Functional morphology of the Crocodilian Diaphragm

Abstract

In the American alligator (Alligator mississippiensis) there is an intracoelomic septum, herein termed the diaphragm, which anatomically partitions the intracoelomic cavity. The diaphragm adheres to the capsule of the liver caudally and to the visceral pleura of the lung cranially; the ventrolateral portions of the diaphragm are invested with smooth muscle, the remainder is tendinous. Differential pressure transducers were implanted into the intrathoracic and intraperitoneal cavities; these allowed for the direct recording of intrathoracic (ITP) and intraperitoneal (IPP) pressures, and the determination of transdiaphragmatic pressure (TDP). Sub-adult specimens were anesthetized with isoflurane, then a mechanical ventilator delivered discrete pulses of Oxygen (and isoflurane) to the lungs. Each ventilatory pulse resulted in a pulse in ITP and in IPP; however, the IPP pulses had significantly lower amplitude, meaning that a TDP was established, and that the diaphragm was functionally capable of isolating the pleural and peritoneal cavities. Power spectral analysis of the ITP and IPP pressure traces revealed the same dominant frequency (the frequency of the ventilatory pulses) and similar harmonics which appear to be oscillations of the diaphragm. Crocodilians, including Alligator, have a highly mobile liver that can function in ventilation. The anesthetized alligators were tilted 30° head-up or head-down in order to displace the liver caudally or cranially. Head-up rotations caused a significant increase in IPP, and a significant decrease in ITP (which became negative). Head-down rotations caused a significant increase in ITP, and a significant decrease in IPP (which became negative). These rotations were maintained for a duration of 30s during which the ITP and IPP were always stable. During these rotations, the diaphragm maintained opposite pressures (positive or negative) in the pleural and peritoneal cavities, and established TDPs greater than have been reported for some mammals. Analysis of the IPP traces before and after rotation revealed shifts in the power spectral density in opposite directions depending on the direction of rotation; these shifts appear to reflect changes in the tensile state of the diaphragm. During these experiments the specimens would occasional make voluntary movements similar to a breath. Two types of “breaths” were seen. The first was interpreted as a contraction of the diaphragmaticus muscle, which displaces the liver caudally; these breaths had the same effect as the head-up rotations. The second type of breath was interpreted as constriction of the thoracic and abdominal body walls; this type of breath produced pronounced, long-duration, roughly parallel, increases in ITP and IPP. These experiments demonstrate that the coelomic cavity of Alligator mississippiensis is functionally separated by a dynamic diaphragm. The presence of smooth muscle within the diaphragm indicates the possibility of higher-order adjustment or tuning of the tensile state of the diaphragm. These experiments focused on the ability of the diaphragm to maintain separate pressures in the peritoneal and pleural cavities, they were not intended to document the ventilatory, or non-ventilatory, functional role(s) of the diaphragm.