73

Journal of Physiology (1991), 439, pp. 73-88 With 10 figures Printed in Great Britain

DIFFERENTIAL CONTROL OF THE INSPIRATORY INTERCOSTAL MUSCLES DURING AIRWAY OCCLUSION IN THE DOG

BY ANDRE DE TROYER From the Respiratory Research Unit and Chest Service, Erasme University Hospital, Brussels School of Medicine, Brussels 1070, Belgium and the Thoracic Diseases Research Unit, Mayo Clinic, Rochester, MN 55905, USA

(Received 30 August 1990) SUMMARY

1. The effect of airway occlusion on the electrical activity of the three groups of inspiratory intercostal muscles (external intercostal, levator costae, parasternal intercostal) situated in the cranial portion of the rib-cage has been studied in thirty anaesthetized, spontaneously breathing dogs. 2. The three muscles were active during normal inspiration, and their activity was prolonged similarly during airway occlusion. However, a comparison of activity during occluded and unoccluded inspirations indicated that airway occlusion caused a facilitation of external intercostal and levator costae activities but an inhibition of parasternal intercostal activity. 3. The facilitation of external intercostal and levator costae activities was markedly reduced after section of the phrenic nerves and completely suppressed after section of the appropriate thoracic dorsal roots. 4. The inhibition of parasternal intercostal activity was not affected by section of the phrenic nerves or by section of the thoracic dorsal roots. This phenomenon, however, was abolished after bilateral cervical vagotomy. 5. Activation of the external intercostals and levator costae during inspiratory efforts are thus highly dependent on segmental reflexes arising in these muscles.. In contrast, activation of the parasternal intercostals resembles that of the diaphragm in the sense that it depends primarily on the central respiratory drive. INTRODUCTION

Elevation of the ribs and expansion of the rib-cage are prominent features of the inspiratory phase of the breathing cycle. In man the scalene muscles play an important role in causing this phenomenon (De Troyer & Estenne, 1984; Estenne & De Troyer, 1985), but in anaesthetized dogs these muscles in general are not active during resting breathing (Robertson, Foster & Johnson, 1977; De Troyer & Kelly, 1982; De Troyer & Farkas, 1989a). A number of electromyographic studies in anaesthetized cats (Sears, 1964a; Hilaire, Nicholls & Sears, 1983) and dogs (De Troyer & Ninane, 1986; De Troyer & Farkas, 1989b) have shown that inspiration MS 8764

74

A. DE TR 0 YER

in these animals is associated with activation of three groups of intercostal muscles,

namely the internal intercostals of the parasternal region (the parasternal intercostals), the external intercostals of the cranial interspaces and the levator costae. Additional measurements have also shown that when any one of these three muscles is selectively activated by electrical stimulation, it produces elevation of the ribs into which it inserts (De Troyer & Kelly, 1982; De Troyer & Farkas, 1989b). In anaesthetized dogs these three groups of intercostal muscles thus play a major role in the act of breathing: they overcome the expiratory effect of pleural pressure on the rib-cage and elevate the ribs. Changes in inspiratory mechanical load are well known to alter the timing and magnitude of motor discharges to the inspiratory intercostal muscles. In anaesthetized animals, when the inspiratory airflow resistance is suddenly increased or when the airway is occluded at end-expiration for a single breath, the duration of inspiration increases, and the peak electrical activity of the inspiratory intercostals

increases as well. The mechanism of this increase in intercostal muscle activity, however, remains controversial. Corda, Eklund & von Euler (1965) have reported that the rate of rise of intercostal muscle activity in cats increases during occluded inspiration. These investigators have further shown that this increased rate of rise was associated with an increased afferent discharge from the intercostal muscle spindles and was abolished after section of the appropriate dorsal roots. Similar observations were subsequently reported by Sant'Ambrogio & Widdicombe (1965) in rabbits and by Shannon & Zechman (1972) in cats. Hence, it would appear that the response of the inspiratory intercostal muscles to increased mechanical loads is highly dependent on stretch reflexes arising in these muscles. However, an increase in the rate of rise of intercostal muscle activity during loaded breathing was observed only occasionally by Bradley (1972) in cats and by D'Angelo (1982) in rabbits, and this increase, when present, was of small magnitude. In general, the increases in peak electrical activity during loading resulted only from the increase in inspiratory time, thus suggesting that segmental mechanisms have little influence on the response of the inspiratory intercostals to loading. We speculated that part of this controversy

might be related to a difference in the

intercostal muscles investigated. Corda et al. (1965), Sant'Ambrogio & Widdicombe (1965) and Shannon & Zechman (1972) made all their electrical recordings from external intercostal muscles. In contrast, Bradley (1972) and D'Angelo (1982) recorded from both parasternal and external intercostals. In these two reports, no distinction was made between the two muscle groups, presumably because these traditionally been considered to have a similar function and similar control hiave mechanisms. Histological studies in cats, however, have established that there is a marked structural difference between the external intercostals and the parasternal intercostals; the former contain large numbers of muscle spindles, whereas the latter are poorly supplied with muscle spindles (Duron, Jung-Caillol & Marlot, 1978). Measurements of the changes in intercostal muscle length in dogs have also suggested that the external intercostals, but not the parasternal intercostals, may be a primary determinant of postural movements such as trunk rotation (Decramer, Kelly & De Troyer, 1986). Consequently, the different inspiratory intercostals might respond differently to loading. The purpose of the present studies was to test this hypothesis.

