Introduction
Cardiac autonomic nervous system (ANS) consists of components from parasympathetic and sympathetic systems (1). As first glance, while parasympathetic activity causes negative chronotropic and dromotropic effects, the sympathetic system primarily affects cardiac contractility and regulates peripheral vasoconstriction (1). Although the two components are usually considered to be antagonistic, autonomic control of the heart is regulated via several levels of feedback loops with a fine balance of sympathetic and parasympathetic signals between the heart and the peripheral and central nervous systems (2).
Structurally, the ANS of any visceral organ is represented by a complex neural plexus formed by extrinsic and intrinsic parts according to localization of postganglionic neurons that provide fibers from the ganglion to the effector organ (3, 4). For the heart, large numbers of neurons are associated with ganglia and their interconnecting nerves on atria and ventricles, and this intrinsic cardiac ANS has been collectively referred to as the heart’s “little brain”(4-8). Extrinsic and intrinsic cardiac structures in animal models and humans are species-dependent and translating experimental data obtained from animals to humans can be challenging.
Excessive parasympathetic tone may be an important cause of several clinical bradyarrhythmias such as functional atrioventricular block, some forms of sinus node dysfunction, and vasovagal syncope (VVS) (9). Similarly, a causal relationship between intrinsic cardiac ANS and atrial tachyarrhythmias has been demonstrated both in animals and in humans (10-16). Furthermore, electrophysiological effects of ANS may be exacerbated in diseased hearts (11, 12). In recent years, catheter based autonomic neuromodulation has emerged as an important novel therapy for VVS and for atrial fibrillation (AF), and has been shown to have better efficacy when compared with pharmacological therapies (9, 10). However, the full extent of the distribution of intrinsic cardiac nerve plexus on the human heart and the functional properties of human intrinsic cardiac neural elements remain insufficiently understood by many clinical electrophysiologists. In this review, we focus on anatomical and functional characteristics of the intrinsic cardiac ANS in non-human mammals and in humans, and attempt to present neuroanatomical terminologies in an attempt to fill this knowledge gap.