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MS 8916 Received 2 November 1998; accepted after revision 16 March 1999.
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ABSTRACT |
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INTRODUCTION |
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Although it is now well established that the interchondral portion of the internal intercostal muscles (the so-called parasternal intercostals) elevates the ribs and inflates the lung when it contracts (De Troyer & Kelly, 1982; De Troyer et al. 1996), the actions of the external intercostals and the interosseous portion of the internal intercostals remain uncertain. The current conventional view is based on the theory proposed 250 years ago by Hamberger (1749). According to this theory, the fibres of the external intercostals slope obliquely caudad and ventrally from the rib above to the rib below, and so their lower insertion is further from the centre of rotation of the ribs (i.e. the costo-vertebral articulations) than their upper insertion. Consequently, when this muscle contracts with its force equal and opposite at both insertions, the torque acting on the lower rib, which tends to raise it, is greater than that acting on the upper rib, which tends to lower it. The net effect of the muscle, therefore, would be to raise the ribs and to inflate the lung. In contrast, the fibres of the internal interosseous intercostals slope obliquely caudad and dorsally from the rib above to the one below so that their lower insertion is closer to the centre of rotation of the ribs than the upper one. As a result, the net effect of their contraction would be to lower the ribs and to deflate the lung. This theory, however, has not been verified, and computations based on the orientation of the muscle fibres and on descriptions of rib displacement in dogs (Margulies et al. 1989) have recently suggested that the actions of the external and internal intercostal muscles on the lung might vary between the dorsal and the ventral aspects of the rib cage as well as between the rostral and caudal interspaces (Wilson & De Troyer, 1993).
In the present studies, we have examined the effects of the canine external and internal interosseous intercostals on the lung by using a standard theorem of mechanics, the Maxwell reciprocity theorem. When applied to the respiratory system (Wilson & De Troyer, 1992, 1993), this theorem predicts that the respiratory effect of a particular muscle (that is, the potential change in airway pressure - 
Pao - produced by the muscle contracting alone against a closed airway) is related to the mass (m) of the muscle, the maximal active muscle tension per unit cross-sectional area (
), and the fractional change in muscle length (L/L) per unit volume increase of the relaxed chest wall (VL)Rel, such that:
Pao = m (L/(L VL))Rel. (1)
For a machine, such as a lever, mechanical advantage is defined as the ratio of the force delivered at the load to the force applied at the handle. By analogy, the mechanical advantage of a respiratory muscle may therefore be defined as Pao/m and, according to eqn (1), could be evaluated by measuring (L/(L VL))Rel. In other words, a muscle that shortens during passive inflation would have an inspiratory mechanical advantage and would cause a fall in Pao when it contracts. Conversely, a muscle that lengthens during passive inflation would have an expiratory mechanical advantage and would cause a rise in Pao during contraction.
The validity of this equation has been tested experimentally on a number of canine inspiratory muscles, including the parasternal intercostals, and on the triangularis sterni, an important expiratory muscle of the rib cage (De Troyer et al. 1996; Legrand et al. 1996, 1997; De Troyer & Legrand, 1998). For all these muscles, there was a unique relationship between Pao/m and (L/(L VL))Rel. Furthermore, the coefficient of proportionality () between these two variables was 3·0 kg cm-2, in close agreement with values of maximal active muscle tension measured in vitro (Close, 1972; Farkas et al. 1985; Farkas, 1991). These observations therefore confirmed eqn (1) in all respects, and this implies that the respiratory effect of any muscle can be estimated simply by measuring its fractional change in length during passive inflation and its mass. Based on this principle, we thus set out: (1) to examine the changes in length of the canine external and internal interosseous intercostals throughout the rib cage during passive inflation; and (2) to determine external and internal intercostal muscle mass. The respiratory effects of the muscles were then calculated.
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METHODS |
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The studies were carried out on eight adult mongrel dogs (15-25 kg body weight), as approved by the Animal Ethics and Welfare Committee of the Brussels School of Medicine. The animals were anaesthetized with pentobarbitone sodium (initial dose, 30 mg kg-1 I.V.), placed in the supine posture, and intubated with a cuffed endotracheal tube. A venous cannula was inserted in the forelimb to give maintenance doses of anaesthetic (3-5 mg kg-1 h-1 I.V.), and a catheter was inserted into the left femoral artery to monitor blood pressure and heart rate. The rib cage and intercostal muscles were then exposed on both sides of the sternum from the first to the eleventh rib.
