Triangularis sterni: a primary muscle of breathing in the dog

The isolated action, pattern of neural activation, and mechanical contribution to eupnea of the triangularis sterni (transversus thoracis) muscle were studied in supine anesthetized dogs. Linear displacement transducers were used to measure the axial displacements of the ribs and sternum. Tetanic stimulation of the triangularis sterni in the apneic animal caused a marked caudal displacement of the ribs, a moderate cranial displacement of the sternum, and a decrease in lung volume. During quiet breathing, there was invariably a rhythmic activation of the muscle in phase with expiration that was independent of the presence or absence of activity in the abdominal and internal interosseous intercostal muscles. This phasic expiratory activity in the triangularis sterni was of large amplitude and caused the ribs to be more caudal and the sternum to be more cranial during the spontaneous expiratory pause than during relaxation. Additional studies on awake animals showed that rhythmic activation of the triangularis sterni occurs in all body positions and is not caused by anesthesia. These findings indicate that expiration in the dog is not a passive process and that the end-expiratory volume of the rib cage is not determined by an equilibrium of static forces alone. Rather, it is actively determined and maintained below its relaxation volume by contraction of the triangularis sterni throughout expiration. The use of this muscle is likely to facilitate inspiration by increasing the length of the parasternal intercostals and taking on a portion of their work.     

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.     

Mechanics of the parasternal intercostals during occluded breaths in dogs

The electrical activity and the respiratory changes in length of the third parasternal intercostal muscle were measured during single-breath airway occlusion in 12 anesthetized, spontaneously breathing dogs in the supine posture. During occluded breaths in the intact animal, the parasternal intercostal was electrically active and shortened while pleural pressure fell. In contrast, after section of the third intercostal nerve at the chondrocostal junction and abolition of parasternal electrical activity, the muscle always lengthened. This inspiratory muscle lengthening must be related to the fall in pleural pressure; it was, however, approximately 50% less than the amount of muscle lengthening produced, for the same fall in pleural pressure, by isolated stimulation of the phrenic nerves. These results indicate that 1) the parasternal inspiratory shortening that occurs during occluded breaths in the dog results primarily from the muscle inspiratory contraction per se, and 2) other muscles of the rib cage, however, contribute to this parasternal shortening by acting on the ribs or the sternum. The present studies also demonstrate the important fact that the parasternal inspiratory contraction in the dog is really agonistic in nature.     

Mechanical advantage of sternomastoid and scalene muscles in dogs

Legrand, Alexandre, Vincent Ninane, and André DeTroyer. Mechanical advantage of sternomastoid and scalene muscles in dogs. J. Appl. Physiol. 82(5):1517-1522, 1997.Theoretical studies have led to the predictionthat the maximal effect of a given respiratory muscle on airway openingpressure (Pao) is the product of muscle mass, the maximal active muscletension per unit cross-sectional area, and the fractional change inmuscle length per unit volume increase of the relaxed chest wall. It has previously been shown that the parasternal intercostals behave inagreement with this prediction (A. De Troyer, A. Legrand, and T. A. Wilson. J. Physiol. (Lond.) 495:239-246, 1996; A. Legrand, T. A. Wilson, and A. DeTroyer. J. Appl. Physiol. 80:2097-2101, 1996). In the present study, we have tested theprediction further by measuring the response to passive inflation andthe pressure-generating ability of the sternomastoid and scalenemuscles in eight anesthetized dogs. With 1-liter passive inflation, thesternomastoids and scalenes shortened by 2.03 ± 0.17 and 5.98 ± 0.43%, respectively, of their relaxation length(P < 0.001). During maximalstimulation, the two muscles caused similar falls in Pao. However, thesternomastoids had greater mass such that the change in Pao (Pao)per unit muscle mass was 0.19 ± 0.02 cmH₂O/g for the scalenes and only0.07 ± 0.01 cmH₂O/g forthe sternomastoids (P < 0.001).After extension of the neck, there was a reduction in both the muscleshortening during passive inflation and the fall in Pao duringstimulation. The Pao per unit muscle mass was thus closely relatedto the change in length; the slope of the relationship was 3.1. These observations further support the concept that the fractional changes inlength of the respiratory muscles during passive inflation can be usedto predict their pressure-generating ability.

     

Effect of lithium on diffusion chamber granulopoiesis

We studied the effect of lithium on diffusion chamber (DC) granulopoiesis. When DC loaded with bone marrow cells were implanted into the peritoneal cavity of mice previously injected with lithium carbonate, more proliferative and nonproliferative granulocytes were produced as compared to DC implanted into control hosts. The number of DC CFU-c was increased significantly in the lithium-treated group, but there was no difference in the number of DC CFU-s. Levels of DC fluid CSF showed no evident correlation with DC myelopoiesis. These data suggest that a humoral factor other than CSF mediates the action of lithium in DC granulopoiesis, and that lithium's influence on DC hematopoietic stem cell proliferation occurs mainly at the CFU-c level.     

Action of intercostal muscles on the lung in dogs

The action on the lung of interosseous intercostal muscles located in the third and the seventh interspaces was studied in 15 anesthetized-curarized supine dogs. Changes in pleural pressure, airflow rate, and lung volume produced by maximal stimulation of both intercostal muscle layers were measured at and above functional residual capacity (FRC). In five animals measurements were also obtained during isolated stimulation of the internal layer. At FRC, intercostal stimulation in the upper interspaces had invariably an inspiratory effect on the lung but no effect was detectable in the lower interspaces. Qualitatively similar results were obtained during isolated stimulation of the internal layer. Increasing lung volume reduced the inspiratory action of the upper intercostals and conferred an expiratory action to the lower intercostals. These results indicate the following: 1) when contracting in a single interspace, the external and internal intercostals have a qualitatively similar action on the lung; and 2) this action, however, depends critically on their location along the cephalocaudal axis of the rib cage: in the upper portion of the rib cage, both muscle layers have an inspiratory effect at and above FRC; in the lower portion of the rib cage, they have no respiratory action at FRC and act in the expiratory direction at higher lung volumes.     

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.