DIFFERENTIAL CONTROL OF INSPIRATORY INTERCOSTALS 75 The responses of the external intercostals, levator costae and parasternal intercostals to airway occlusion were thus elevated separately, and the mechanisms of these responses were assessed. METHODS

The experiments were carried out on thirty-two adult mongrel dogs (17-28 kg) anaesthetized with sodium pentobarbitone (initial dose: 20 mg/kg i.v.). The animals were placed in the supine posture and intubated with a cuffed endotracheal tube, and a venous cannula was inserted in the forelimb to give maintenance doses of anaesthetic. The animals appeared to remain at a constant, reasonable depth of anaesthesia throughout the experiments; they had no spontaneous movement other than those involved in the act of breathing and did not respond to painful stimuli. However, because anaesthesia might have affected the phenomenon herein studied, great care was taken to keep the animals lightly anaesthetized; the conjunctival and corneal reflexes were kept present throughout the measurements; and end-tidal Pco2 was maintained within reasonable limits. Rectal temperature also was maintained constant at 37 0+0 5 °C with a heating pad or infra-red lamps. In each animal we recorded the electromyograms of the external intercostal, levator costae and parasternal intercostal muscles situated in the third or fourth right interspace. The surgical approach was the same as the one used in previous investigations (De Troyer & Farkas, 1989b, 1990). The rib-cage was thus exposed on the right side of the chest from the first to seventh rib through a mid-line incision over the sternum and deflection of the pectoralis and transversus costarum muscles. The serratus ventralis, serratus dorsalis cranialis and iliocostalis muscles were then detached from the ribs into which they insert, and a pair of silver hook electrodes spaced 3-4 mm apart was implanted, under direct vision, in the levator costae of the interspace selected for investigation. The medial and dorsal bundles of the scalenes were also deflected, and similar pairs of hook electrodes were implanted in the external intercostal, midway between the angle of the rib dorsally and the costochondral junction ventrally, and in the parasternal intercostal 1-2 cm lateral to the sternum. In nineteen animals, a mid-line incision of the neck was also performed to isolate bilaterally either the C5, C6 and C7 phrenic nerve roots (nine animals) or the vagi (ten animals); loose ligatures were placed around the isolated nerves so that these could be identified easily later. The three electromyographic (EMG) signals were processed using amplifiers (Medelec PA63, Surrey, UK), band-pass filtered below 80 and above 1600 Hz, and rectified prior to their passage through leaky integrators with a time constant of 01 or 0-2 s. In addition, we measured tidal volume by electronic integration of the flow signal derived from a heated Fleisch pneumotachograph and a Celesco differential pressure transducer, airway pressure with an additional differential pressure transducer (± 100 cmH2O) connected to the side port of the endotracheal tube, and end-tidal Pco2 with an infra-red C02 analyser (Beckman, model LB-2). All signals were recorded on an eight-channel hot-pen recorder (Graphtec WR 3101; Ankersmit, Brussels, Belgium). Each animal was allowed to recover for 30 min after instrumentation, after which baseline measurements of EMG activity, volume and end-tidal Pc. were made. The animal was spontaneously breathing room air throughout. Every twenty to twenty-five breaths, however, the inspiratory line of a Hans-Rudolph valve attached to the tracheal tube and the pneumotachograph was blocked during the expiratory pause for a single inspiratory effort. At least ten occluded breaths were obtained in each animal. Three experiments protocols were then followed. Experiment 1. In nine animals, a complete paralysis of the diaphragm was induced by sectioning the three phrenic nerve roots on both sides of the neck. Ten to fifteen minutes were allowed to elapse for recovery before the measurements were repeated. As in the control condition, at least ten occluded breaths were obtained after section of the phrenic nerve roots. Experiment 2. In thirteen animals, thoracic dorsal rhizotomies were performed so as to produce deafferentation of the intercostal space selected for investigation. In these animals, a catheter had been placed in the left femoral artery to monitor blood pressure and analyse arterial blood periodically for Po, Pc0 and pH (Radiometer model ABL-2, Copenhagen, Denmark); these variables were measured to ensure that the animal was in a stable general physiological condition throughout the experiment. So, after completion of the control measurements, the animal was moved from the supine to the