The changes in intercostal muscle length during passive inflation were assessed by using the same procedure as in our previous investigations on the parasternal intercostals and triangularis sterni (De Troyer & Legrand, 1995, 1998; De Troyer et al. 1996). Thus in each animal, the length of the second intercostal space was measured from the angle of the ribs dorsally to the costochondral junctions ventrally, the orientations of the external intercostal muscle were carefully defined, and three muscle bundles representing the dorsal third, the middle third, and the ventral third of the interspace were selected. Pairs of small screws were then inserted into the second and third ribs at the points of insertions of these bundles, and the animal was paralysed with an intravenous injection of 2 mg pancuronium and ventilated mechanically. The ventilation was stopped and the chest wall was allowed to relax to equilibrium, and the linear distance between the screws of each pair (i.e. the length of each external intercostal muscle bundle at functional residual capacity (FRC)) was measured with callipers. The tracheal cannula was then connected to a calibrated syringe, lung volume was increased by 1 l, and the measurements were repeated.
The process was subsequently repeated for the fourth, sixth, eighth and tenth interspaces, after which small segments of external intercostals were sectioned along their caudal insertions. Internal intercostal muscle bundles representing the dorsal third, the middle third, and the ventral third of the second, fourth, sixth, eighth and tenth interspaces were selected, and the procedure was repeated. All measurements of muscle length were made in triplicate. No changes in blood pressure or heart rate occurred during the course of the experiments; in addition, the pupils in each animal remained constricted and unresponsive to light throughout, thus indicating a deep level of anaesthesia.
After completion of these measurements, the animals were given an overdose (50 mg kg-1) of anaesthetic, and in six of them, the entire layer of external and internal intercostal muscles situated in the dorsal third, the middle third, and the ventral third of the five interspaces studied was harvested on both sides of the chest and weighed.
Data analysis
For each muscle bundle in each animal, the three measurements of length performed at FRC and after passive inflation were averaged, and the changes in muscle length caused by passive inflation were expressed as percentage changes relative to the muscle length at FRC (LFRC). Data were averaged for the animal group, and they are presented as means ± S.E.M. Comparisons between the fractional changes in length and the masses of the external and internal intercostal muscles from the rib angles to the costochondral junctions in the different interspaces were made by analysis of variance (ANOVA) with repeated measures, and multiple comparison testing of the mean values was performed, when appropriate, using Student-Newman-Keuls tests. The criterion for statistical significance was taken as P < 0·05.
In addition, a linear multiple regression analysis was performed to determine the dependence of the dependent variable y (length change or respiratory effect) on each of the three independent variables x1, x2 and x3 that describe the anatomy of the muscle. The first independent variable x1 was taken to be the fibre orientation, the value +1 being assigned to the external layer and -1 to the internal layer. The second independent variable x2 was taken to be the position of the muscle fibres along the arc of the ribs and the values +1, 0 and -1 were assigned to the dorsal, middle and ventral positions, respectively. The third variable x3 was the interspace number.
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RESULTS |
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Changes in muscle length
The effects of passive inflation on external intercostal muscle length are shown in Fig. 1A for the eight animals studied. Passive inflation caused in every animal a large fractional shortening of the external intercostal in the dorsal third of the second interspace. However, the shortening of this dorsal part decreased continuously and rapidly toward the base of the rib cage (P < 0·001), so that it was abolished in the eighth interspace and was then reversed into a significant lengthening in the tenth interspace. In addition, the muscle in any given interspace showed less fractional shortening or more fractional lengthening as one moved away from the angle of the ribs toward the costochondral junctions (P < 0·05 or less). As a result, the external intercostal muscles in the ventral third of the sixth interspace and in the middle and ventral thirds of the eighth and tenth interspaces invariably lengthened as well.
