Anatomie et physiologie : recherches en cours
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The Influence of Gravitational Gradients on the American Alligator (Alligator mississippiensis)

Lucas Knoche, Bruce A. Young, Tatyana Kondrashova

The cardiovascular systems of terrestrial and aquatic snakes cannot accommodate gravitational gradients, while blood flow in arboreal snakes is not disrupted by tilting. Alligators have a very dynamic cardiovascular system, with multiple means of active regulation; however, the response of alligators to gravitational gradients has never been documented. Gravitational gradients were induced by rotating sub-adult specimens of the American alligator (Alligator mississippiensis) between 45° head-up to 45° head-down in 15° increments. Intracranial pressure was assayed by quantifying the diameter of the optic nerve sheath using ocular ultrasonography; each increment of head-up rotation produced a significant decrease in nerve sheath diameter, while head-down rotations resulted in corresponding significant increases in nerve sheath diameter. Vascular ultrasonography revealed a consistent pattern. Head-down rotation resulted in vasodilation of the carotid artery and jugular vein, and head-up rotation resulted in a decrease in the luminal area of these vessels. In contrast to these manifestations of orthostatic pressure gradients, instantaneous heart rate (determined by EKG) revealed no evidence for a barostatic reflex in A. mississippiensis. This is the first report from a non-mammalian vertebrate of how intracranial pressure varies under gravitational gradients. The alligator has a unique response to gravitational gradients, characterized by the lack of a barostatic reflex.