Structure of canine Bachmann's bundle related to propagation of excitation

Impulse propagation and histology were studied in adult and neonatal canine Bachmann's bundle. Both showed non-uniform electrical anisotropy: effective longitudinal conduction velocity (θ(L)) markedly exceeded effective transverse conduction velocity (θ(T)), and extracellular potential waveforms with transverse propagation were polyphasic. An age difference in θ(L) (0.80 m/s neonate, 1.31 m/s adult) was found; it could be largely accounted for by a difference in myocyte diameter (4.7 μm neonate, 17.1 μm adult). Close apposition of myocytes in the neonate and development of transverse tubules in the adult may have influenced θ(L) at each stage. Perimysial septa separated fascicles in both neonatal and adult bundles; however, endomysial septa between individual myocytes were completely developed only in adult bundles. Thus perimysial septa were much more responsible for nonuniform anisotropy and low θ(T) than endomysial septa. Fascicle diameter and length were greater in the adult, which may have affected transverse propagation. Specialized conduction system cells could not be identified.