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Mean ± S.E.M. data obtained from eight animals. The changes in muscle length produced by passive inflation (1 l) are expressed as percentage changes relative to the muscle length at FRC (LFRC); negative changes in length represent muscle shortening (inspiratory mechanical advantage), whereas positive changes in length represent muscle lengthening (expiratory mechanical advantage). | ||
The internal intercostal muscle in the dorsal third of any given interspace showed more fractional lengthening than the external intercostal, but these muscles also demonstrated prominent dorsoventral and rostrocaudal gradients (Fig. 1B). Thus in a given interspace, the fractional lengthening decreased progressively from the angle of the ribs to the costochondral junctions (P < 0·001); the internal intercostal muscle lengthening also decreased gradually from the eighth to the second interspace (P < 0·001). Consequently, the internal intercostals situated in the ventral third of the eighth and sixth interspaces showed less fractional lengthening than the corresponding external intercostals. The internal intercostals in the middle and ventral thirds of the second interspace even had a small but consistent fractional shortening.
The multiple regression analysis of these data indicated that the orientation of the muscle fibres (external vs. internal) accounted for only 19 % of the total variance of length change. The interspace number was the most significant variable, accounting for 66 % of the variance, and the position of the muscle fibres along the arc of the ribs accounted for another 12 %.
Muscle mass
The values of bilateral external and internal intercostal muscle mass in the different interspaces are shown in Fig. 2. For the six animals, external intercostal muscle mass (Fig. 2A) in the dorsal third of the intercostal spaces decreased progressively (P < 0·02) from 2·9 ± 0·5 g in the second interspace to 1·4 ± 0·4 g in the tenth. Except for the tenth interspace, the mass of external intercostal muscle in each interspace also decreased gradually (P < 0·005) from the angle of the ribs toward the costochondral junctions. Conversely, although the mass of internal intercostal muscle did not show any clear-cut dorsoventral gradient, it increased progressively (P < 0·05) from 0·8 ± 0·2 g in the dorsal third of the second interspace to 2·1 ± 0·4 g in the dorsal third of the tenth interspace.
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Same symbols as in Fig. 1. Mean data obtained from six animals on both sides of the sternum; the S.E.M. are small and included in the symbols. | ||
Computed respiratory effects
According to eqn (1), the maximum Pao that a muscle can produce can be calculated by multiplying mechanical advantage by muscle mass and maximum active stress. In vitro studies on a variety of limb and respiratory muscles from different animal species, including the canine external and internal interosseous intercostal muscles, have shown that for any muscle, maximum active stress is 
3·0 kg cm-2 (Close, 1972; Farkas et al. 1985; Farkas, 1991). Recent studies have yielded a similar value in vivo for the canine parasternal intercostals and triangularis sterni (De Troyer et al. 1996; De Troyer & Legrand, 1998). To calculate the Pao that each area of external and internal intercostal muscle would produce during isolated, maximal contraction, it seemed therefore reasonable to assume that maximum active stress for these muscles in vivo is also 3·0 kg cm-2.
The values of Pao thus computed are shown in Fig. 3. As anticipated from the data reported in Figs 1 and 2, the distributions of maximum Pao had the same shape as the distributions of mechanical advantage and mass but showed stronger gradients. Thus the external intercostals in the dorsal third of the second and fourth interspaces had a clear-cut inspiratory effect, but the muscles in the middle third and the ventral third of the eighth and tenth interspaces had a definite expiratory, rather than inspiratory, effect. On the other hand, the internal intercostals in the dorsal third of the sixth through tenth interspaces had a clear-cut expiratory effect, but the muscle in the ventral third of the second interspace had an inspiratory, rather than expiratory, effect.
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Same symbols as in Fig. 1. These data are the computed maximal changes in airway pressure ( | ||
The multiple regression analysis of these data indicated that the interspace number was also the most important determinant of the respiratory effect of the intercostal muscles; this factor alone accounted for 55 % of the total variance. The orientation of the muscle fibres and the position of the fibres along the rib circumference accounted, respectively, for 27 and 10 % of this variance.
