WILLIAM F. FRIEDMAN 
745 
Figure 16. — Choloinergic fibers at the same magnification in the AV node (top) and interventricular septum (bot- 
tom) of the fetal lambs close to term (left) and adult sheep (right). No apparent age-related differences were 
observed. 
contain abundant catacholamine stores. More- 
over, at a stage in development when a reduc- 
tion exists in sympathetic nerves and in cardiac 
norepinephrine stores and the intraneuronal en- 
zymes concerned with norepinephrine biosyn- 
thesis and degradiation, there appear to be no 
age-related differences in parasympathetic in- 
nervation. 
The heart of the adult mammal is richly sup- 
plied with sympathetic nerves and the release of 
norepinephrine from the endings of these 
nerves provides one of the fundamental mecha- 
nisms for the modulation of cardiac contrac- 
tility.^^ Although there is evidence to suggest 
that regulation of the heart rate and arterial 
blood pressure in the newborn is under some 
autonomic control,^ * the extent of which sympa- 
thetic nerves contribute to the contractile state 
of the heart at this stage of development has 
not been previously defined. It has been shown 
that the ordinary activity of the adrenergic 
nervous system may have minimal effects on 
the normal heart, and that the intrinsic con- 
tractile state of the myocardium may not be 
influenced by alterations in endogenous catacho- 
lamine stores.^'^ The force of contraction of the 
heart may be stimulated profoundly by an in- 
crease in the number of impulses traversing the 
sympathetic nerves whenever an imbalance ex- 
ists between the cardiac output and the perfu- 
sion requirements of the peripheral tissues. 
When the latter occurs in the perinatal period, 
the interaction between a supersensitive my- 
ocardium and the adrenal release of catacho- 
