Early stages in the organization of cardiovascular control.

Our studies have revealed a number of unexpected characteristics of the immature cardiovascular system of the infant rat. Sympathetic and parasympathetic divisions of the autonomic system appear to have different timetables for the development of tonic and phasic activities. These different timetables of development result in unusual physiological organizations at particular stages in postnatal life. We have described some of the features of one of these: the infant rat's cardiac rate responses in the second week after birth include a high resting rate, decreased rates in response to several (but not all) forms of activation, and two forms of phasic activity, a bradycardia and a tachycardia, that are related to naturally occurring behavioral states and have not been described in adults. The nature of this age-specific autonomic organization was explored further and found to be embedded in the infant's relationship with its mother and in her role as a supplier of nutrient in particular. The mechanisms by which nutrient intake regulates autonomic cardiovascular control during this stage of life have been partially explored by analytic experiments and the results are described along with the possible adaptive value of this regulatory system.