76

A. DE TROYER

prone position. The spine was approached by a mid-line incision of the skin and deflection of the muscles of the lower cervical and upper thoracic region, and the spinal cord was exposed, through a dorsal laminectomy, from the 1st to the 6th thoracic segments. Previous studied by Eccles, Sears & Shealy (1962) have shown that the a-motoneurones driving the inspiratory intercostal muscles in a given segment receive monosynaptic excitation not only from the same segment but also from the adjacent segments. Therefore, after the dura was opened, the dorsal roots corresponding to the intercostal space being studied and those corresponding to the spaces immediately above and below were sectioned bilaterally; depending on whether the intercostal muscles investigated were situated in the third or fourth interspace, the dorsal rhizotomy thus extended from T2 to T4 or from T3 to T5, respectively. The incision was then loosely closed and covered with warm mineral oil, and the animal was moved back to the supine position. The animal was allowed to recover from the trauma for at least 30 min, after which measurements of EMG activity during unimpeded and occluded breaths were repeated. Systemic blood pressure and arterial blood gases showed only minimal changes in eleven of the thirteen animals in which the rhizotomies were performed. This suggests that these animals remained in a stable physiological condition throughout the experiment. Two animals, however deteriorated markedly during the course of the surgical procedure, developing severe hypotension and metabolic acidosis; these two animals were not considered in the data analysis. Experiment 3. In the remaining ten animals, the phrenic nerves and the dorsal roots were left intact, but the cervical vagi were infiltrated with 2 % lidocaine and sectioned. Measurements of EMG activity during unimpeded breathing and during occluded breaths were repeated, here too, after a 10-15 min recovery period. Phasic inspiratory electrical activity in the external intercostal, levator costae and parasternal intercostal muscles during unimpeded breathing and during airway occlusion was quantified first by measuring the peak height of the integrated EMG signal in arbitrary units. To allow comparison between the different animals of the study, EMG activity during occluded breaths was then expressed as a percentage of the activity recorded during unimpeded breathing. Occluded inspiration, however, elicited different responses in the three muscles studied (see Results). Therefore, in each animal, the traces of integrated EMG signals during each occluded breath were subsequently superimposed on the traces obtained during the immediately preceding unoccluded breath (Younes, Iscoe & Milic-Emili, 1975; van Lunteren, Strohl, Parker, Bruce, van de Graaf & Cherniack, 1984; Strohl, 1985). As shown in Fig. 1, timing of events in the respiratory cycle was related to the onset of parasternal intercostal activity, and for each of the three muscles studied, the activity during the occluded breath was calculated at peak parasternal intercostal a6tivity and expressed as a fraction of the activity during the unoccluded breath. The amount of facilitation or inhibition during the occluded breath was thus defined as follows: (electrical activity during the occluded breath/electrical activity during the unoccluded breath) x 100 %. Superimposing the traces recorded during occluded and unoccluded breaths also allowed the onset of facilitation or inhibition of electrical activity during airway occlusion to be assessed. To allow comparison between the different animals, this onset was divided by the unoccluded inspiratory time (Tl), as measured from the start of the parasternal intercostal electromyogram until its peak activity. The response for each muscle in each animal was obtained by averaging the responses to all occluded breaths recorded in each condition of the study. Data were then averaged for the group of animals, and they are presented as means + s.E.M. Statistical analysis was performed using the Wilcoxon signed-rank test. The criterion for statistical significance was taken as P < 0 05.

RESULTS

Baseline data When breathing at rest, the thirty animals studied had phasic inspiratory activity in the external intercostal, levator costae and parasternal intercostal muscles. The three integrated signals had a similar pattern; that is, the activity in each muscle increased gradually in amplitude as inspiration proceeded, reached its peak at the

DIFFERENTIAL CONTROL OF INSPIRATORY INTERCOSTALS 77 end of inspiration, and then declined rather abruptly. These observations confirm previous recordings in cats (Sears, 1964 a; Hilaire, Nicholls & Sears, 1983) and in dogs (De Troyer & Ninane, 1986; De Troyer & Farkas, 1989 b). Tidal volume in the thirty animals averaged 336 + 22 ml, and end-tidal Pco, ranged between 3-14 and 4-56 kPa (3-89+O007 kPa). A

B

Volume Pressure

I a

b

.

Parasternal

External intercostal

.

Levator costae

is

Fig. 1. Traces of electrical activity (integrated signal) of the parasternal intercostal, external intercostal and levator costae muscles are shown for one animal breathing room air (end-tidal Pco = 3-95 kPa). A, traces obtained originally during an unoccluded and an occluded breath. Volume calibration, 250 ml; pressure calibration, 10 cmH2O. B, the traces of integrated signals during the occluded breath (continuous lines) are superimposed on the traces obtained during the unoccluded breath (dashed lines). Timing of events was related to the onset of parasternal activity (line a), and comparison of activity was made at peak parasternal activity (line b). The small arrow on each trace marks the point where the occluded and unoccluded trajectories start departing from each other.

