpnoeas revealed that there were no differences in the activities of any of the muscles between the two forms of stimulation. We conclude that peripheral chemoreceptor stimulation causes significant and sustained recruitment of expiratory muscles even in the absence of pulmonary feedback and that both expiratory and inspiratory muscles are recruited to the same extent during peripheral chemoreceptor stimulation as they are during an identical hyperpnoea caused by central chemoreceptor stimulation.

Respiratory muscle recruitment in awake ponies during exercise and CO2 inhalation

We measured respiratory muscle electromyograms (EMG), inspiratory (I) and expiratory (E) airflow patterns and functional residual capacity (FRC) in six ponies at rest, during treadmill walking at 1.8 mph-5, 10, and 15% grades, and during 2, 4, and 6% CO2 inhalation. There were several similarities in the responses to exercise and CO2 inhalation. The shapes of the I and E flow patterns were not changed appreciably from the respective control patterns during either condition. Mean diaphragm EMG increased from control (P less than 0.05) at 1.8 mph-10 and 15% grades, and during 4 and 6% inhalation. However, mean transversus abdominis EMG did not change significantly from control (P greater than 0.10) during either condition. Exercise did not have an effect (P greater than 0.05) on FRC, and there was only a slight (P less than 0.05) increase in FRC (100 ml or 2%) during 6% CO2 inhalation. Based on the fact that we did not find major differences between exercise and CO2 inhalation in mean diaphragm and transversus abdominis EMG, I and E flow patterns, and FRC we conclude that factor(s) other than the ventilatory stimulus and the non-ventilatory functions of the respiratory muscles are important determinants of the pony's respiratory muscle recruitment pattern under these conditions.

Phasic respiratory muscle patterns and sleep-disordered breathing during rapid eye movement sleep in the English bulldog

Basic mechanisms of sleep-disordered breathing (SDB) during rapid-eye-movement sleep (REMS) have been little investigated, despite the fact that events are often more prolonged and SaO2 nadirs lower during REMS. We predicted that the mechanisms of SDB in REMS would be related to the normal phasic changes in respiratory control in that state, rather than to cyclic arousals or responses to hypoxia as postulated for non-REMS SDB. Recordings of the EMG of the diaphragm (DIA) and the sternohyoid (SH), an upper airway dilating muscle, were made in five English bulldogs during sleep. We found that, as predicted, SDB events were associated with phasic influences rather than with arousals or response to hypoxia. The onset of SDB was significantly related to suppression of drive to both the DIA (p less than 0.01) and the SH (p less than 0.01). The mean drive of the DIA was suppressed to 42% of normal and of the SH to 17% of normal; the suppression of the SH was significantly greater than that of the DIA (p less than 0.05). Events were associated with changes in respiratory muscle EMG patterns typical of phasic REMS (p less than 0.01 for each muscle). The occurrence and duration of events exhibited no clear pattern or relationship to arousal or SaO2. Rather, as would be expected of phenomena associated with phasic REMS, the onset and termination of events were unpredictable. The association of SDB in REMS with phasic REMS influences rather than arousal or hypoxia suggests new directions for therapeutic approaches.

Electromyography of some respiratory muscles in the horse

To investigate activity in respiratory muscles, insulated wire electrodes were used to record electromyographic activity in the costal diaphragm and in the intercostal, serratus ventralis, internal abdominal oblique, transversalis and rectus abdominis muscles in conscious horses and in the same animals when anaesthetised. Electromyographic activity was related to respiratory phases as recorded by a stethograph around the chest wall. The costal diaphragm showed tonic and inspiratory activity in both conscious and anaesthetised animals. The principal muscle actively involved in expiration was the transversalis muscle. Intercostal muscle activity, and any increased activity in the second part of either inspiration or expiration recorded in the conscious animal, was absent under anaesthesia. The very marked tonic activity found in the serratus ventralis muscle in standing horses disappeared during anaesthesia. It was concluded that any stabilisation of the chest wall contributed by activity in the serratus ventralis and intercostal muscles in conscious, standing horses is greatly reduced during anaesthesia.

