Respiratory changes in nasal muscle length

Respiratory changes in alae nasi muscle length were recorded using sonomicrometry in pentobarbital sodium-anesthetized tracheostomized dogs spontaneously breathing 100% O2. Piezoelectric crystals were inserted via small incisions into the alae nasi of 11 animals, and bipolar fine-wire electrodes were inserted contralaterally in nine of the same animals. The alae nasi shortened during inspiration in all animals. The mean amount of shortening was 1.33 +/- 0.22% of resting length (LR), and the mean velocity of shortening during the first 200 ms was 4.60 +/- 0.69% LR/S. The onset of alae nasi shortening preceded inspiratory flow by 77 +/- 18 ms (P less than 0.002), at which time both alae nasi shortening and the moving average of electromyographic (EMG) activity had reached approximately one-third of their peak values. In contrast, there was a relative delay in alae nasi relaxation relative to the decay of alae nasi EMG at the end of expiration. Single-breath airway occlusions at end expiration changed the normally rounded pattern of alae nasi shortening and moving average EMG to a late-inspiratory peaking pattern; both total shortening and EMG were increased by similar amounts. The onset of vagally mediated volume-related inhibition of alae nasi shortening occurred synchronously with the onset of inhibition of alae nasi EMG; both occurred at lung volumes substantially below tidal volume. These results indicate that the pattern of inspiratory shortening of this nasal dilating muscle is reflected closely in the pattern of EMG activity and that vagal afferents cause substantial inhibition of alae nasi inspiratory shortening.     

Rib cage and abdominal expiratory muscle responses to CO2 and esophageal distension

The present study compared the responses of rib cage and abdominal expiratory muscles to chemical and mechanical stimuli. In pentobarbital-anesthetized spontaneously breathing dogs, electromyograms (EMG) were recorded from the triangularis sterni (TS) and transverse abdominis (TA) muscles using bipolar intramuscular wire electrodes. During resting oxygen breathing, both muscles were electrically active during expiration. Progressive hyperoxic hypercapnia significantly augmented the expiratory activity of both the TA and the TS. However, the mean percent increases in electrical activity in response to CO2 were substantially greater for the TA than for the TS at all PCO2 levels greater than 50 Torr (P less than 0.01). Occlusion of the airway at end inspiration significantly delayed the onset of TS EMG (from 0.35 +/- 0.07 to 3.35 +/- 0.67 sec; P less than 0.002) and decreased TS EMG rate of rise (P less than 0.002), but did not significantly alter these parameters for the TA. Esophageal distension increased TS EMG in all dogs (by mean of 220 +/- 64%; P less than 0.01), but in contrast decreased TA EMG in all dogs (by a mean of 63 +/- 12%; P less than 0.001). The response to esophageal distention occurred in a graded manner and appeared to be mediated predominantly via vagal afferents. We concluded that expiratory muscles of the rib cage and abdomen manifest substantial differences in their electrical responses to chemoreceptor, pulmonary stretch receptor, and esophageal mechanoreceptor stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vagal contributions to respiratory muscle activity during eupnea in the awake dog.

We examined the effects of reversible vagal cooling on respiratory muscle activities in awake chronically instrumented tracheotomized dogs. We specifically analyzed electromyographic (EMG) activity and its ventilatory correlates, end-expiratory lung volume (EELV) and diaphragmatic resting length via sonomicrometry. Elimination of phasic and tonic mechanoreceptor activity by vagal cooling doubled the EMG activity of the costal, crural, and parasternal muscles, with activation occurring sooner relative to the onset of inspiratory flow. Diaphragmatic postinspiration inspiratory activity in the intact dog coincided with a brief mechanical shortening of the diaphragm during early expiration; vagal blockade removed both the electrical activity and the mechanical shortening. Vagal blockade also doubled the EMG activity of a rib cage expiratory muscle, the triangularis sterni, but reduced that of an abdominal expiratory muscle, the transversus abdominis. Within-breath electrical activity of both muscles occurred sooner relative to the onset of expiratory flow during vagal blockade. Vagal cooling was also associated with a 12% increase in EELV and a 5% decrease in end-expiratory resting length of the diaphragm. We conclude that vagal input significantly modulates inspiratory and expiratory muscle activities, which help regulate EELV efficiently and optimize diaphragmatic length during eupneic breathing in the awake dog.     

