Abstract
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.
Highlights
The cranial fluids of vertebrate animals include the arterial blood, venous blood, and the cerebrospinal fluid
Measuring intracranial pressure using ocular ultrasonography originated for neurological emergency care [30,31,32] but has found application in veterinary medicine [33]
Studies employing both ocular sonography and direct recording of intracranial pressure have found a significant relationship between intracranial pressure and both the optic nerve and the optic nerve sheath [34,35]
Summary
The cranial fluids of vertebrate animals include the arterial blood, venous blood, and the cerebrospinal fluid. The dynamic balance between these three fluids is well known. Arterial blood pressure in the choroid plexus generates cerebrospinal fluid [1], and intracranial pressure leads to fluid loss from the cerebrospinal fluid to the cerebral venous plexus [2]. In humans and other mammals, the cranial balance among these three fluids can be altered by orthostatic pressures; head-down postures can elevate arterial blood pressure, venous blood pressure, and intracranial pressure and elevation of the head can reduce arterial perfusion, venous blood pressure, and intracranial pressure [3,4]. Orthostatic transitions can produce marked changes in the venous luminal area, best known in the “collapse” of the jugular veins during head elevation [5]. The same basic pattern of orthostatic response likely occurs in all terrestrial vertebrates [6]; relatively little experimental work has been done on non-mammalian vertebrates
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