Multilevel Interactions Between the Sympathetic and Parasympathetic Nervous Systems: a Minireview

In order to allow precise regulation of bodily functions, the activity of the autonomic nervous system must be precisely regulated. The traditional model concerning the regulation of norepinephrine and acetylcholine release in target tissues suggests that the activities of the efferent arms of the autonomic nervous system are more or less independent of each other. However, plenty of experimental and clinical studies have demonstrated the presence of multiple interactions between the sympathetic and parasympathetic nervous system that are mediated through several pathways and mechanisms at both central and peripheral levels of the neuraxis. Interactions within the central nervous system are mediated predominantly by neurons within the nucleus of the solitary tract and paraventricular hypothalamic nucleus. Peripheral interactions are based on the morphological-functional organization of the sympathetic and parasympathetic pathways at the levels of the sympathetic prevertebral ganglia or neuroeffector connections. Furthermore, evidence suggests that neuroeffector connections may be realized at the axo-axonal, presynaptic, postsynaptic, and post-receptor levels. Alterations in interactions between the sympathetic and parasympathetic nervous system can lead to unbalanced autonomic activities, which may influence the development of various disorders, including cardiovascular, inflammatory, metabolic, neurological, and psychiatric diseases. The aim of this article is to illustrate the complexity of interaction between the sympathetic and parasympathetic nervous systems and to describe the role of these interactions in the heart, adrenal medulla, and vagal trunk.