Role of triangularis sterni during coughing and sneezing in dogs

Studies in mammals have found that during breathing the triangularis sterni (TS) muscle regulates expiratory airflow and the end-expiratory position of the rib cage and furthermore that the respiratory activity of this muscle is influenced by a variety of chemical and mechanical stimuli. To assess the role of the TS during coughing and sneezing, electromyograms (EMGs) recorded from the TS were compared with EMGs of the transversus abdominis (TA) in eight pentobarbital-anesthetized dogs. During coughing induced by mechanically stimulating the trachea or larynx (n = 7 dogs), peak EMGs increased from 23 +/- 2 to 74 +/- 5 U (P less than 0.00002) for the TS and from 21 +/- 6 to 66 +/- 4 U (P less than 0.0002) for the TA. During sneezing induced by mechanically stimulating the nasal mucosa (n = 3 dogs), peak EMG of the TS increased from 10 +/- 3 to 66 +/- 7 U (P less than 0.005) and peak EMG of the TA increased from 10 +/- 2 to 73 +/- 7 U (P less than 0.02). For both muscles the shape of the EMG changed to an early peaking form during coughs and sneezes. Peak expiratory airflow during coughs of different intensity correlated more closely with peak TS EMG in three dogs and with peak TA EMG in four dogs; peak expiratory airflow during sneezes of different intensity correlated more closely with peak TS than TA EMG in all three animals. These results suggest that the TS is actively recruited during coughing and sneezing and that different neuromuscular strategies may be utilized to augment expiratory airflow.     

Coupling between triangularis sterni and parasternals during breathing in dogs

The purpose of the present studies was to assess the functional coupling between the parasternal intercostals and the triangularis sterni (transversus thoracis) muscles during resting breathing, and we measured the electrical activity and the respiratory changes in length of these two muscles in 13 supine anesthetized dogs. The changes in muscle length were defined relative to their respective in situ relaxation length (Lr). During inspiration, the parasternal intercostals were active and shortened below Lr, causing the triangularis sterni to be passively stretched above Lr. Shortly after the cessation of parasternal contraction, the triangularis sterni became active and shortened below Lr, and in nine animals this active shortening was associated with a forcible distension of the parasternal intercostals above Lr. Deactivation of the triangularis sterni at end expiration caused both muscles to return to their respective Lr. This pattern was essentially unchanged after supplemental anesthesia and bilateral phrenicotomy. We conclude that in dogs breathing quietly the length of the rib cage muscles during the expiratory pause is not passively determined as conventionally thought.     

Tracheal smooth muscle mechanics in vivo

We applied the technique of sonomicrometry to directly measure length changes of the trachealis muscle in vivo. Pairs of small 1-mm piezoelectric transducers were placed in parallel with the muscle fibers in the posterior tracheal wall in seven anesthetized dogs. Length changes were recorded during mechanical ventilation and during complete pressure-volume curves of the lung. The trachealis muscle showed spontaneous fluctuations in base-line length that disappeared after vagotomy. Before vagotomy passive pressure-length curves showed marked hysteresis and length changed by 18.5 +/- 13.2% (SD) resting length at functional residual capacity (LFRC) from FRC to total lung capacity (TLC) and by 28.2 +/- 16.2% LFRC from FRC to residual volume (RV). After vagotomy hysteresis decreased considerably and length now changed by 10.4 +/- 3.7% LFRC from FRC to TLC and by 32.5 +/- 14.6% LFRC from FRC to RV. Bilateral supramaximal vagal stimulation produced a mean maximal active shortening of 28.8 +/- 14.2% resting length at any lung volume (LR) and shortening decreased at lengths above FRC. The mean maximal velocity of shortening was 4.2 +/- 3.9% LR.S-1. We conclude that sonomicrometry may be used to record smooth muscle length in vivo. Vagal tone strongly influences passive length change. In vivo active shortening and velocity of shortening are less than in vitro, implying that there are significant loads impeding shortening in vivo.     