Influence of phasic volume feedback on abdominal expiratory nerve activity

Our purpose was to examine the influence of phasic lung volume feedback on the activities of motor nerves innervating the diaphragm and transversus abdominis muscles during hypercapnia and hypoxia. We studied seventeen decerebrate cats that were paralyzed and ventilated with a servo-respirator controlled by the integrated phrenic neurogram. The effects of phasic lung volume feedback were assessed by withholding pulmonary inflation during the central inspiratory period. Withholding lung inflation for a single respiratory cycle under hyperoxic, normocapnic conditions consistently prolonged the durations of the inspiratory and expiratory periods, and caused marked increases in the peak electrical activities of both phrenic and abdominal nerves. Hyperoxic hypercapnia (PaCO2 50-80 mmHg) and isocapnic hypoxia (PaO2 60-35 mmHg) increased peak phrenic and abdominal neural activities, and withholding pulmonary inflation under these conditions caused even greater augmentations of inspiratory and expiratory motor output. The augmentation of expiratory activity by withholding lung inflation was proportionately greater than the concomitant prolongation of the central expiratory period. All responses to non-inflation maneuvers were abolished following bilateral cervical vagotomy. The results indicate that vagally mediated volume feedback during inspiration can attenuate the output of abdominal motoneurons in the subsequent expiratory period. Moreover, hypoxia, which attenuates abdominal motor activity in vagotomized animals, enhances this activity when the vagi are intact.

The influence of carotid body chemoreceptors on expiratory muscle activity

Respiratory muscle EMG responses to transient, specific carotid body excitation (NaCN, hypoxia) and inhibition (dopamine, hyperoxia) were studied in 7 unanesthetized standing dogs while intact (N = 7) and during bilateral cold block of the vagi (N = 3). In all dogs carotid body excitation augmented the EMG activities of both expiratory muscles (triangularis sterni, TS; transversus abdominis, TA) and the (inspiratory) crural diaphragm (CR); carotid body inhibition significantly reduced phasic EMG activities in all muscles. With vagal blockade the response of the TS to carotid body stimulation was increased; that of the TA was essentially unchanged from the intact state. In all dogs expiratory muscle recruitment in response to carotid body excitation/inhibition could either precede or follow that of the CR. We conclude that in response to specific, transient carotid body excitation or inhibition: (1) Changes in CR, TS and TA EMGs are qualitatively similar. (2) Vagal feedback is strongly inhibitory to TS and excitatory to TA. (3) Changes in expiratory muscle EMGs do not require preceding changes in the CR. Carotid body stimulation contributes significantly to the generation of phasic expiratory activity during eupnea.

The effects of locomotion on respiratory muscle activity in the awake dog

Using 6 chronically-instrumented awake dogs, we characterized the response of the respiratory muscles to mild and moderate treadmill exercise in terms of (1) the magnitude and timing of the EMGs of the costal and crural diaphragm, triangularis sterni and transverse abdominal muscles, and (2) the concomitant changes in esophageal (PE) and gastric (PG) pressures during treadmill exercise. Dogs wore a bivalved breathing mask which constrained breathing frequency and prevented some of the exercise-induced hypocapnia. Inspiratory and chest expiratory muscle EMGs increased linearly with a 1.5-fold tidal volume (VT) change during exercise. Abdominal muscle recruitment occurred during most exercise levels and exhibited (1) phasic activity coincident with footplant and (2) tonic activation evidenced by a baseline shift in PG and EMG activities. During control, inspiratory and expiratory flow preceded the onset of respiratory muscle EMG activity, but during exercise, electrical activation of these muscles coincided with the onset of mechanical flow. We conclude that phasic and tonic activation of expiratory muscles during exercise in the dog is significantly influenced by both respiratory and locomotory requirements. These patterns of expiratory muscle recruitment may assist inspiration by lengthening inspiratory muscles through decrements in end-expiratory lung volume and by passive recoil of the rib cage and abdominal compartments.

The effects of chemical versus locomotory stimuli on respiratory muscle activity in the awake dog

