The functional development of the adrenal-sympathetic nervous system in neonatal and adolescent swine

The development of the animal after birth is marked by complex changes. Certain periods are particularly important and have been defined as critical periods (Krecek, 1971). Two processes are important to the survival of the animals during these times. The process of adaptation involves the maturation of specific physiological systems according to the needs of the neonatal animal, and is dictated by the external environment. The process of tolerance renders the neonate relatively unresponsive to a stressor as compared to the adult and protects the animals from over-responding the depleting vital resources until it is physiologically mature. An example of tolerance is the resistance to anoxia of immature neonates. The newborn pig has a high rate of mortality. The number of pigs born 114 days after conception represent only about 55% of the original ova released. Besides having a large number of embryo losses during the gestation period, 13-25% of the live born piglets die before weaning as shown in Table I. The low viability of the neonatal piglets is undoubtedly related to their inability to maintain homeostasis during stress. The sympathetic division of the autonomic nervous system is a major coordinator of homeostasis. Therefore, a study of the development of the sympathetic nervous system could provide some clues to explain the high mortality rate of the newborn pigs. Since the coordinating ability of the sympathetic nervous system is not only dependent on the maturity of the system, but also on the development of the effector organs and their responsiveness to activation, it is logical to examine their relative development