Mechanical role of expiratory muscle recruitment during eupnea in supine anesthetized dogs

To assess the mechanical role of the expiratory musculature during eupnea, we recorded the electromyographic (EMG) activity of the triangularis sterni, the external oblique, and the transversus abdominis in eight supine lightly anesthetized dogs and have measured the volume generated by the phasic activation of the expiratory muscles. Activation of the expiratory muscles was invariably associated with a decrease in lung volume below the relaxed position of the respiratory system, a volume equal to 41.3 +/- 8.4 ml. This volume represented roughly 20% of tidal volume generated during spontaneous breathing. The fractional expiratory contribution to the tidal volume was unrelated to the size of the animal. Traction on the forelimbs (limb extension), however, tended to enhance the phasic expiratory activation of both the triangularis sterni and the transversus abdominis in the majority of animals. Moreover, after limb extension, the fractional contribution of tidal volume attributed to the phasic activation of the expiratory muscles increased in all but one animal. During spontaneous breathing with the forelimbs extended, roughly 25% of tidal volume was found to be due directly to phasic expiratory muscle contraction. The present observations firmly establish that in supine lightly anesthetized dogs breathing at rest the expiratory component of tidal volume represents a substantial contribution.     

Mechanical role of expiratory muscles during breathing in prone anesthetized dogs

The purpose of the present study was to assess the mechanical role of the expiratory muscles during spontaneous breathing in prone animals. The electromyographic (EMG) activity of the triangularis sterni, the rectus abdominis, the external oblique, and the transversus abdominis was studied in 10 dogs light anesthetized with pentobarbital sodium. EMGs were recorded during spontaneous steady-state breathing in supine and prone suspended animals both before and after cervical vagotomy. We also measured the end-expiratory lung volume [functional residual capacity (FRC)] in supine and prone positions to assess the mechanical role of expiratory muscle activation in prone dogs. Spontaneous breathing in the prone posture elicited a significant recruitment of the triangularis sterni, the external oblique, and the transversus abdominis (P less than 0.05). Bilateral cervical vagotomy eliminated the postural activation of the external oblique and the transversus abdominis but not the triangularis sterni. Changes in posture during control and after cervical vagotomy were associated with an increase in FRC. However, changes in FRC, on average, were 132.3 +/- 33.8 (SE) ml larger (P less than 0.01) postvagotomy. We conclude that spontaneous breathing in prone anesthetized dogs is associated with a marked phasic expiratory recruitment of rib cage and abdominal muscles. The present data also indicate that by relaxing at end expiration the expiratory muscles of the abdominal region are directly responsible for generating roughly 40% of the tidal volume.     

Abdominal muscle use during breathing in unanesthetized dogs

The pattern of abdominal muscle use during breathing in unanesthetized dogs is unknown. Therefore, we have recorded the electromyograms of the rectus abdominis, external oblique, and transversus abdominis in eight conscious animals breathing quietly in the sitting, standing, and prone postures. During quiet breathing in the sitting posture, all animals invariably had a large amount of phasic expiratory activity in the transversus abdominis. In contrast, only four animals showed some expiratory activity in the external oblique, and only one animal had expiratory activity in the rectus abdominis. A similar pattern was observed when the animals were standing or lying prone, although the amount of expiratory activity was less in this posture. Bilateral cervical vagotomy in four animals did not affect the degree of transversus abdominis expiratory activation or the influence of posture. We conclude that in conscious dogs 1) the abdominal muscles play an important role during breathing and make spontaneous quiet expiration a very active process, 2) the transversus abdominis is the primary respiratory muscle of the abdomen, and 3) unlike in anesthetized animals, extrapulmonary receptors play a major role in promoting abdominal expiratory contraction.     

