Atrioventricular node innervation in ultrastructural three dimensions

Atrioventricular nodal tissue was obtained from a two‐week‐old mouse heart which was fixed with phosphate‐buffered osmium tetroxide, embedded in Epon 812, serially sectioned for both light and electron microscopy and stained with uranyl acetate. From study of electron micrographic montages of the first 115 serial sections of the most inferior portion of the atrioventricular node it was determined that the intercellular spaces of this portion of the node contain fibrocytes, bundles of collagen fibrils, capillaries and a plexus of unmyelinated nerve bundles which are insheathed by Schwann cells. Relatively large nerve bundles divide to form smaller ones, the vesiculated nerve processes of which make intimate contact with the sarcolemmae of nodal cells. These neuromuscular contiguities, on the basis of the physical interrelationships of their respective vesiculated nerve processes and nodal cells, exist in various forms. They may be categorized as follows: (1) individual vesiculated nerve processes which dwell in sarcolemma‐lined tunnels or in culs‐de‐sac inside the nodal cells; (2) individual vesiculated nerve processes which lie within shallow or deep grooves on the surfaces of nodal cells; (3) vesiculated nerve processes which remain part of a small bundle while dwelling in a groove on the surface of an individual nodal cell or in grooves on the surfaces of two or more nodal cells; and (4) small individual bundles of vesiculated nerve processes and their Schwann cell investments which reside within sarcolemma‐lined tunnels inside nodal cells. These neuromuscular contiguities are fully described and their significance is discussed in relation to the present status of the nature and organization of the postganglionic ramifications of the autonomic nervous system within the conduction system of the heart.