Innervation and Neuronal Control of the Mammalian Sinoatrial Node: A Comprehensive Atlas

Abstract

Cardiac function is under exquisite intrinsic cardiac neural control. Neuroablative techniques to modulate control of cardiac function are currently being studied in patients, albeit with variable and sometimes deleterious results. Recognizing the major gaps in our understanding of cardiac neural control, we sought to evaluate neural regulation of impulse initiation in the sinoatrial node as an initial discovery step. Here, we report an in-depth, multi-scale structural and functional characterization of the innervation of the sinoatrial node (SAN) by the right atrial ganglionated plexus (RAGP) in porcine and human hearts. Combining intersectional strategies including tissue clearing, immunohistochemical and ultrastructural techniques, we have delineated a comprehensive neuroanatomic atlas of the RAGP-SAN complex. The RAGP shows significant phenotypic diversity of neurons while maintaining predominant cholinergic innervation. Cellular and tissue-level electrophysiologic mapping and ablation studies demonstrate interconnected ganglia with synaptic convergence within the RAGP to modulate SAN automaticity, atrioventricular (AV) conduction and left ventricular (LV) contractility. For the first time, we demonstrate that intrinsic cardiac neurons influence the pacemaking site in the heart. This provides an experimental demonstration of a discrete neuronal population controlling a specific geographic region of the heart (SAN) that can serve as a framework for further exploration of other parts of the intrinsic cardiac nervous system (ICNS) in mammalian hearts and for developing targeted therapies.

The Heart’s Pacemaker Mimics Brain Cytoarchitecture and Function: Autonomic innervation, a peripheral glial cell web, and a novel S100B expressing interstitial cell type impart structural and functional complexity to the sinoatrial node

“…Pacemaker cells, parasympathetic fibers, and sympathetic fibers of the SAN neuronal plexus immunoreactive for HCN4, VChAT, and TH, respectively, were readily identifiable up to 350 µm in depth in the SAN tissue in all whole mount preparations ( n=3 ). Compared with atrial areas adjacent to the root of SVC, the density of nerve fibers within the HCN4-positive meshwork of pacemaker cells was 2–3 fold higher ( n=3 ) consistent with previous publications (18, 19).…”

“…Images of ICG stained for TH, VAChT, and HCN4, confirmed that most neurons in these ganglia were cholinergic (i.e., immunoreactive for VAChT), while others were adrenergic, and that some neurons within ICG were reactive for neither VAChT nor TH ( Fig. 2 ), suggesting the presence of other neurotransmitters within ganglionic neurons (18). We also observed HCN4+ immunoreactivity on the shell of ICG suggesting that some HCN4+ satellite cells may be present around the ganglia.…”

Chemical phenotypes of intrinsic cardiac neurons in the newborn pig (Sus scrofa domesticus Erxleben, 1777)

“…Similarly, the proportion of adrenergic and nitrergic NS is also almost unfluctuating between the different sGPi. Presumably, the neonatal ICNs are not specialized functionally yet as this is predictable in adult animals (Brack, 2015; Hanna et al, 2021). Nevertheless, one shift from uniform distribution of ICNs was noticed in the piglet VLA sGP, in which the abundance of cholinergic NS was comparatively lesser due to an increase of adrenergic and nitrinergic NS in this sGP.…”

“…Chemical phenotype of a neuron may be indicative of its function and, therefore, the shift in proportions of particular neuronal phenotypes provides the anatomical basis for adaptability of ICNs as this was clearly demonstrated for the heart rate decreases in respect to age of some animal species following shift in abundance of neurons in the sympathetic chain ganglia (Woods et al, 1977; Yiallourou et al, 2012). Since both, the chemical phenotype and the location of ICNs, may be strongly associated with their function, an analysis of distribution of the phenotypically distinct ICNs should contribute to disclosure of the genuine cause of different structural patterns of GP in diverse animal species (Aksu et al, 2021; Brack, 2015; Hanna et al, 2021). Furthermore, the widespread neural communications with and between ICNs ensure an integration of the extrinsic and intracardiac signals within GP in which the interplay between cholinergic, adrenergic, and other chemical phenotypes of ICNs occurs.…”

Autonomic modulation of ventricular electrical activity: recent developments and clinical implications

“…In addition, satellite glial cells are also abundantly present within the intrinsic cardiac nervous system, [ 94 ] and have been demonstrated to become injured upon catheter ablation of atrial fibrillation [ 95 ]. Interestingly, higher serum concentrations of S100B after catheter ablation of atrial fibrillation were associated with a lower recurrence rate [ 95 ].…”