The effect of graded increases in inspiratory airflow resistance and airway occlusion on the electrical activity and the mechanical behaviour of the levator costae and external intercostal muscles situated in the caudal interspaces (zone of apposition of the diaphragm to the rib-cage) has been studied in spontaneously breathing dogs. 2. The external intercostal and levator costae muscles in the cranial interspaces were invariably active during unloaded inspiration and showed progressive facilitation of activity with increases in inspiratory resistance. In contrast, whether in the supine or in the prone position, the levator costae muscles of the caudal interspaces did not show any facilitation of activity, and the caudal external intercostal muscles never showed any inspiratory electrical activity, including during airway occlusion. 3. With graded increases in inspiratory airflow resistance, the cranial external intercostals demonstrated a gradual inspiratory lengthening and the cranial ribs were progressively displaced in the caudal direction. The caudal ribs, however, were invariably displaced in the cranial direction. As a result, the caudal external intercostals showed a progressive inspiratory shortening. 4. Shortening of the caudal external intercostals and cranial displacement of the caudal ribs were reproduced by isolated stimulation of the phrenic nerves. Thus, as inspiratory resistance increases, contraction of the diaphragm causes unloading, rather than loading, of the spindles present in the caudal external intercostal muscles. 5. After the phrenic nerves were sectioned, however, the caudal external intercostals invariably lengthened a substantial amount during inspiration, but they still did not show any inspiratory electrical activity. Accentuating the inspiratory lengthening of these muscles by external rib fixation and increasing the chemical respiratory drive did not elicit any inspiratory electrical activity either. The alpha-motoneurones of the external intercostal muscles in the caudal interspaces thus have very small central respiratory drive potentials with respect to their critical firing threshold.