Using 6 chronically instrumented awake dogs, we contrasted the ventilatory and respiratory muscle EMG responses to steady-state normoxic hypercapnia with those occurring at similar levels of ventilation during steady-state treadmill exercise. During hypercapnia, increases in ventilatory output were primarily due to increments in VT whereas during exercise, increases in minute ventilation were due primarily to increases in frequency. With either hyperpnea, augmentation of both inspiratory and expiratory muscle EMG activity occurred but only diaphragmatic EMG activity was strongly correlated with tidal volume changes in both conditions. Using both EMGs and thoraco-abdominal pressures, we found that with increasing chemical or locomotory stimuli, during inspiration (1) diaphragmatic EMG activation occurred sooner relative to the onset of mechanical flow and (2) that respiratory muscles other than the diaphragm contributed significantly to the generation of inspiratory flow and VT either actively or passively through recoil. Post-inspiratory inspiratory activity of the crural diaphragm shortened relative to control in hypercapnia but did not change in exercise. Finally, during mild hyperpneas, end-expiratory lung volume did not change significantly in either hypercapnia or exercise but decreased at the higher levels of hyperpnea only during exercise. This may reflect differences not in the magnitude of phasic expiratory muscle activity, but rather in the degree of tonic abdominal muscle activation between the two conditions.

Motor unit regulation of mammalian pharyngeal dilator muscle activity

The present study examined the cellular regulation of one of the pharyngeal dilator muscles, the geniohyoid, by assessing its motor unit (MU) behavior in anesthetized cats. During spontaneous breathing, MU that (a) were active during inspiration only (I-MU) and (b) were active during both inspiration and expiration (I/E-MU) were identified. I-MU had a later inspiratory onset time and a shorter duration of inspiratory firing than did I/E-MU (P less than 0.002 and P less than 0.0001, respectively). I-MU were usually quiescent whereas I/E-MU were usually active during the last 20% of inspiration. I/E-MU fired more rapidly (P less than 0.00001) and for relatively longer periods of time (P less than 0.00001) during inspiration than during expiration. End-expiratory airway occlusion (preventing lung expansion during inspiration) augmented the inspiratory activity of both I-MU and I/E-MU. Conversely, end-expiratory airway occlusion reduced the absolute and relative firing durations (P less than 0.002 and P less than 0.00002, respectively) and the firing frequency (P less than 0.001) of I/E-MU activity during expiration. These results indicate that (a) the complex pattern of pharyngeal dilator muscle activity is due to the integrated activity of a heterogeneous group of MU, (b) changes in the degree to which pharyngeal dilator muscles are active result from combinations of MU recruitment/decruitment and modulations of the frequency and duration of MU firing, and (c) gating of lung-volume afferent information occurs during the respiratory cycle.

Inhibition of expiratory muscle EMG and motor unit activity during augmented breaths in cats

To test the hypothesis that expiratory muscle activity is reduced during augmented breaths, electromyographic activity (EMG) of the triangularis sterni (TS) was recorded from eight pentobarbital anesthetized cats. Augmented breaths significantly increased tidal volume and peak diaphragm EMG, and prolonged inspiratory time and the first phase of expiration. However, the duration of the second phase of expiration was unchanged. Peak TS EMG was reduced during sighs in all animals, from 25 +/- 5 to 12 +/- 2 arbitrary units (P less than 0.005). Furthermore, the onset of TS activity during expiration was significantly delayed during augmented breaths (P less than 0.002), whereas the duration of expiratory firing tended to decrease but not significantly. Electrical activity was recorded from eight motor units of the TS in five cats. During resting breathing the motor units had a mean relative expiratory onset time of 46 +/- 4% of expiration, and a mean firing frequency of 19 +/- 2 impulses/sec. Two motor units became quiescent during augmented breaths. Of the remaining six motor units, three minimally shortened their duration of activity (by less than 15%) while three substantially abbreviated their period of firing (by 50% or more). In addition, all TS motor units reduced their mean firing frequency (P less than 0.05) and number of impulses per breath (P less than 0.002) during sighs. We conclude that expiratory activity of the triangularis sterni muscle is reduced during augmented breaths, due to a combination of motor unit derecruitment and a slowing of motor unit firing frequency.

Discharge of abdominal muscle alpha and gamma motoneurons during expiratory loading in cats

Expiratory responses were elicited in abdominal alpha and gamma motoneurons of anesthetized cats by continuous positive airway pressure, tracheal occlusion, lung inflation, or step-changes in expiratory airflow resistance. Abdominal alpha motoneurons were silent during quiet breathing but became rhythmically active whenever expiration was opposed. In addition, the responses of abdominal alpha motoneurons to an increase in expiratory airflow resistance included an increase in discharge frequency, an earlier firing time of individual neurons, recruitment of successively larger motor fibers, and an increased duration of discharge. Abdominal gamma motoneurons discharged continuously during quiet breathing and an increased frequency of discharge during the expiratory phase of the respiratory cycle was evident in approximately one-third of the fibers. This respiratory modulation was enhanced, or initiated if absent, by imposing a load during expiration. Bilateral cervical vagotomy abolished both the respiratory modulation of abdominal gamma motoneurons and expiratory activity in abdominal alpha motoneurons indicating the importance of supraspinal structures. Coactivation of abdominal alpha and gamma motoneurons during active expiration also suggests that a segmental reflex involving abdominal muscle spindles may be capable of providing automatic compensation for changes in expiratory load. In conclusion, both vagal and dorsal root information appear to contribute to the proprioceptive control of abdominal muscle expiratory activity.

