Development of the aortic baroreflex system under conditions of microgravity

The mechanism of autonomic regulation of the cardiovascular system develops postnatally like as various functions of the somatic system in the mammal including human. Normal development of the mechanisms is undoubtedly fundamental for maturation of individual. Therefore it is one of important problems to be urgently solved for the human life in the coming space era to understand the process of development of the regulation mechanism of the blood circulation in the growing animal in space. We have studied ontogenesis of the aortic baroreflex in the rabbit and rat to clarify the development process of the autonomic regulation of the cardiovascular system, and found the following facts. 1) Myelinated fibers of the aortic nerve which is the afferent of the reflex continue to increase in number, thickness of myelin and the diameter of axon after birth. 2) In the rabbit the increase of the number of myelinated fibers correlates lineally with the age related increase of the basal mean arterial pressure which is probably caused by in part increases of cardiac output accompanied with the growth of body. 3) Response of the integrated activity of the aortic nerve to increases in arterial pressure is smaller in the young rat and rabbit than in the mature animals. 4) Bradycardia response of the aortic baroreflex is less or slower in the young rabbit than in the mature animal. 5) The composition of the aortic nerve in both animals and bradycardia response in the rabbit become the mature type around eight weeks of age. On the basis of the background of these evidences we have investigated the effect of microgravity on the development of the aortic baroreflex system. The following is the outlines of our investigation.