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DISCUSSION |
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The current conventional view of intercostal muscle actions, based on the theory of Hamberger (1749), maintains that as a result of the orientation of the muscle fibres, the external intercostals have an inspiratory action on the lung and the internal interosseous intercostals have an expiratory action. According to this theory, all the external intercostals should therefore shorten as the relaxed respiratory system is inflated and the ribs rotate cranially, whereas all the internal interosseous intercostals should lengthen. The present observation that passive inflation induces shortening of the external intercostals in the dorsal third of the second through sixth interspaces and lengthening of the internal intercostals over a large fraction of the rib cage (Fig. 1) is in agreement with the theory, and this therefore confirms that the orientation of the intercostal muscle fibres is a significant determinant of their mechanical advantage. However, the current studies have also established the important fact that in the dog, this factor alone accounts for only 19 % of the mechanical advantage of the muscles. Thus other determinants operate (1) to decrease both the inspiratory mechanical advantage of the external intercostals and the expiratory mechanical advantage of the internal intercostals as one moves from the angle of the ribs toward the costochondral junctions, and (2) to decrease the inspiratory mechanical advantage of the external intercostals from the rostral to the caudal interspaces and simultaneously increase the expiratory mechanical advantage of the internal intercostals. Although these deviations from Hamberger's theory appear to be complex, they actually point to the two biases of the theory.
First, the theory of Hamberger is based on a two-dimensional model of the rib cage; the ribs are modelled as straight bars and assumed to rotate around axes that lie perpendicular to the plane of the ribs. In fact, the ribs are curved, and for two parallel curved ribs that rotate by equal amounts around parallel axes, the change in length of muscles connected to the ribs is proportional to only one component of rib rotation, the component normal to the surface of the ribs (Wilson & De Troyer, 1993). As shown in Fig. 4A, the axis of rib rotation is oriented dorsally and laterally, and so the component of rib rotation perpendicular to the surface of the ribs is greatest in the dorsal part of the rib cage, decreases gradually as one moves away from the spine, and is eventually reversed in the ventral part of the rib cage. With passive inflation, therefore, the external intercostal (continuous line in Fig. 4B) would shorten most in the dorsal aspect of the interspace and least in the ventrolateral aspect. The internal intercostal (dashed line in Fig. 4B) would show opposite length changes, with a large lengthening in the dorsal aspect of the rib cage, a smaller lengthening in the lateral aspect, and a shortening in the vicinity of the sternum. It has long been recognized that the internal intercostal muscles in the parasternal region have a different function than those in the dorsal aspect of the rib cage, but it was not appreciated that this is a progressive, continuous change that results from the continuous change in the orientation of the surface of the ribs.
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A shows the plan form of a typical rib in the dog and the axis of rib rotation (bold vector). If the ribs were parallel and rotated cranially by equal amounts around parallel axes, the magnitude of intercostal muscle length change would be largely determined by the component of rib rotation normal to the surface of the ribs (light vectors). This component varies with the angular position ( | ||
The second shortcoming of the theory of Hamberger is its implication that all the ribs rotate by equal amounts around parallel axes. During passive inflation, in fact, the cranial displacement of ribs 4-6 is larger than the displacements of the more rostral and more caudal ribs (Margulies et al. 1989; De Troyer et al. 1996). As a result, the rostral interspaces are compressed as the ribs move cranially, so that both the external and internal intercostal muscles in these interspaces tend to shorten; in contrast, the caudal interspaces are expanded, so that both the external and internal intercostals tend to lengthen. On the basis of the data shown in Fig. 1 and the analysis of variance performed on these data, it appears that the location of the muscles along the rostrocaudal axis of the rib cage is the single most important determinant of their mechanical advantage.
As the animals in this study were investigated in the supine posture, we cannot exclude the possibility that the distributions of mechanical advantage would be different in the standing posture. In particular, passive inflation in this posture might induce more uniform cranial displacement of the ribs, such that the rostrocaudal gradients of mechanical advantage could be smaller. One of the most prominent features of the current studies, however, is the remarkable similarity between the distributions of intercostal muscle mass (Fig. 2) and the distributions of the magnitudes of mechanical advantage. Indeed, the external intercostals were thickest in the dorsal portion of the rostral interspaces, that is in the areas where their inspiratory mechanical advantage in the supine posture is greatest. The internal intercostals were also thickest in the caudal interspaces where their expiratory mechanical advantage in the supine posture is greatest. This distribution of muscle mass suggests that the external and internal intercostal muscles have significant rostrocaudal and dorsoventral gradients of mechanical advantage in all body positions.
