Iguana Breathing

Rib kinematics & intercostal muscle strain during breathing in Iguana iguana

E.L. Brainerd, D.A. Ritter, M.M. Dawson, and A. Sullivan

Published abstract (PDF,74KB).

Marker-based and markerless X-ray Reconstruction of Moving Morphology (XROMM) were used to create precise and accurate animations of 3D rib motion in green iguanas during deep breathing. Muscle fascicle strain was measured by mapping the attachment points of external intercostal (EI) and internal intercostal (II) muscle fascicles onto the 3D rib models (Figure), and running the XROMM animations to measure the change in distance between the attachment points (Movie). Green iguanas have four cervical (Cv) ribs, four sternal (St) ribs that articulate with the sternum via long costal cartilages, and three xiphisternal ribs (Figure). XROMM analysis shows that the dorsal osseous ribs and ventral costal cartilages move as separate rigid elements, connected by thin cartilaginous regions that act as joints (Movie). XROMM animations reveal little movement of Cv1 and Cv2, increasing amounts of rotation in Cv3-St1, the greatest rotation in St2, and gradually decreasing rotation in St3-4 and more caudal ribs. In the intercostal space between the osseous portions of St1 and St2, the EI lengthen during exhalation and shorten during inhalation, whereas between St3 and St4, the EI shorten during exhalation and lengthen during inhalation. The costal cartilages show no craniocaudal rotation gradient, but they fold back during exhalation such that the parasternal II fibers lengthen during exhalation and shorten during inhalation (Movie). These results are consistent with the previously described EMG patterns of the EI and II in iguanas (Carrier, 1989), and are strikingly similar to recent consensus developing on the functions of EI and parasternal II in dogs (Carrier, 1996; De Troyer et al., 2005).

Pictures

Green iguana, Iguana iguana. Green iguanas have four cervical (Cv) ribs, four sternal (St) ribs that articulate with the sternum via long costal cartilages, and three xiphisternal ribs. XROMM analysis shows that the dorsal osseous ribs (gold) and ventral costal cartilages (green) move as separate rigid elements, connected by thin cartilaginous regions that act as joints. Still image from biplanar X-ray video of lung ventilation in a green iguana. Snout-vent length of this animal is approximately 50 cm. To measure intercostal muscle fascicle strain, a 3D digitizer is used to map the fascicles from an iguana cadaver onto the 3D models of the ribs. Then the XROMM animation is used to measure intercostal fascicle strain. External intercostal (red) and internal intercostal (blue) fascicles are shown here. Rib rotation gradient with cool colors (blue) indicating less motion during breathing and hot colors (red) more motion. XROMM animations reveal little movement of Cv1 and Cv2, increasing amounts of rotation in Cv3-St1, the greatest rotation in St2, and gradually decreasing rotation in St3-4 and more caudal ribs. In the intercostal space between the osseous portions of St1 and St2, the EI lengthen during exhalation and shorten during inhalation, whereas between St3 and St4, the EI shorten during exhalation and lengthen during inhalation.

Movies

Iguana Breathing: Biplanar x-ray videoIguana Breathing: XROMM animationIguana Breathing: XROMM animation of sternal ribs 1 and 2Iguana Breathing: XROMM animation of costal cartilages 1 and 2

References

Carrier D.R. (1989). Ventilatory action of the hypaxial muscles of the lizard Iguana iguana: a function of slow muscle. Journal of Experimental Biology. 143: 435–457.

Carrier D.R. (1996). Function of the intercostal muscles in trotting dogs: ventilation or locomotion? Journal of Experimental Biology. 199: 1455–1465.

De Troyer A., Kirkwood P.A., and Wilson T.A. (2005). Respiratory action of the intercostal muscles. Physiological Reviews. 85: 717–756.