Effect of hypercapnia and PEEP on expiratory muscle EMG and shortening

The present study examined the effects of hypercapnia and positive end-expiratory pressure (PEEP) on the electromyographic (EMG) activity and tidal length changes of the expiratory muscles in 12 anesthetized, spontaneously breathing dogs. The integrated EMG activity of both abdominal (external oblique, internal oblique, rectus abdominis, and transverse abdominis) and thoracic (triangularis sterni, internal intercostal) expiratory muscles increased linearly with increasing PCO2 and PEEP. However, with both hypercapnia and PEEP, the percent increase in abdominal muscle electrical activity exceeded that of thoracic expiratory muscle activity. Both hypercapnia and PEEP increased the tidal shortening of the external oblique and rectus abdominis muscles. Changes in tidal length correlated closely with simultaneous increases in muscle electrical activity. However, during both hypercapnia and PEEP, length changes of the external oblique were significantly greater than those of the rectus abdominis. We conclude that both progressive hypercapnia and PEEP increase the electrical activity of all expiratory muscles and augment their tidal shortening but produce quantitatively different responses in the several expiratory muscles.     

Diaphragm, genioglossus, and triangularis sterni responses to poikilocapnic hypoxia

Both isocapnic and poikilocapnic hypoxia may elicit a biphasic respiratory response, during which an initial ventilatory stimulation is followed by a reduction in breathing and diaphragm (DIA) electrical activity. To ascertain whether during adulthood other respiratory muscles have biphasic hypoxic responses similar to the DIA, in nine anesthetized cats electromyograms (EMG) were recorded from the DIA, genioglossus (GG), and triangularis sterni (TS) (n = 7) muscles during poikilocapnic hypoxia. DIA and GG EMG started at 60 +/- 4 and 9 +/- 3 units, respectively, during O2 breathing, increased to a maximum of 100 units during the 10-min hypoxic stimulus, and subsequently declined to 81 +/- 6 and 58 +/- 12 units, respectively, by the end of 10 min of hypoxia. The time course of the increase and subsequent decline was similar for the DIA and GG and for GG activity during both inspiration and expiration. Furthermore the degree to which GG EMG declined after reaching its peak activity level correlated with the magnitude of the respective decline in DIA EMG (r = 0.79, P less than 0.02). The TS, in contrast, was maximally active either during O2 breathing or very early during hypoxia, and its activity declined progressively thereafter (to 13 +/- 6% of its peak value at the end of 10 min of hypoxia). The degree to which TS EMG declined did not correlate with the degree to which DIA or GG EMG declined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Parasternal and external intercostal responses to various respiratory maneuvers.

Recent studies suggest that the external intercostal (EI) muscles of the upper rib cage, like the parasternals (PA), play an important ventilatory role, even during eupneic breathing. The purpose of the present study was to further assess the ventilatory role of the EI muscles by determining their response to various static and dynamic respiratory maneuvers and comparing them with the better-studied PA muscles. Applied interventions included 1) passive inflation and deflation, 2) abdominal compression, 3) progressive hypercapnia, and 4) response to bilateral cervical phrenicotomy. Studies were performed in 11 mongrel dogs. Electromyographic (EMG) activities were monitored via bipolar stainless steel electrodes. Muscle length (percentage of resting length) was monitored with piezoelectric crystals. With passive rib cage inflation produced either with a volume syringe or abdominal compression, each muscle shortened; with passive deflation, each muscle lengthened. During eupneic breathing, each muscle was electrically active and shortened to a similar degree. In response to progressive hypercapnia, peak EMG of each intercostal muscle increased linearly and to a similar extent. Inspiratory shortening also increased progressively with increasing PCO2, but in a curvilinear fashion with no significant differences in response among intercostal muscles. In response to phrenicotomy, the EMG and degree of inspiratory shortening of each intercostal muscle increased significantly. Again, the response among intercostal muscles was not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sonomicrometric regional diaphragmatic shortening in awake sheep after thoracic surgery