Effects of airway occlusion The effects of occluding the airway for a single breath on the EMG activity of the external intercostal, levator costae and parasternal intercostal muscles are illustrated by the records of a representative animal in Fig. IA. In each animal, the three muscles studied reached their peaks at end-inspiration, and they all increased (P < 0-01) their peak activity relative to the preceding unoccluded breath. The increases in external intercostal and levator costae activity were similar. As shown in Fig. 2, however, these were both considerably greater (P < 001) than the increase in parasternal intercostal activity. This difference between the external intercostals and levator costae on the one hand and the parasternal intercostals on the other suggested that the shape of the integrated signals during airway occlusion had changed differently when compared to the unoccluded breaths. Changes observed in three animals are shown in Fig. 3,

A. DE TROYER

78

in which the occluded and unoccluded trajectories are superimposed. With airway occlusion, the integrated signals of the external intercostal and the levator costae showed a clear-cut facilitation; that is, at any comparable point in time, the amount of electrical activity recorded in these two muscles was substantially greater during 500

v6 400 0

300

200

(L100

El

LC

PS

Fig. 2. Effect of airway occlusion on peak external intercostal (El), levator costae (LC) and parasternal intercostal (PS) inspiratory electromyographic (EMG) activity. The peak activity during occluded breaths is expressed as a percentage of the peak activity during unoccluded breaths. Average data of thirty animals; bars show S.E.M.

the occluded breath than during the preceding unoccluded breath. Facilitation of external intercostal and levator costae activities during airway occlusion was seen in all animals (Fig. 4), and as shown in Fig. 3 it appeared in general early after the onset of the occluded breath. For the thirty animals studied, facilitation of external intercostal activity started at 27-9 + 3.7 % of Ti, and facilitation of levator costae activity started at 23-2 + 3-4 % of Ti. Facilitation was not seen in the case of the parasternal intercostals. With airway occlusion, the integrated signal of parasternal intercostal activity showed few changes in its pattern in the first part of the unoccluded trajection. At this point, however, comparison of the two trajectories showed inhibition of parasternal intercostal activity (Fig. 3). In general, the inhibition became apparent in the third or fourth quarter of inspiration (Fig. 3, left), but in eight animals it appeared consistently earlier, being already obvious in the second quarter of inspiration (Fig. 3, middle). Only three of the thirty animals studied did not have any inhibition of parasternal activity; in these three animals, the integrated signal of parasternal activity during the occluded breaths remained consistently superimposed on the one recorded during the unoccluded breaths until the very end of the breaths (Fig. 3, right). The inhibition of parasternal intercostal activity in the thirty animals studied

DIFFERENTIAL CONTROL OF INSPIRATOR Y INTERCOSTALS

I

Parasternal-

External intercostal .-

Levator costae 1 s

l

1 s

s

Fig. 3. Superimposed traces of electrical activity (integrated signal) of the parasternal intercostal, external intercostal and levator costae muscles during an unoccluded breath (dashed lines) and an occluded breath (continuous lines) are shown for three representative animals. The animal shown in the left panel corresponds to the most frequent pattern, with an early facilitation of external intercostal and levator costae activities during the occluded breath and a late inhibition of the parasternal intercostal activity. In some animals, however, the inhibition of parasternal intercostal activity started very early in the course of the breath (middle panel), and in three animals inhibition of parasternal intercostal activity was not observed (right panel). Whereas all animals had facilitation of external intercostal and levator costae activities, no animal had facilitation of parasternal intercostal activity. 250 T_-

200-

CD

150

>0

C.)

Fig. 4. Average values of external intercostal (El), levator costae (LC) and parasternal intercostal (PS) activity during occluded breaths at peak PS activity in thirty animals; bars show S.E.M. The activity during occluded breaths is expressed as a percentage of the peak activity during the unoccluded breaths.

79

A. DE TR OYER

80

thus started at 59-7 +3 4% of T1, and its magnitude was such that the amount of activity recorded during occluded breaths averaged 87-8 + 1-7 % of the peak activity during unoccluded breaths (Fig. 4).

Effects of phrenicotomy Sectioning the phrenic nerve roots bilaterally in the neck caused a reduction in tidal volume (before, 364 + 46 ml; after 267 + 27 ml; P < 0 01) and an increase in

Parasternal

.......

External intercostal

Levator costae

is

1s

Fig. 5. Traces of electrical activity (integrated signal) of the parasternal intercostal, external intercostal and levator costae muscles during an unoccluded breath (dashed lines) and an occluded breath (continuous lines) are shown for one animal before (left) and after (right) bilateral section of the phrenic nerve roots in the neck.

