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.     

Mechanical advantage of the canine triangularis sterni

De Troyer, André, and Alexandre Legrand.Mechanical advantage of the canine triangularis sterni.J. Appl. Physiol. 84(2): 562-568, 1998.Recent studies on the canine parasternal intercostal,sternomastoid, and scalene muscles have shown that the maximal changesin airway opening pressure (Pao) obtained per unit muscle mass(Pao/m) during isolatedcontraction are closely related to the fractional changes in musclelength per unit volume increase of the relaxed chest wall. In thepresent study, we have examined the validity of this relationship for the triangularis sterni, an important expiratory muscle of the rib cagein dogs. Passive inflation above functional residual capacity (FRC)induced a virtually linear increase in muscle length, such that, with a1.0-liter inflation, the muscle lengthened by 17.9 ± 1.6 (SE) % of its FRC length. When the muscle in one interspace wasmaximally stimulated at FRC, Pao increased by 0.84 ± 0.11 cmH₂O. However, in agreement withthe length-tension characteristics of the muscle, when lung volume wasincreased by 1.0 liter before stimulation, the rise in Pao amounted to1.75 ± 0.12 cmH₂O. At thehigher volume, Pao/m thereforeaveraged + 0.53 ± 0.05 cmH₂O/g, such that the coefficientof proportionality between the change in triangularis sterni lengthduring passive inflation and Pao/m was the same as that previously obtained for the parasternalintercostal and neck inspiratory muscles. These observations,therefore, confirm that there is a unique relationship between thefractional changes in length of the respiratory muscles, bothinspiratory and expiratory, during passive inflation and theirPao/m. Consequently, the maximal effect of a particular muscle on the lung can be predicted on the basisof its change in length during passive inflation and its mass. Ageometric analysis of the rib cage also established that thelengthening of the canine triangularis sterni during passive inflationis much greater than the shortening of the parasternal intercostalsbecause, in dogs, the costal cartilages slope downward from thesternum.

     

Characterization of expiratory intercostal activity to triangularis sterni in cats

Our purpose was to characterize activity of the intercostal nerve branch innervating the triangularis sterni muscle and the motoneuronal activities comprising this nerve discharge. In decerebrate, vagotomized, paralyzed, and ventilated cats, phasic triangularis sterni neural activity was evident in normocapnia. In most cats, activity did not commence until midexpiration. Activity then rose progressively to terminate at end expiration. Peak neural activities increased in parallel with phrenic activity in hypercapnia and fell in hypocapnia. The progressive increase in triangularis sterni neural activity within each respiratory cycle resulted from recruitment of motoneuronal activities throughout expiration. Once recruited, many motoneurons had a decrementing or constant discharge frequency. In hypercapnia, motoneuronal discharge frequencies increased, and additional activities were recruited. The number of active motoneurons and their discharge frequencies fell in hypocapnia. A similar pattern of motoneuronal activities and responses to stimuli was observed in cats with intact vagi. Factors are considered that may underlie the recruitment pattern of triangularis sterni motoneuronal activities and the inhibition of these in early expiration.     

Ex vivo imaging of motor axon dynamics in murine triangularis sterni explants

We provide a protocol that describes an explant system that allows the dynamics of motor axons to be imaged. This method is based on nerve-muscle explants prepared from the triangularis sterni muscle of mice, a thin muscle that covers the inside of the thorax. These explants, which can be maintained alive for several hours, contain long stretches of peripheral motor axons including their terminal arborizations and neuromuscular junctions. Explants can be prepared from transgenic mouse lines that express fluorescent proteins in neurons or glial cells, which enables direct visualization of their cellular and subcellular morphology by fluorescence microscopy. Time-lapse imaging then provides a convenient and reliable approach to follow the dynamic behavior of motor axons, their surrounding glial cells and their intracellular organelles with high temporal and spatial resolution. Triangularis sterni explants can be prepared in 15 min, imaged ex vivo for several hours and processed for immunohistochemistry in about 2 h.     

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)     

Size-related differences in the branching pattern of the motor nerve terminals in triangularis sterni muscle of the mouse

A light microscopy morphometric study was performed in singly innervated synaptic areas of the triangularis sterni muscle of the normal adult Swiss mouse. Investigating mechanisms of the motor nerve growth control, we tested the hypothesis that significant differences in the nerve terminal branching pattern can be detected between different populations of nerve endings classified according to their arborization complexity or size. The main observations of this morphometric study are first, that the mean segment length of the terminal arborization between branch points behaves as an independent variable from the remaining parameters; the mean value of this parameter did not change in nerve endings of differing size and complexity. Secondly, the increase in size of the nerve endings is accompanied by a significant reduction in the mean length of the distal free-end segments. Results are discussed in the context of the possible regulatory mechanisms governing nerve terminal growth and remodelling.     

