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 nonuniform electrical anisotropy: effective longitudinal conduction velocity (theta L) markedly exceeded effective transverse conduction velocity (theta T), and extracellular potential waveforms with transverse propagation were polyphasic. An age difference in theta 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 microns neonate, 17.1 microns adult). Close apposition of myocytes in the neonate and development of transverse tubules in the adult may have influenced theta 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 theta 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.