end-tidal Pco2 (before, 3-99+0412 kPa; after, 4-75+0-14 kPa; P < 001) in the nine animals studied. The procedure also elicited an increased inspiratory activation of the parasternal intercostal, external intercostal and levator costae during unimpeded breathing. These effects of phrenicotomy are similar to those previously described (Ninane, Farkas, Baer & De Troyer, 1989; De Troyer & Farkas, 1989b). Phrenicotomy also resulted in major alterations in the responses of the external intercostal and levator costae to airway occlusion. These alterations are illustrated by the tracings of a representative animal in Fig. 5, and the results obtained in the nine animals studied are summarized in Fig. 6. After phrenicotomy, the facilitation of external intercostal and levator costae activities during airway occlusion was reduced or abolished (P < 001 for both), such that the integrated signals during occluded breaths were virtually superimposed on those recorded during unoccluded breaths. In contrast, phrenicotomy did not affect the inhibition of parasternal intercostal activity. After phrenicotomy, the magnitude of the inhibition increased in three, remained similar in four, and diminished in two animals. Hence, for the animal group

DIFFERENTIAL CONTROL OF INSPIRATORY INTERCOSTALS

81

as a whole, the amount of parasternal intercostal activity recorded during occluded breaths before and after phrenicotomy averaged, respectively, 88-5 + 43 and 876 + 20 % of the peak activity during unoccluded breaths (not significant). The timing of the parasternal intercostal inhibition remained unchanged as well; for the

300 2500-

200

--11\

_- -' 10

---

0

El LC PS

El LC PS

Fig. 6. Average values of external intercostal (El), levator costae (LC) and parasternal intercostal (PS) activity during occluded breaths at peak PS activity before (left panel) and after (right panel) bilateral section of the phrenic nerve roots in nine animals; bars show S.E.M. The activity during occluded breaths is expressed as a percentage of the peak activity recorded during the unoccluded breaths.

nine animals, the inhibition started at 52-2 + 8 1 % T1 after phrenicotomy while it started at 573 + 75 % Til during control (not significant).

Effects of dorsal rhizotomy Although sectioning the dorsal roots T2-T4 or T3-T5 did not affect end-tidal Pco, (before, 3-73 + 011 kPa; after, 3-70 + 0-08 kPa, not significant), it produced a small but significant reduction in tidal volume (before, 375+36 ml; after, 317 + 23 ml; P < 0 05), and in five of eleven animals it caused abolition of electrical activity in the external intercostal and levator costae. In three of these five animals, external intercostal activity returned after periods ranging from 45 to 90 min; simultaneous recovery of levator costae activity occurred in two animals. In the other two animals, however, no recovery of external intercostal and levator costae activities was seen after intervals of up to 2 h following the dorsal root section. Such a disappearance of external intercostal activity after dorsal rhizotomy has previously been reported in rabbits (Sant'Ambrogio & Widdicombe, 1965) and in cats (Shannon & Zechman, 1972). In contrast, abolition of parasternal intercostal activity was never observed, even temporarily. The effects of dorsal rhizotomy on the responses of the external intercostal, levator costae and parasternal intercostal muscles to airway occlusion are illustrated by the tracings of a representative animal in Fig. 7, and the results obtained in the eleven animals studied are summarized in Fig. 8. After dorsal rhizotomy, the facilitation of

A. DE TROYER

82

II F

Parasternal---

External intercostal-

"-'..

7t

1

Levator

1

costae 1s

Fig. 7. Traces of electrical activity (integrated signal) of the parasternal intercostal, external intercostal and levator costae muscles during an unoccluded breath (dashed lines) and an occluded breath (continuous lines) are shown for one animal before (left) and after (right) section of the dorsal roots in the corresponding segment and in the segments situated immediately above and below it.

C)

en

_

-0

L) cu

C.:

-0

.El

LC PS

Fig. 8. Average values of external intercostal (EI), levator costae (LC) and parasternal intercostal (PS) activity during occluded breaths at peak PS activity before (left panel) and after (right panel) section of the appropriate thoracic dorsal roots in eleven animals; bars show S.E.M. The activity during occluded breaths is expressed as a percentage of the peak activity recorded during the unoccluded breaths. Note, however, that dorsal root section caused abolition of external intercostal activity in two animals and of levator costae activity in three animals; consequently, the values shown for these muscles in the right panel are average data of only nine and eight animals, respectively.

external intercostal and levator costae activities during occluded breaths was considerably reduced or abolished in all animals. In three of the eight animals that retained inspiratory activity in the two muscles, the facilitation of external

DIFFERENTIAL CONTROL OF INSPIRATOR Y INTERCOSTALS

83

intercostal and levator costae activities was even reversed into an inhibition. As a result, for the animal group as a whole, the amount of activity during occluded breaths was no longer different from that recorded during unoccluded breaths. On the other hand, dorsal rhizotomy had no consistent effect on the inhibition of parasternal intercostal activity. The amount of parasternal intercostal inhibition

Parasternal---

External

--

.- ---

intercostal

---

Levator costae 1 s

s ~~~~~~1

Fig. 9. Traces of electrical activity (integrated signal) of the parasternal intercostal, external intercostal and levator costae muscles during an unoccluded breath (dashed lines) and an occluded breath (continuous lines) are shown for one animal before (left) and after (right) bilateral cervical vagotomy.

decreased in three animals but remained unchanged in three others and increased in the remaining five animals. As a result, the amount of parasternal activity during occluded breaths in the eleven animals was still 86-8 + 2-6 % of the peak activity during unoccluded breaths (before, 866 + 26 %; not significant). Dorsal rhizotomy did not affect the onset of parasternal inhibition either; the inhibition in the anirual group started at 60-2 + 6-2 % Ti during control and at 60-4 + 6-5 % Til after rhizotomy (not significant).