As a result of this matching between muscle mass and mechanical advantage, the distributions of respiratory effect showed marked gradients in the dorsoventral direction. Thus the computed Pao values for the external intercostals situated in the dorsal aspect of the second to fourth interspaces (Fig. 3) were 8 to 9 times more negative than those computed for the muscles in the ventral aspect of the same interspaces; the computed Pao values for the internal intercostals in the dorsal aspect of the eighth and sixth interspaces were also 2 to 4 times more positive than those computed for the muscles in the ventral aspect. The distributions of respiratory effect showed even stronger gradients in the rostrocaudal direction. As the fibres of the external or internal intercostal muscle in a given interspace are parallel to each other, the forces they develop on the ribs are additive. Therefore, the Pao values generated by the areas of external or internal intercostal muscle in the dorsal third, the middle third, and the ventral third of a given segment must be additive as well, and the results of such additions in the different interspaces are shown in Fig. 5. Whereas the total Pao for the external intercostal in the second interspace amounts to -1·24 cmH2O, the total Pao for the external intercostal in the sixth interspace is only -0·11 cmH2O and that for the external intercostal in the tenth interspace amounts to +1·17 cmH2O. In contrast, the total Pao for the internal intercostal decreases gradually from +3·31 cmH2O in the tenth interspace to -0·11 cmH2O in the second interspace.
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These data were obtained by adding the changes in airway pressure ( | ||
The only data with which the computed Pao values can be compared are those of Ninane et al. (1991). These investigators measured the changes in pleural pressure (Ppl) during electrical stimulation of the canine external and internal interosseous intercostals in the third and seventh intercostal spaces. When the two muscles in the third interspace were simultaneously activated, Ppl fell by 0·5 to 1·9 cmH2O. If one assumes that the pressure changes produced by the external and internal intercostals in a given interspace are additive, our computed values for the second and fourth interspaces (Fig. 5) are -1·35 and -0·50 cmH2O, respectively. These values agree reasonably well with the measured values. However, when Ninane et al. (1991) stimulated the external and internal intercostals in the seventh interspace, they recorded no change in Ppl, whereas our computed Pao values for the sixth and eighth interspaces amount to +0·99 and +3·43 cmH2O, respectively.
Two explanations may account for this difference. First, Ninane et al. (1991) stimulated the external and internal intercostal muscles by inserting copper thread wires between the muscle layers from the costochondral junctions to the angles of the ribs. We have confirmed that this procedure indeed elicits forceful contraction of the two muscles (Legrand et al. 1998). However, the levator costae muscle in the same interspace inserts just into the angle of the rib and lies, therefore, in the immediate vicinity of the tip of the stimulating electrodes. As this muscle has a definite inspiratory mechanical advantage, its activation would obscure or at least reduce the expiratory effect of the external and internal intercostals in the caudal segments. Second, Ninane et al. (1991) performed their muscle stimulation at FRC, whereas our computed values of Pao are the maximal changes in Pao that the muscles can generate. These changes, therefore, are those produced by the muscles when maximally activated at optimal length (Lo), and our recent studies on the canine triangularis sterni have clearly illustrated the critical importance of muscle length on Pao (De Troyer & Legrand, 1998). Specifically, when this muscle in a single interspace was bilaterally stimulated at FRC, Pao averaged +0·80 cmH2O. However, when the lung was inflated by 1·0 l above FRC before stimulation such that the muscle then was in the vicinity of Lo, Pao amounted to +1·75 cmH2O. In view of the substantial lengthening of the external and internal intercostals in the caudal segments during passive inflation (Fig. 1), it is most likely that the lung (chest wall) volume corresponding to the optimal length of these muscles is also well above FRC. Consequently, their force-generating ability at FRC should be less than maximum. Ninane et al. (1991) did not stimulate the muscles after maximal inflation, yet they observed that the Ppl produced by the external and internal intercostals in the seventh interspace increased from 0 to +0·4 cmH2O when lung volume was moderately increased above FRC by a positive pressure of 10-15 cmH2O at the airway opening. Irrespective of the possible co-activation of the adjacent levator costae muscle, this result supports the current conclusion that the intercostal muscles in the caudal segments, when activated at appropriate lung volumes, do have an expiratory effect on the lung.
