3-D CHARACTERIZATION OF GANGLION-CELLS OF THE TERMINAL NERVE PLEXUS OF RAT ATRIOVENTRICULAR JUNCTION

Three-dimensional (3-D) morphology of neurons of the terminal nerve pl exus of the atrioventricular junction was examined in a scanning elect ron microscope. Distributions of different cell types encountered as w ell as their relations to different structures of the atrioventricular specialized tissue were also studied. Most neurons were found dissemi nated in a thin connective tissue layer separating different segments of the atrioventricular conductive tissue from the interventricular se ptum. Sometimes, they formed small pluricellular ganglia (up to 5 neur ons) but, frequently, they occurred isolated in the terminal ramificat ions of the intramural nerve plexus of specialized tissue. Some intran odal neurons could also be identified. According to their 3-D morpholo gy, nerve cells of the perinodal ganglionated plexus could be divided into three categories: (1) Large unipolar neurons were scattered throu ghout the atrioventricular junction. Their long and thin axonal projec tions were often directed towards the interventricular septum. (2) Lar ge pseudounipolar or bipolar neurons were located at a few specific lo ci, namely all along the bundle of His and its bifurcation into the ri ght and left bundle branches. Frequently, they occurred solitary and i mmersed amongst strands of surrounding muscle cells. Only occasional s ynaptic impacts could be identified on the surface of neuronal bodies of these bipolar neurons. On the other hand, their dendritic varicosit ies were richly innervated. Due to their irregular shape, intimate ass ociation with muscular elements and their topographical superposition with occasional spindle-like structures, these nerve cells recall pros pective sensory neurons involved in integration of mechanical and neur al stimuli to the heart. (3) Small multipolar interneurons could be id entified in the retronodal ganglion and within right and left bundle b ranches. The present description of morphological heterogeneity of int ramural nerve cells agrees with recent morphological and functional cl assifications of autonomic neurons and supports the idea that, at the level of the atrioventricular junction, a self-governed neuronal netwo rk may be operating. (C) 1998 Elsevier Science B.V. All rights reserve d.