Morphologic pattern of nerve endings in the hearts of five mammalian species and humans

Heart areas enriched in endings of sensory nerve fibres (NEs) play a role as reflexogenic zones (RZs), where primary afferent signals initiate neural mechanisms controlling heart activity. Distribution, appearance, and chemical phenotypes of cardiac NEs remain under consideration. Study aims to characterise the morphological diversity of NEs in mammalian and human hearts, exposing links between their structural patterns and chemical phenotypes.

Hearts from 12 rats, 12 mice, 8 minipigs, 8 bats of two species, and 14 humans of both genders and various ages were used. Intracardiac neural structures were visualised using acetylcholinesterase histochemistry and immunohistochemistry for pan-neuronal, cholinergic, nitrergic, and sensory neuronal markers.

Two types of NEs were identified in the examined specimens. First type was flower-spray endings (FS-NEs) scattered both epicardially and endocardially. Epicardial FS-NEs were recurrently observed on ventricular walls of rodents, bats, and mini-pigs, whereas in humans, FS-NEs were found limitedly in atrial endocardium and were desperately larger than in mammalian species. Second type of NEs had a basket-like pattern that wraps around the bodies of cardiac ganglionic cells. Basket-type NEs (B-NEs) were immunoreactive to neuronal markers of typical sensory neurons.

Two types of NEs suggests that cardiac RZs involve not only abundant epicardial and endocardial FS-NEs but also B-NEs within intrinsic cardiac ganglia, in which neurons may be directly modulated via afferent nerve fibres comprising B-NEs. The revealed structural organisation of RZs provides an anatomical basis for understanding local sensory-autonomic control of the heart and is relevant for clinical cardiac neuromodulatory interventions.