Presence of synaptic and muscular spindle-like structures in the atrioventricular junction of the rat heart: an electron microscopic study

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 plexus of the atrioventricular junction was examined in a scanning electron microscope. Distributions of different cell types encountered as well as their relations to different structures of the atrioventricular specialized tissue were also studied. Most neurons were found disseminated in a thin connective tissue layer separating different segments of the atrioventricular conductive tissue from the interventricular septum. Sometimes, they formed small pluricellular ganglia (up to 5 neurons) but, frequently, they occurred isolated in the terminal ramifications of the intramural nerve plexus of specialized tissue. Some intranodal neurons could also be identified. According to their 3-D morphology, nerve cells of the perinodal ganglionated plexus could be divided into three categories: (1) Large unipolar neurons were scattered throughout the atrioventricular junction. Their long and thin axonal projections were often directed towards the interventricular septum. (2) Large pseudounipolar or bipolar neurons were located at a few specific loci, namely all along the bundle of His and its bifurcation into the right and left bundle branches. Frequently, they occurred solitary and immersed amongst strands of surrounding muscle cells. Only occasional synaptic impacts could be identified on the surface of neuronal bodies of these bipolar neurons. On the other hand, their dendritic varicosities were richly innervated. Due to their irregular shape, intimate association with muscular elements and their topographical superposition with occasional spindle-like structures, these nerve cells recall prospective sensory neurons involved in integration of mechanical and neural stimuli to the heart. (3) Small multipolar interneurons could be identified in the retronodal ganglion and within right and left bundle branches. The present description of morphological heterogeneity of intramural nerve cells agrees with recent morphological and functional classifications of autonomic neurons and supports the idea that, at the level of the atrioventricular junction, a self-governed neuronal network may be operating.

The sinoatrial ring bundle: a cardiac neural communication system?

The sinoatrial ring bundle (SARB), was originally described as a "whitish bundle of tissue which describes an almost complete loop around the two venae cavae and the coronary sinus" in the adult rabbit heart (Paes de Carvalho et al., 1959). The histologically and electrophysiologically differentiated structure, derived from the embryonic venous valves, was suitably placed for rapid conduction from sinoatrial (SA) to atrioventricular (AV) node, but no evidence was found for this role. Today, the function of the SARB remains obscure. Cholinesterase/silver staining reveals the neural pattern associated with the SARB and suggests a function. Throughout its extent, the SARB contains a bundle of parallel muscle fibers and accompanying long nerves. The nerves distribute to structures at either side of the loop: superolaterally to pectinate muscle and inferomedially to the region of the AV node. Along the curve of the right SARB, the nerves contribute to a dense neural plexus with nerves coiled around muscle. The plexus communicates with the nearby SA node and with the ganglia inferior to the node near the inferior vena cava. The morphological pattern of neural elements is suitably organized to suggest tension monitoring and internodal, neural communication.

Catecholamine-synthesizing enzymes and neuropeptides in rat heart epicardial ganglia; an immunohistochemical study

The subepicardial atrial ganglia of rat hearts were examined using immunohistochemical techniques and antibodies against the catecholamine-synthetic enzymes tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH), and the neuropeptides substance P (SP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP) and met-5-enkephalin (ENK). Some of the ganglion cells present in the ganglia exhibited DBH-like immunoreactivity (LI) and NPY-LI, whilst these cells never exhibited TH-, VIP-, CGRP-, SP- or ENK-LI. Groups of small cells exhibiting an intense TH-LI, corresponding to cells referred to as catecholamine-containing cells and sometimes small intensely fluorescent cells in the literature, were observed in the ganglia. A subpopulation of these cells exhibited immunoreactivity to one of the neuropeptides tested, namelyu SP. Only a few of the cells showing TH-LI displayed DBH-LI. Nerve fibres showing SP-, CGRP-, DBH- and TH-LI were present in the ganglia; some of these fibres being closely associated with the ganglion cells or with the cells showing TH-LI. The observation provide new information on the catecholamine-synthetic enzyme/neuropeptide expression of the ganglion and catecholamine-containing cells and of the associated nerve fibres of rat heart subepicardial ganglia.

Catecholaminergic and peptidergic nerve components of intramural ganglia in the rat heart. An immunohistochemical study

