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Laboratory of Cardio-Respiratory Physiology, Brussels School of Medicine, Belgium.

1. The effect of graded increases in inspiratory airflow resistance on the electrical activity and the mechanical behaviour of the three groups of inspiratory intercostal muscles (parasternal intercostal, external intercostal, levator costae) situated in the cranial portion of the rib-cage has been studied in ten anaesthetized, spontaneously breathing dogs. The mechanical behaviour of the muscles was determined by measuring the respiratory changes in muscle length and the displacements of the rib. 2. During unloaded inspiration, the three muscles were active, the rib moved in the cranial direction, and the parasternal intercostal and levator costae muscles shortened; in most animals, the external intercostals shortened as well. 3. Graded increases in inspiratory airflow resistance elicited a progressive inhibition of parasternal intercostal activity and a gradual facilitation of external intercostal and levator costae activities. Concomitantly, the parasternal intercostals continued to shorten during inspiration. However, both the external intercostals and the levator costae progressively lengthened, and the rib was gradually displaced in the caudal direction. This pattern persisted after increases in chemical respiratory drive had developed. 4. Sectioning the phrenic nerve roots did not alter the electrical or the mechanical response of the parasternal intercostal muscles to loading, but it markedly affected the response of the external intercostals and levator costae. After phrenicotomy, the external intercostals and levator costae continued to shorten during loaded breaths, the rib continued to be displaced in the cranial direction, and although the rate of inspiratory muscle shortening and of rib motion decreased, the facilitation of external intercostal and levator costae activities was markedly reduced or abolished. 5. Lengthening of the external intercostals and caudal displacement of the rib was reproduced by isolated stimulation of the phrenic nerves. 6. The reflex facilitation of external intercostal and levator costae activities that takes place during inspiratory resistive loading thus results primarily from the collapsing action of the diaphragm on the cranial portion of the rib-cage and the consequent lengthening of these muscles. The mechanical effectiveness of this reflex facilitation, however, appears to be relatively small.

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