Functional development of cardiac sympathetic nerves in newborn dogs: evidence for asymmetrical development

The functional development of cardiac sympathetic nerves was investigated in 52 puppies, 1 to 6 weeks of age. The effect of nerve stimulation on refractory period shortening at eight epicardial sites was used as criterion. Zero or minimal effect was observed at the first week of life for all the nerves tested, then a sharp increase of effect was observed at the second week, while at the third week, an unexpected decrease was observed for all nerves. At the fourth week, the effect increased again and remained high for the subsequent weeks. Thus functional sympathetic innervation of the heart is not complete at birth, but continues to develop throughout the first 6 weeks of life, following an irregular pattern of progression. The nonuniform maturation of cardiac nerves coupled with the localised distribution of the nerves may provide a basis for regional sympathetic imbalance and subsequent arrhythmiogenesis in early life.