The main implication of the present studies, therefore, is that the actions of the external and internal intercostals on the lung during breathing are largely determined by the topographic distribution of neural drive among the muscles. A number of electrical recordings from intercostal muscles and nerves in anaesthetized cats (Sears, 1964; Bainton et al. 1978; Kirkwood et al. 1982, 1984; Greer & Martin, 1990) and dogs (De Troyer & Ninane, 1986) have shown that the external intercostals are active during inspiration. The muscles also appeared to display greater inspiratory activity in the dorsal than in the ventral portion of the rib cage and greater inspiratory activity in the rostral than in the caudal segments. In contrast, the internal intercostals were active during expiration and displayed greater activity in the caudal than the rostral segments (Bainton et al. 1978; De Troyer & Ninane, 1986; Greer & Martin, 1990). In view of the topographic distributions of mechanical advantage among the muscles, such distributions of activity suggest that the external intercostals have an inspiratory action on the lung during breathing and that the internal intercostals have an expiratory action. On the other hand, efferent discharges to the internal intercostal in the second interspace during inspiration have also been recorded during strenuous respiratory efforts in decerebrate cats (Le Bars & Duron, 1984), in agreement with the inspiratory mechanical advantage of this muscle area. Efferent discharges to the external intercostals in the caudal segments during expiration have been recorded also (Le Bars & Duron, 1984), in agreement with their expiratory mechanical advantage, and this raises the possibility that the external and internal intercostals may not have distinct inspiratory and expiratory effects during breathing. This issue has prompted us to re-examine precisely the spatial distribution of activity among these muscles in a variety of respiratory manoeuvers; the results of these studies are reported in the accompanying communication (Legrand & De Troyer, 1999).
Another implication of the current studies is that the linkage between the ribs and the lung is not uniform. Indeed, we have previously shown in dogs that when the external or the internal interosseous intercostal muscle in a single rostral or caudal segment is selectively activated at FRC, the cranial displacement of the rib below is always greater than the caudal displacement of the rib above (De Troyer et al. 1983). Similar rib displacements were found by Ninane et al. (1991) during stimulation of the external and internal intercostals in the third and seventh interspaces, yet the external and internal intercostals in the caudal segments do not have any inspiratory effect on the lung . As shown in Figs 3 and 5, the effect of these muscles on the lung can only be expiratory. This apparent paradox suggests that in the caudal interspaces, a given displacement of the rib below has a much smaller effect on lung volume than the same displacement of the rib above.
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REFERENCES |
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| Bainton, C. R., Kirkwood, P. A. & Sears, T. A. (1978). On the transmission of the stimulating effects of carbon dioxide to the muscles of respiration. The Journal of Physiology 280, 249-272 | [Abstract] |
| Close, R. I. (1972). Dynamic properties of mammalian skeletal muscles. Physiological Reviews 52, 129-197 | [Medline] |
| De Troyer, A. & Kelly, S. (1982). Chest wall mechanics in dogs with acute diaphragm paralysis. Journal of Applied Physiology 53, 373-379 | [Medline] |
| De Troyer, A., Kelly, S. & Zin, W. A. (1983). Mechanical action of the intercostal muscles on the ribs. Science 220, 87-88 | [Medline] |
| De Troyer, A. & Legrand, A. (1995). Inhomogeneous activation of the parasternal intercostals during breathing. Journal of Applied Physiology 79, 55-62 | [Medline] |
| De Troyer, A. & Legrand, A. (1998). Mechanical advantage of the canine triangularis sterni. Journal of Applied Physiology 84, 562-568 | [Abstract/Full Text] |
| De Troyer, A., Legrand, A. & Wilson, T. A. (1996). Rostrocaudal gradient of mechanical advantage in the parasternal intercostal muscles of the dog. The Journal of Physiology 495, 239-246 | [Abstract] |
