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.