Effects of vagotomy Bilateral cervical vagotomy caused an increase in tidal volume (before, 259 + 17 ml; after, 521 + 66 ml; P < 0-01) and a reduction in end-tidal Pc02 (before, 400+0-14 kPa; after, 3-77+0'13 kPa; P = 005) in the ten animals studied, but it did not affect the responses of the external intercostal and levator costae to airway occlusion. This is illustrated by the tracings of a representative animal in Fig. 9. After vagotomy, facilitation of external intercostal and levator costae activities persisted in all animals (Fig. 10), and the magnitude and timing of this facilitation were unchanged relative to the control condition. In contrast, vagotomy produced dramatic alterations in the response of the parasternal intercostals (Figs 9 and 10). In eight of the ten animals, the inhibition of

A. DE TR OYER

84

parasternal intercostal activity during airway occlusion was abolished after vagotomy, and in three of them the inhibition was even reversed into a definite, albeit small, facilitation. Hence, whereas during control the amount of parasternal intercostal activity during occluded breaths averaged 88-5+225% of the peak

300-

*~250200

0-

o..

1500

10

Ii

50 El LC PS

El LC PS

Fig. 10. Average values of external intercostal (EI), levator costae (LC) and parasternal intercostal (PS) activity at peak PS activity before (left panel) and after (right panel) bilateral cervical vagotomy in ten animals; bars show S.E.M. The activity during occluded breaths is expressed as a percentage of the peak activity recorded during the unoccluded breaths.

activity recorded during unoccluded breaths, it was 104-3 + 3-7 % of the peak activity after vagotomy (P < 0-01). DISCUSSION

The present studies have established that occluding the airway for a single breath affects differently the different inspiratory intercostals. Specifically, the response of the parasternal intercostals differs from that of the external intercostals and levator costae. The increases in the peak activity of the two latter muscles were considerably larger than the increase in peak parasternal activity. Furthermore, when the integrated electrical signals recorded during occluded breaths were superimposed on those obtained during unoccluded breaths, such that the influence of inspiratory duration was eliminated, it was apparent that occluded breaths elicited a facilitation of the external intercostals and levator costae but caused at the same time an inhibition of the parasternal intercostals. Facilitation of external intercostal activity during single-breath airway occlusion or during a step increase in inspiratory airflow resistance has previously been reported in cats (Corda et al. 1965; Shannon & Zechman, 1972) and in rabbits (Sant'Ambrogio & Widdicombe, 1965). This response of the external intercostals has been referred to as the 'load-compensating' reflex, and the present observations that it is abolished after section of the appropriate thoracic dorsal roots and persists after vagotomy are in perfect agreement with the idea that the muscle spindles are the primary determinant of this phenomenon. Thus, airway occlusion causes stretch

DIFFERENTIAL CONTROL OF INSPIRATORY INTERCOSTALS

85

reflexes in the external intercostal muscles of the cranial interspaces, resulting in excitatory postsynaptic potentials (EPSP) in the corresponding oc-motoneurones; these EPSPs then superimpose on the normal central respiratory drive potentials (Eccles et al. 1962; Sears, 1964b), such that the global, efferent a-motor activity to these muscles is increased. It is worth noting, however, that facilitation of external intercostal activity during occluded breaths was markedly reduced or abolished after phrenicotomy as well (Figs 5 and 6). This finding suggests that the excitatory effect of airway occlusion on the external intercostal muscle spindles is closely related to the expiratory action of the diaphragm on the cranial portion of the rib-cage. The levator costae muscles responded to airway occlusion in the same way as the external intercostals did. All animals exhibited a facilitation of levator costae activity, and, on average, the magnitude of this facilitation was similar to the magnitude of external intercostal facilitation. The responses of the levator costae to dorsal root section and to vagotomy further indicate that facilitation of the muscle's activity during occluded breaths is governed by a proprioceptive mechanism closely similar to the one causing the facilitation of external intercostal activity. This similarity was not unexpected, since the levator costae, like the external intercostals, contain a relatively large number of spindles (Hilaire et al. 1983). On the other hand, it is well established that the parasternal intercostals are poorly supplied with muscle spindles (Duron et al. 1978). In view of this paucity of the parasternal intercostals in muscle spindles, one would have predicted a smaller facilitation of activity in these muscles during occlusion. In the animals of the current study, however, the parasternal intercostals did not show any facilitation. Most of them, in fact, exhibited a clear-cut inhibition. Clearly, this inhibition could not be accounted for by the low density in muscle spindles. We initially postulated that the inhibition of parasternal intercostal activity during occluded breaths resulted from the activation of tendon organs. This hypothesis was based on two lines of evidence. First, it is well known that the intercostal muscles contain tendon organs (Critchlow & von Euler, 1963), and that these receptors, when triggered, cause inhibition of the corresponding a-motoneurones. Second, recent measurements of intramuscular pressure in the canine respiratory muscles have indicated that pressure, in the parasternal intercostals increases considerably during occluded breaths as compared to unimpeded breathing (Leenaerts & Decramer, 1990). Hence, airway occlusion might well trigger tendon organs in the parasternal intercostals, the inhibitory effect of which could outweigh the excitatory effect of the few spindles present in these muscles. To test this hypothesis, we first sectioned the phrenic nerve roots. In this condition, the diaphragm is paralysed, such that the inspiratory intercostal muscles are responsible for the entire inspiratory effort. Therefore, the tension developed by the contracting parasternal intercostals presumably increases, and this should result in greater activation of the tendon organs in these muscles. As a result, one would expect that after section of the phrenic nerves the inhibition of parasternal intercostal activity during occluded breaths would be accentuated. As shown in Figs 5 and 6, this was not the case. After section of the phrenic nerves, the inhibition of parasternal intercostal activity remained unchanged. To further test the role played by the tendon organs, we next produced