Through a right thoracotomy in seven sheep we chronically implanted sonomicrometry crystals and electromyographic electrodes in the costal and crural diaphragmatic regions. Awake sheep were studied during recovery for 4-6 wk, both during quiet breathing (QB) and during CO2 rebreathing. Tidal volume, respiratory frequency, and esophageal and gastric pressures were studied before and after surgery. Normalized resting length (LFRC) was significantly decreased for the costal segment on postoperative day 1 compared with postoperative day 28. Fractional costal shortening both during QB and at 10% end-tidal CO2 (ETCO2) increased significantly from postoperative days 1 to 28, whereas crural shortening did not change during QB but progressively increased at 10% ETCO2. Maximal costal shortening during electrophrenic stimulation was constant at 40% LFRC during recovery, although maximal crural shortening increased from 23 to 32% LFRC. Minute ventilation, tidal volume, and transdiaphragmatic pressure at 10% ETCO2 increased progressively after thoracotomy until postoperative day 28. Our results suggest there is profound diaphragmatic inhibition after thoracotomy and crystal implantation in sheep that requires at least 3-4 wk for stable recovery.     

In vivo length-force relationship of canine diaphragm

Diaphragmatic length was measured by sonomicrometry and transdiaphragmatic pressure (Pdi) by conventional latex balloons in eight dogs anesthetized with pentobarbital sodium under passive conditions and during supramaximal phrenic stimulation. The passive length-pressure relationship indicates that the crural part of the diaphragm is more compliant than the costal part. With supramaximal stimulation the costal diaphragm showed a length-pressure relationship similar in shape to in vitro length-tension curves previously described for the canine diaphragm. The crural part has a smaller pressure-length slope than the costal part in the length range from 80% of optimum muscle length (Lo) to Lo. At supine functional residual capacity (FRC) the resting length (LFRC) of the costal and crural diaphragms are not at Lo. The costal part is distended to 105% of Lo, and crural is shortened to 92% of Lo. Tidal shortening will increase the force output of costal while decreasing that of the crural diaphragm. The major forces setting the passive supine LFRC are the abdominal weight (pressure) and the elastic recoil of the lungs. The equilibrium length (resting length of excised diaphragmatic strips) was 79 +/- 3.6% LFRC for the costal diaphragm and 87 +/- 3.9% LFRC for the crural diaphragm. Similar shortening was obtained in the upright position, indicating passive diaphragmatic stretch at supine LFRC.     

Geniohyoid muscle function in awake canines.

The geniohyoid (Genio) upper airway muscle shows phasic, inspiratory electrical activity in awake humans but no activity and lengthening in anesthetized cats. There is no information about the mechanical action of the Genio, including length and shortening, in any awake, nonanesthetized mammal during respiration (or swallowing). Therefore, we studied four canines, mean weight 28.8 kg, 1.5 days after Genio implantation with sonomicrometry transducers and bipolar electromyogram (EMG) electrodes. Awake recordings of breathing pattern, muscle length and shortening, and EMG activity were made with the animal in the right lateral decubitus position during quiet resting, CO2-stimulated breathing, inspiratory-resisted breathing (80 cmH2O. l-1. s), and airway occlusion. Genio length and activity were also measured during swallowing, when it shortened, showing a 9.31% change from resting length, and its EMG activity increased 6.44 V. During resting breathing, there was no phasic Genio EMG activity at all, and Genio showed virtually no movement during inspiration. During CO2-stimulated breathing, Genio showed minimal lengthening of only 0.07% change from resting length, whereas phasic EMG activity was still absent. During inspiratory-resisted breathing and airway occlusion, Genio showed phasic EMG activity but still lengthened. We conclude that the Genio in awake, nonanesthetized canines shows active contraction and EMG activity only during swallowing. During quiet or stimulated breathing, Genio is electrically inactive with passive lengthening. Even against resistance, Genio is electrically active but still lengthens during inspiration.     