Heterogeneous lung emptying during forced expiration

Several lines of evidence suggest that the healthy mammalian lung empties homogeneously during a maximally forced deflation. Nonetheless, such behavior would appear to be implausible if for no other reason than that airway structure is known to be substantially heterogeneous among parallel pathways of gas conduction. To resolve this paradox we reexamined the degree to which lung emptying is homogeneous, and considered mechanisms that might control differential regional emptying. Twelve excised canine lungs were studied. Regional alveolar pressure relative to pleural pressure was used as an index of regional lung volume. By use of a capsule technique, alveolar pressure was measured simultaneously in each of six regions during flow-limited deflations; flow from the lung was measured plethysmographically. The standard deviation of interregional pressure differences, which was taken as an index of nonuniformity, was 0.0, 0.74, 0.64, and 0.90 cmH2O at mean recoil pressures of 30, 8.4, 4.5, and 2.1 cmH2O (0, 25, 50, and 75% expired vital capacity), indicating that interregional pressure differences increased more rapidly earlier in the deflation. When we examined the time rate of change of regional alveolar pressure as an index of regional flow, we observed an intricate pattern of differential regional behavior that was inapparent in the maximum expiratory flow-volume (MEFV) curve. The most plausible interpretation of these findings is that regions of the healthy excised canine lung empty heterogeneously to a small degree, but in an interdependent compensatory pattern that is inapparent in the configuration of the maximum expiratory flow-volume curve.     

Responses of bulbospinal neurons during expiration reflex in cats

The responses of 129 bulbospinal (BS) neurons were investigated an anesthetized cats during the expiration reflex evoked by mechanical stimulation of the vocal folds or electrostimulation of the superior laryngeal nerve. It was found that in addition to expiratory and inspiratory neurons, a large number of nonrespiratory BS cells localized bilaterially, primarily in the giant-cell tegmental fields, are activated. It is shown that expulsive expiration during this reflex is to a considerable extent provided by activation of non-respiratory BS neurons participating also in the transmission of descending influences from high-threshold afferents of other modalities.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 88–98, January–February, 1991.     

Estimation of tracheal sound intensity during forced expiration

The tracheal sounds during forced expiration were studied using the mathematical model of forced expiration. It has been shown that separated flow in the region of dynamic constriction of the trachea during forced expiration may cause the generation of tracheal sounds.     

Factors influencing glottic dimensions during forced expiration

To examine the relationship between expiratory effort, expiratory flow, and glottic aperture, we compared the effects of actively and passively produced changes in flow in six normal subjects. During flow transients of 1.08 +/- 0.08 l/s produced by voluntary expiratory effort, glottic width (dg) increased by 54 +/- 13% (mean +/- SE). In contrast transient increases in expiratory flow, produced passively by chest compression, were not accompanied by increases in glottic dimensions. Similarly, when subjects expired through a resistance, transient passive increases in mouth pressure of 8.1 +/- 0.8 cmH2O failed to increase glottic width. However, when similar positive-pressure transients were produced actively, dg increased by 97 +/- 36% even though the expiratory efforts were accompanied by relatively small increases in flow (0.20 +/- 0.05 l/s). During tidal breathing glottic widening commenced 160 +/- 60 ms before the onset of inspiratory flow, whereas the widening associated with active flow and pressure transients did not measurably precede the onset of the change in flow or pressure. Our results indicate that transient expulsive efforts are associated with synchronous increases in dg, regardless of whether expiratory flow increases. The findings are most readily explained by a centrally determined synchronous recruitment of intrinsic laryngeal and expiratory muscles that facilitates lung emptying by minimizing airway resistance during forced exhalation.     

The expiration reflex during ontogenesis in the rat.

The authors studied the elicitability of the expiration and aspiration reflex and of the respiratory reaction from the tracheobronchial area in 131 anaesthetized rats (aged 1-15 days, adult and biologically old). They found that the expiration reflex could be elicited, in the rat, from the first day of life, at a time when other respiratory reflexes were not yet stable. In young rats, the expiration reflex was often followed by a cough reaction which was absent in adult animals. The findings indicate that the expiration reflex is one of the most important respiratory reflexes of the early postnatal period in the rat, because the aspiration reflex and the respiratory reaction from the bronchi were not stable until the 15th day of life. In biological old rats, the expiration reflex is less frequently elicited and its intensity attains about half the value found in adult animals. The aspiration reflex and the respiratory reaction from the bronchi are likewise less readily elicited than in adult animals, but when the intensity of their maximum expiratory effort is increased, it is far greater.