Immunohistochemical properties of the terminal nerve network in the rat heart were assessed by use of the elution-restaining method. The colocalization of the enzymes involved in catecholamine synthesis (tyrosine hydroxylase--TH. dopamine-beta-hydroxylase--DBH) as well as the respective distributions of the neuropeptides associated with the adrenergic nervous system (neuropeptide tyrosine--NPY, C-terminal flanking peptide of neuropeptide Y--C-PON) were studied in series of serial sections throughout the interatrial septum and the atrioventricular junction. Our data suggest that ganglion cells of sulcus terminalis as well as the epicardial ganglia enclosed between the superior vena cava and ascending aorta are VIP- and TH-negative, but neuropeptide Y- and DBH-immunoreactive. They give rise to three intraseptal nerves directed towards the specialised structures of the atrioventricular junction. These nerve fascicles contain abundant, thick TH-immunoreactive nerve fibres and scarce, thin NPY- and DBH-immunoreactive fibres. The cell bodies of the intramural ganglion cells localized between the right and left branches of the bundle of His (Moravec and Moravec 1984) are strongly TH- and DBH-immunoreactive. They are innervated by thick nerve fibres having the same immunohistochemical properties (NPY- and DBH-immunoreactivities) as those of a subpopulation of the epicardial ganglion cells and seem to supply some of the TH-immunoreactive nerve fibres directed via the intraseptal nerves to the epicardial ganglia. The existence of a multicomponent nerve network, characterized by a reciprocal innervation of the sinus node and atrioventricular node areas, is suggested by our immunohistochemical data.

Adrenergic neurons and short proprioceptive feedback loops involved in the integration of cardiac function in the rat

Serial cryostat and paraffin-embedded sections through the atrioventricular junction of the rat heart were studied at the light-microscopic level after indirect immunohistochemical staining (tyrosine hydroxylase, neuropeptide Y, C-terminal flanking peptide of neuropeptide Y immunoreactivities) or silver impregnation. The distribution of these immunoreactivities in the Hissian ganglion (Moravec and Moravec 1984) as well as the relationships of the Hissian ganglion cells with the surrounding structures have been studied to assess its function. The results suggest that the Hissian ganglion is composed of large multipolar neurons displaying both tyrosine hydroxylase (TH) and related peptide (neuropeptide Y. C-terminal flanking peptide of neuropeptide Y) immunoreactivities. The dendritic projections of these adrenergic cells penetrate the reticular portion of the atrioventricular node and the upper segments of the interventricular septum where they constitute sensory-like corpuscles. The hypothesis that the adrenergic neurons of the atrioventricular junction are involved in short proprioceptive feedback loops necessary for beat-to-beat modulation of cardiac excitability and intracardiac conduction can thus be suggested.

The distribution of nerve fibers showing substance P-like immunoreactivity in the conduction system of the bovine heart: dense innervation in the atrioventricular bundle

There is limited information on the distribution of nerve fibers containing substance P (SP) in the heart conduction system. Therefore, in the present study, the various parts of the conduction system of the bovine heart were examined by the use of an SP-antiserum and immunohistochemistry. Nerve fibers showing SP-like immunoreactivity (SP-LI) occurred in the proximities of conduction cells in all parts of the conduction system, but were present in greatly larger numbers in the AV bundle than in the other parts. The nerve fibers showed a predilection for certain regions of the bundles of conduction cells (Purkinje fiber bundles) in the AV bundle and the bundle branches and their ramifications. Nerve fibers showing SP-LI also occurred in the walls of the arteries and in association with some the ganglionic cells located in the regions of the conduction system. None of the ganglionic cells exhibited SP-LI. The observations are discussed in relation to what is known of the function of SP in the heart and of the distribution of sympathetic and parasympathetic nerve fibers in the conduction system. As SP is regarded as a marker of afferent fibers the observations support the view that afferent nerve fibers are present throughout the conduction system. It is likely that the existence of a significant SP-innervation in the conduction system is of importance for the function of this part of the heart.

Intrinsic innervation of the atrioventricular junction of the rat heart

Alternate serial semithin and thin sections of the glutaraldehyde-fixed interatrial septum and atrioventricular junction of adult rat were examined in light and electron microscopes. The animals were pretreated with a false precursor of catecholamines, i.e., with 5-OH-dopamine, in order to differentiate the adrenergic component of the intrinsic nervous system. According to the light microscope data, two kinds of ganglia can be distinguished at the level of the interatrial septum. Those of the first kind are composed of large pale cells with voluminous nuclei. Those of the other kind resemble acinuslike clusters of small osmiophilic cells. Another small ganglion is invariably associated with the distal edge of the bundle of His. At the electron-microscope level, two types of ganglionic cells are found in the meshes of the peri- and intranodal plexus: 1) small neurons (10 microns) with richly developed neuropiles, and 2) large 5-OH-dopamine contrasted neurosecretory cells (up to 25 microns) containing electron-dense vesicles typical of sympathetic neurons. Numerous glomeruli with dendrodendritic and axodendritic connections are also found in the vicinity of the specialized tissue; and, in the nodal interstitium, several clusters of small chromaffin cells (5 microns) and a network of multipolar satellite cells similar to the interstitial cells of Cajal can be distinguished. Our data suggest that the microanatomical and cytological organization of the terminal innervation of the node of Aschoff-Tawara and of the bundle to His resembles that of the myenteric plexus. The physiological significance of these ultrastructural data for the local control of electrophysiological properties of the atrioventricular junction is briefly considered.