The effect of posture on respiratory activity of the abdominal muscles

The purpose of this study was to determine the influence of posture on the expiratory activity of the abdominal muscles. Fifteen young adult men participated in the study. Activities of the external oblique abdominis, internal oblique abdominis, and rectus abdominis muscles were measured electromyographically in various postures. We used a pressure threshold in order to activate the abdominal muscles as these muscles are silent at rest. A spirometer was used to measure the lung volume in various postures. Subjects were placed in the supine, standing, sitting, and sitting-with-elbow-on-the-knee (SEK) positions. Electromyographic activity and mouth pressure were measured during spontaneous breathing and maximal voluntary ventilation under the respiratory load. We observed that the lung volume changed with posture; however, the breathing pattern under respiratory load did not change. During maximal voluntary ventilation, internal oblique abdominis muscle expiratory activity was lower in the SEK position than in any other position, external oblique abdominis muscle inspiratory activity was lower in the supine position than in any other position, and internal oblique abdominis muscle activity was higher in the standing position than in any other position. During spontaneous breathing, external oblique abdominis muscle activity was higher during expiration and inspiration in the SEK position than in any other position. The internal oblique abdominis muscle activity was higher during both inspiration and expiration in the standing position than in any other position. The rectus abdominis muscle activity did not change with changes in posture during both inspiration and expiration. Increase in the external oblique abdominis activity in the SEK position was due to anatomical muscle arrangement that was consistent with the direction of lower rib movement. On the other hand, increase in the internal oblique abdominis activity in the standing position was due to stretching of the abdominal wall by the viscera. We concluded that differences in activity were due to differences in the anatomy of the abdominal muscles and the influence of gravity.     

Respiratory changes in thoracic muscle length during bronchoconstriction

The purpose of the present study was to assess the effects of bronchoconstriction on respiratory changes in length of the costal diaphragm and the parasternal intercostal muscles. Ten dogs were anesthetized with pentobarbital sodium and tracheostomized. Respiratory changes in muscle length were measured using sonomicrometry, and electromyograms were recorded with bipolar fine-wire electrodes. Administration of histamine aerosols increased pulmonary resistance from 6.4 to 14.5 cmH2O X l-1 X s, caused reductions in inspiratory and expiratory times, and decreased tidal volume. The peak and rate of rise of respiratory muscle electromyogram (EMG) activity increased significantly after histamine administration. Despite these increases, bronchoconstriction reduced diaphragm inspiratory shortening in 9 of 10 dogs and reduced intercostal muscle inspiratory shortening in 7 of 10 animals. The decreases in respiratory muscle tidal shortening were less than the reductions in tidal volume. The mean velocity of diaphragm and intercostal muscle inspiratory shortening increased after histamine administration but to a smaller extent than the rate of rise of EMG activity. This resulted in significant reductions in the ratio of respiratory muscle velocity of shortening to the rate of rise of EMG activity after bronchoconstriction for both the costal diaphragm and the parasternal intercostal muscles. Bronchoconstriction changed muscle end-expiratory length in most animals, but for the group of animals this was statistically significant only for the diaphragm. These results suggest that impairments of diaphragm and parasternal intercostal inspiratory shortening occur after bronchoconstriction; the mechanisms involved include an increased load, a shortening of inspiratory time, and for the diaphragm possibly a reduction in resting length.     

Effects of diaphragmatic ischemia on the inspiratory motor drive.