| De Troyer, A. & Ninane, V. (1986). Respiratory function of intercostal muscles in supine dog: an electromyographic study. Journal of Applied Physiology 60, 1692-1699 | [Medline] |
| Farkas, G. A. (1991). Mechanical properties of respiratory muscles in primates. Respiration Physiology 86, 41-50 | [Medline] |
| Farkas, G. A., Decramer, M., Rochester, D. F. & De Troyer, A. (1985). Contractile properties of intercostal muscles and their functional significance. Journal of Applied Physiology 59, 528-535 | [Medline] |
| Greer, J. J. & Martin, T. P. (1990). Distribution of muscle fiber types and EMG activity in cat intercostal muscle. Journal of Applied Physiology 69, 1208-1211 | [Medline] |
| Hamberger, G. E. (1749). De Respirationis Mechanismo et usu Genuino. Iena. | |
| Kirkwood, P. A., Sears, T. A., Stagg, D. & Westgaard, R. H. (1982). The spatial distribution of synchronisation of intercostal motoneurones in the cat. The Journal of Physiology 327, 137-155 | [Medline] |
| Kirkwood, P. A., Sears, T. A. & Westgaard, R. H. (1984). Restoration of function in external intercostal motoneurones of the cat following partial central deafferentation. The Journal of Physiology 350, 225-251 | [Abstract] |
| Le Bars, P. & Duron, B. (1984). Are the external and internal intercostal muscles synergist or antagonist in the cat? Neuroscience Letters 51, 383-386 | [Medline] |
| Legrand, A. & De Troyer, A. (1999). Spatial distribution of external and internal intercostal activity in dogs. The Journal of Physiology 518, 291-300. | [Abstract/Full Text] |
| Legrand, A., Ninane, V. & De Troyer, A. (1997). Mechanical advantage of sternomastoid and scalene muscles in dogs. Journal of Applied Physiology 82, 1517-1522 | [Abstract/Full Text] |
| Legrand, A., Wilson, T. A. & De Troyer, A. (1996). Mediolateral gradient of mechanical advantage in the canine parasternal intercostals. Journal of Applied Physiology 80, 2097-2101 | [Medline] |
| Legrand, A., Wilson, T. A. & De Troyer, A. (1998). Rib cage muscle interaction in airway pressure generation. Journal of Applied Physiology 85, 198-203 | [Abstract/Full Text] |
| Margulies, S. S., Rodarte, J. R. & Hoffman, E. A. (1989). Geometry and kinematics of dog ribs. Journal of Applied Physiology 67, 707-712 | [Medline] |
| Ninane, V., Gorini, M. & Estenne, M. (1991). Action of intercostal muscles on the lung in dogs. Journal of Applied Physiology 70, 2388-2394 | [Medline] |
| Sears, T. A. (1964). Efferent discharges in alpha and fusimotor fibres of intercostal nerves of the cat. The Journal of Physiology 174, 295-315. | |
| Wilson, T. A. & De Troyer, A. (1992). Effect of respiratory muscle tension on lung volume. Journal of Applied Physiology 73, 2283-2288 | [Medline] |
| Wilson, T. A. & De Troyer, A. (1993). Respiratory effect of the intercostal muscles in the dog. Journal of Applied Physiology 75, 2636-2645 | [Medline] |
This study was supported in part by a grant (HL - 45545) from the National Institutes of Health (USA).
Corresponding author
A. De Troyer: Chest Service, Erasme University Hospital, Route de Lennik, 808, 1070 Brussels, Belgium.
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A. De Troyer, M. Cappello, N. Meurant, and P. Scillia Synergism between the canine left and right hemidiaphragms J Appl Physiol, May 1, 2003; 94(5): 1757 - 1765. [Abstract] [Full Text] [PDF]
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M. Cappello and A. De Troyer On the respiratory function of the ribs J Appl Physiol, April 1, 2002; 92(4): 1642 - 1646. [Abstract] [Full Text] [PDF]
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 W. C. Lancaster and J. R. Speakman Variations in respiratory muscle activity during echolocation when stationary in three species of bat (Microchiroptera: Vespertilionidae) J. Exp. Biol., March 14, 2002; 204(24): 4185 - 4197. [Abstract] [Full Text] [PDF]
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 D. LEDUC, E. BRUNKO, and A. DE TROYER Response of the Canine Internal Intercostal Muscles to Chest Wall Vibration Am. J. Respir. Crit. Care Med., January 1, 2001; 163(1): 49 - 54. [Abstract] [Full Text]
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A. De Troyer and T. A. Wilson Coupling between the ribs and the lung in dogs J. Physiol., April 1, 2002; 540(1): 231 - 236. [Abstract] [Full Text] [PDF]
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) around the rib. As shown in B, the external intercostals (continuous line) would therefore shorten most in the dorsal part of the rib cage (