Phasic stretch reflex of the abdominal muscles

This analysis of the abdominal stretch reflex (ASR) evoked by a tap to the abdomen was designed to explore how abdominal motoneurons process signals from respiratory and nonrespiratory sources. We recorded surface EMGs from the external and internal oblique muscles in standing subjects. Amplitudes of the abdominal stretch reflex varied despite constant tap forces, but strong taps evoked a larger reflex than weak taps. Trunk rotation toward the recording side, or voluntary contraction of the external and internal oblique muscles increased the reflex amplitudes, whereas contralateral rotation reduced their occurrence. An abdominal stretch reflex during a voluntary contraction was followed by a silent period of 40 to 80 ms. Often a late wave followed a reflex by 20 to 40 ms. Amplitudes during breathholds at residual lung volume were larger than those evoked during a breathhold at functional residual capacity, suggesting that abdominal stretch reflex amplitudes are inversely proportional to static lung volume. During quiet breathing the reflex amplitude reached a maximum slightly before end-expiration and decreased progressively to a minimum close to end-inspiration. During rebreathing, background abdominal activity was augmented with highest activity in late expiration. Abdominal stretch reflex amplitudes continued to wax and wane in phase with respiration, and the maximal reflex occurred progressively earlier in expiration. In summary, the abdominal stretch reflex reflects strong control from abdominal muscle spindles, lung proprioceptors, and chemoreceptors. The relative contributions of these inputs need to be determined.

Caudal medullary expiratory neurone and internal intercostal nerve discharges in the cat: effects of lung inflation

In midcollicular decerebrate, thoracotomized, paralysed cats that were ventilated by a cycle-triggered pump and had an expiratory load, recordings were taken from expiratory neurones in the nucleus retroambigualis of the caudal medulla and from the internal intercostal nerves at T8-T9 levels. Expiratory neurone and internal intercostal activities had augmenting patterns of two types: (a) step-ramp (one-third of the neurones): a large initial increase of activity synchronous with inspiratory termination, followed by a ramp increase throughout the expiratory phase; (b) ramp (two-thirds of the neurones): a steady rise of activity without a sharp initial increase, discharge usually starting after a delay (as much as several hundred milliseconds) from the onset of expiration. Both types of unit pattern could occur together with each type of internal intercostal pattern. At the end of expiration, unit activity shut off abruptly just prior (0-120 ms) to the onset of phrenic discharge. The effects of pulmonary afferent discharge on unit and internal intercostal activities were evaluated by use of inflation tests: withholding inflation during the preceding inspiratory phase; maintaining inflation at the end-inspiratory level during expiration. Both tests produced lengthening of expiratory phase duration (TE), but their effects on activity differed. Following no-inflation during inspiration, the discharge onset delay was lengthened for most ramp neurones, but for only a minority of step-ramp neurones; the slope of activity augmentation did not change on the average; and the peak (end-expiratory) discharge frequency was only slightly increased. The predominant effect of maintained expiratory inflation was reduction of activity slope for ramp neurones and for a minority of step-ramp neurones, as well as increase of peak frequency; there was a moderate increase of discharge onset delay for ramp neurones, but not for step-ramp neurones. The lengthening of TE produced by no-inflation in the preceding inspiration was associated with lengthening of the durations of both the discharge onset delay and the discharge burst, but there was no correlation between changes of these two variables. We observed a 'reversal phenomenon': moderate inflation facilitated activity, whereas higher inflation levels depressed activity, as demonstrated by: comparison of effects of maintained expiratory deflation (removal of the expiratory load) and of maintained expiratory inflation, both of which reduced activity; comparison of effects of phasic expiratory inflations having different magnitudes.(ABSTRACT TRUNCATED AT 400 WORDS)