86

A. DE TROYER

deafferentation of the parasternal intercostal investigated. If the inhibition of parasternal intercostal activity during occluded breaths resulted from the excitation of tendon organs, it should be abolished, together with the facilitation of external intercostal and levator costae activities, after section of the appropriate thoracic dorsal roots. After section of the dorsal roots, however, the inhibition of parasternal intercostal activity during occluded breaths was unchanged with respect to both its magnitude and its timing. These observations thus lead to the conclusion that tendon organs are not involved in producing the observed inhibition of parasternal intercostal activity. Volume-related vagal afferents from the lungs modulate the motor discharges to the diaphragm, and removing these influences by blocking the vagal nerves or by occluding the airway for a single breath in anaesthetized animals has recently been reported to produce inhibition of diaphragmatic activity. Such a vagal facilitatory effect of phasic volume feedback on the diaphragm has been shown in the cat (Bartoli, Cross, Guz, Huszczuk & Jefferies, 1975; Di Marco, von Euler, Romaniuk & Yamamoto, 1981) and in the dog (van Lunteren et al. 1984; Strohl, 1985). To the extent that the inhibition of parasternal intercostal activity during airway occlusion was known not to be caused by tendon organs, we thus speculated that it was related to the removal of vagal afferent inputs. We thus tested the effect of vagotomy on this inhibition, and indeed, after bilateral cervical vagotomy, the phenomenon was no longer observed. In some animals, the inhibition was even reversed into a small facilitation. It is concluded, therefore, that the inhibition of parasternal intercostal activity during occluded breaths is due to a vagal mechanism. Thus, whereas the response of the cranial external intercostals and levator costae to airway occlusion is closely related to segmental reflexes, the response of the parasternal intercostals is governed primarily by a supraspinal mechanism similar to the one modulating the response of the diaphragm. The parasternal intercostals clearly differ from the diaphragm with respect to their anatomical insertions and their respiratory function. These two muscle groups, however, have a common structural characteristic in the sense that they are both poorly supplied with muscle spindles. In addition, the diaphragm is the main muscle of respiration in mammals, and likewise the parasternal intercostals in the dog are the main determinant of the inspiratory elevation of the ribs (De Troyer, Farkas & Ninane, 1988; De Troyer, & Farkas, 1990); when the canine parasternal intercostals are selectively denervated, the rhythmic cranial motion of the ribs is considerably reduced despite the compensatory increased inspiratory activation of the external intercostals and levator costae (De Troyer, 1991). In the dog, the parasternal intercostals and the diaphragm are thus the predominant force generators in the act of breathing, and it is remarkable that their rhythmic activation and their response to airway occlusion is governed by similar control mechanisms. I am very grateful for the assistance of S. Blecic and G. A. Farkas in some of the experiments reported in this study. The work was supported in part by the National Institutes of Health (HL 21584) and the Fonds National de la Recherche Scientifique (Belgium).

