The sinoatrial ring bundle: a cardiac neural communication system?

The distribution of sinoatrial nodal cells and their innervation in the pig

The sinoatrial node (SAN) has been the object of interest of various studies. In experimental neurocardiology, the real challenge is the choice of the most appropriate animal model. Pig is routinely used animal due to its size and physiological features. Despite this, the anatomy and innervation of the pig SAN are not completely examined. This study analyses the distribution of SAN cells and their innervation in whole-mount preparations and the cross-sections of the pig right atrium. Our findings revealed the differences in the distribution of the SAN cells and their innervation pattern between pigs and other animals. The pig SAN myocytes were distributed around the root of the anterior vena cava. A meshwork of nerve fibers (NFs) in this area was four-fold denser compared to other right atrial areas and contained the adrenergic (positive for TH), cholinergic (positive for ChAT), nitrergic (positive for nNOS), and potentially sensory (positive for SP) NFs. The SAN area contained 98 ± 10 ganglia that involved 21 ± 2 neuronal somata per ganglion. The determined chemical phenotypes of ganglionic cells demonstrate their diversity in the pig SAN area as there were identified neuronal somata positive for ChAT, nNOS, TH, and simultaneously for ChAT/nNOS and ChAT/TH. Small intensively fluorescent cells were also abundant. The broad distribution of SAN cells, the chemical diversity, and the high density of neural components in the SAN area are comparable to the human one and, therefore, the pig may be considered as the appropriate animal model for experimental cardiology.

Distribution and morphology of sensory and autonomic fibres in the subendocardial plexus of the rat heart

Cardiac reflexes originating from sensory receptors in the heart ensure blood supply to vital tissues and organs in the face of constantly changing demands. Atrial volume receptors are mechanically sensitive vagal afferents which relay to the medulla and hypothalamus, affecting vasopressin release and renal sympathetic activity. To date, two anatomically distinct sensory endings have been identified which may subserve cardiac mechanosensation: end-nets and flower-spray endings. To map the distribution of atrial receptors in the subendocardial space, we have double-labelled rat right atrial whole mounts for neurofilament heavy chain (NFH) and synaptic vesicle protein 2 (SV2) and generated high-resolution maps of the rat subendocardial neural plexus at the cavo-atrial region. In order to elucidate the nature of these fibres, double labelling with synaptophysin (SYN) and either NFH, calcitonin gene-related peptide (CGRP), choline acetyltransferase (ChAT) or tyrosine hydroxylase (TH) was performed. The findings show that subendocardial nerve nets are denser at the superior cavo-atrial junction than the mid-atrial region. Adluminal plexuses had the finest diameters and stained positively for synaptic vesicles (SV2 and SYN), CGRP and TH. These plexuses may represent sympathetic post-ganglionic fibres and/or sensory afferents. The latter are candidate substrates for type B volume receptors which are excited by stretch during atrial filling. Deeper nerve fibres appeared coarser and may be cholinergic (positive staining for ChAT). Flower-spray endings were never observed using immunohistochemistry but were delineated clearly with the intravital stain methylene blue. We suggest that differing nerve fibre structures form the basis by which atrial deformation and hence atrial filling is reflected to the brain.