To assess the effect of diaphragmatic ischemia on the inspiratory motor drive, we studied the in situ isolated and innervated left diaphragm in anesthetized, vagotomized, and mechanically ventilated dogs. The arterial and venous vessels of the left diaphragm were catheterized and isolated from the systemic circulation. Inspiratory muscle activation was assessed by recording the integrated electromyographic (EMG) activity of the left and right costal diaphragms and parasternal intercostal and alae nasi muscles. Tension generated by the left diaphragm during spontaneous breathing attempts was also measured. In eight animals, left diaphragmatic ischemia was induced by occluding the phrenic artery for 20 min, followed by 10 min of reperfusion. This elicited a progressive increase in EMG activity of the left and right diaphragms and parasternal and alae nasi muscles to 170, 157, 152, and 128% of baseline values, respectively, an increase in the frequency of breathing efforts, and no change in left diaphragmatic spontaneous tension. Thus the ratio of left diaphragmatic EMG to tension rose progressively during ischemia. During reperfusion, only the frequency of breathing efforts and alae nasi EMG recovered completely. In four additional animals, left diaphragmatic ischemia was induced after the left phrenic nerve was sectioned. Neither EMG activity of inspiratory muscles nor respiratory timing changed significantly during ischemia. In conclusion, diaphragmatic ischemia increases inspiratory motor drive through activation of phrenic afferents. The changes in alae nasi activity and respiratory timing indicate that this influence is achieved through supraspinal pathways.     

Triangularis sterni muscle use in supine humans

The electrical activity of the triangularis sterni (transversus thoracis) muscle was studied in supine humans during resting breathing and a variety of respiratory and nonrespiratory maneuvers known to bring the abdominal muscles into action. Twelve normal subjects, of whom seven were uninformed and untrained, were investigated. The electromyogram of the triangularis sterni was recorded using a concentric needle electrode, and it was compared with the electromyograms of the abdominal (external oblique and rectus abdominis) muscles. The triangularis sterni was usually silent during resting breathing. In contrast, the muscle was invariably activated during expiration from functional residual capacity, expulsive maneuvers, "belly-in" isovolume maneuvers, static head flexion and trunk rotation, and spontaneous events such as speech, coughing, and laughter. When three trained subjects expired voluntarily with considerable recruitment of the triangularis sterni and no abdominal muscle activity, rib cage volume decreased and abdominal volume increased. These results indicate that unlike in the dog, spontaneous quiet expiration in supine humans is essentially a passive process; the human triangularis sterni, however, is a primary muscle of expiration; and its neural activation is largely coupled with that of the abdominals. The triangularis sterni probably contributes to the deflation of the rib cage during active expiration.     

Mechanical function of hyoid muscles during spontaneous breathing in cats

We assessed the mechanical behavior of the geniohyoid and sternohyoid muscles during spontaneous breathing using sonomicrometry in anesthetized cats. When the animals breathed O2, the hyoid muscles either became longer or did not change length (but never shortened) during inspiration. During progressive hyperoxic hypercapnia, transient increases in geniohyoid muscle inspiratory lengthening occurred in many animals; however, at high PCO2 the geniohyoid invariably shortened during inspiration (mean 4.9% of resting length at the end of CO2 rebreathing; P less than 0.001). The PCO2 at which geniohyoid inspiratory lengthening changed to inspiratory shortening was significantly higher than the CO2 threshold for the onset of geniohyoid electrical activity (P less than 0.01). For the sternohyoid muscle, hypercapnia caused inspiratory lengthening in 13 of 17 cats and inspiratory shortening in 4 of 17 cats; on average the sternohyoid lengthened by 1.6% of resting length at the end of CO2 rebreathing (P less than 0.01). Sternohyoid lengthening occurred in spite of this muscle being electrically active. These results suggest that the relationship between hyoid muscle electrical activity and respiratory changes in length is very complex, so that the presence of hyoid muscle electrical activity does not necessarily indicate muscle shortening, and among the geniohyoid and sternohyoid muscles, the geniohyoid has a primary role as a hypopharyngeal dilator in the spontaneously breathing cat, with the sternohyoid muscle acting in an accessory capacity.