DIFFEREiNTIAL CONTROL OF INSPIRATORY INTERCOSTALS

87

REFERENCES

BARTOIL1 A., CROSS, B. A., Guz, A., HUSZCZUK, A. & JEFFERIES, R. (1975). The effect of varying tidal volume on the associated phrenic motoneurone output: studies of vagal and chemical feedback. Respiration Physiology 25, 135-155. BRADLEY, G. W. (1972). The response of the respiratory system to elastic loading in cats. Respiration Physiology 16, 142-160. CORDA, M., EKLUND, G. & VON EULER, C. (1965). External intercostal and phrenic motor responses to changes in respiratory load. Acta Physiologica Scandinavica 63, 391-400. CRITCHLOW, V. & VON EULER, C. (1963). Intercostal muscle spindle activity and its y motor control. Journal of Physiology 168, 820-847. D'ANGELO, E. (1982). Inspiratory muscle activity during rebreathing in intact and vagotomized rabbits. Respiration Physiology 47, 193-218. DECRAMER, M., KELLY, S. & DE TROYER, A. (1986). Respiratory and postural changes in intercostal muscle length in supine dogs. Journal of Applied Physiology 60, 1686-1691. DE TROYER, A. (1991). The inspiratory elevation of the ribs in the dog: primary role of the parasternals. Journal of Applied Physiology (in the Press). DE TROYER, A. & ESTENNE, M. (1984). Coordination between rib cage muscles and diaphragm during quiet breathing in humans. Journal of Applied Physiology 57, 899-906. DE TROYER, A. & FARKAS, G. A. (1989a). Passive shortening of canine parasternal intercostals during breathing. Journal of Applied Physiology 66, 1414-1420. DE TROYER, A. & FARKAS, G. A. (1989b). Inspiratory function of the levator costae and external intercostal muscles in the dog. Journal of Applied Physiology 67, 2614-2621. DE TROYER, A,. & FARKAS, G. A.(1990). Linkage between parasternals and external intercostals during resting breathing. Journal of Applied Physiology 69, 509-516. DE TROYER, A., FARKAS, G. A. & NINANE, V. (1988). Mechanics of the parasternal intercostals during occluded breaths in dogs. Journal of Applied Physiology 64, 1546-1553. DE TROYER, A. & KELLY, S. (1982). Chest wall mechanics in dogs with acute diaphragm paralysis. Journal of Applied Physiology 53, 373-379. DE TROYER, A. & NINANE, V. (1986). Respiratory function of intercostal muscles in supine dog: an electromyographic study. Journal of Applied Physiology 60, 1692-1699. Di MARCO, A. F., VON EULER, C., ROMANIUK, J. R. & YAMAMOTO, Y. (1981). Positive feedback facilitation of external intercostal and phrenic inspiratory activity by pulmonary stretch receptors. Acta Physiologica Scandinavica 113, 375-386. DURON, B., JUNG-CAILLOL, M. C. & MARLOT, D. (1978). Myelinated nerve fiber supply and muscle spindles in the respiratory muscles of cat: quantitative study. Anatomy and Embryology 152, 171-192. ECCLES, R. M., SEARS, T. A. & SHEALY, C. N. (1962). Intracellular recording from respiratory motoneurones of the thoracic spinal cord of the cat. Nature 193, 844-846. ESTENNE, M. & DE TROYER, A. (1985). Relationship between respiratory muscle electromyogram and rib cage motion in tetraplegia. American Review of Respiratory Disease 132, 53-59. HILAIRE, G. G., NICHOLLS, J. G. & SEARS, T. A. (1983). Central and proprioceptive influences on the activity of levator costae motoneurones in the cat. Journal of Physiology 342, 527-548. LEENAERTS, P. & DECRAMER, M. (1990). Respiratory changes in parasternal intercostal intramuscular pressure. Journal of Applied Physiology 68, 868-875. NINANE, V., FARKAS, G. A., BAER, R. E. & DE TROYER, A. (1989). Mechanism of rib cage inspiratory muscle recruitment in diaphragmatic paralysis. American Review of Respiratory Disease 139, 146-149. ROBERTSON, C. H. JR, FOSTER, G. H. & JOHNSON, R. L. JR (1977). The relationship of respiratory failure to the oxygen consumption of, lactate production by, and distribution of blood flow among respiratory muscles during increasing inspiratory resistance. Journal of Clinical Investigation 59, 31-42. SANT'AMBROGIO, G. & WIDDICOMBE, J. G. (1965). Respiratory reflexes acting on the diaphragm and inspiratory intercostal muscles of the rabbit. Journal of Physiology 180, 766-779. SEARS, T. A. (1964 a). Efferent discharges in alpha and fusimotor fibres of intercostal nerves of the cat. Journal of Physiology 174, 295-315.

88

A. DE TROYER

SEARS, T. A. (1964b). The slow potentials of thoracic respiratory motoneurones and their relation to breathing. Journal of Physiology 175, 404-424. SHANNON, R. & ZECHMAN, F. W. (1972). The reflex and mechanical response of the respiratory muscles to an increased airflow resistance. Respiration Physiology 16, 51-69. STROHL, K. P. (1985). Respiratory activation of the facial nerve and alar muscles in anaesthetized dogs. Journal of Physiology 363, 351-362. VAN LUNTEREN, E., STROHL, K. P., PARKER, D. M., BRUCE, E. N., VAN DE GRAAF, W. B. & CHERNIACK, N. S. (1984). Phasic volume-related feedback on upper airway muscle activity. Journal of Applied Physiology 56, 730-736. YOUNES, M., ISCOE, S. & MILIC-EMILI, J. (1975). A method for the assessment of phasic vagal influence on tidal volume. Journal of Applied Physiology 38, 335-343.