Modern Scientific Views on the Composition and Structure of the Autonomic Nervous System (Literature Review)
Authors:   VN Karazin Kharkiv National University, School of Medicine, Human Anatomy and Physiology Department. 6, Svobody Sq., Kharkiv 61022, Ukraine, Polstianoi A. O. 
Published: May 2022

The purpose of the study was to analyze literary sources to summarize modern scientific views on the structure and possible changes in the existing classification of the autonomic nervous system. Results and discussion. According to classical views, the anatomically and functionally autonomic nervous system is divided into sympathetic and parasympathetic parts, and the last one includes cranial and sacral centers. This classification, in addition to ontogenetic, anatomical and physiological justification, has historical roots and is associated with the research of J. Langley. For more than a century, such a classification of the autonomic nervous system was considered commonplace. However, modern views on this issue emphasize the mechanisms of development and molecular mechanisms of these systems. More attention was paid to such elements as gene expression, features of embryogenesis and development, as well as the general functions of neurons localized in these parts of the autonomic nervous system. In recent years, a group of researchers Espinosa-Medina et al. determined the differential genetic features and relationships of sympathetic and parasympathetic preganglionic and postganglionic neurons. The aim of the study was to compare the relationships and genetic characteristics of lumbar and sacral preganglionic neurons with cranial (parasympathetic) and thoracic (sympathetic) neurons. In general, 15 phenotypic and ontogenetic features are given that distinguish the pre- and postganglionic neurons of the cranial part of the parasympathetic part from the sympathetic neurons of the thoracolumbar region. Each of the features, according to Espinosa-Medina et al., proves that the sacral section must be considered as part of the sympathetic, and not the parasympathetic, section of the autonomic nervous system. This assumption is based on the study on mice at 11.5, 13, 165 days of embryonic development of several transcription factors common to both sacral and thoracolumbar preganglionic neurons, but absent in cranial preganglionic neurons. When revising the existing classification, cranial vegetative nervous structures are proposed to be considered parasympathetic, and spinal – sympathetic. However, a number of researchers were critical of the proposed changes to the existing classification. They consider the interpretation of the findings controversial, point to possibly misleading conclusions due to misinterpretations of the evidence, and suggest that the phenotypes of common thoracic and sacral preganglionic neurons may simply share a common spinal identity. Conclusion. In recent years, thanks to the latest research, there has been a rethinking and possible change in the ideas that have existed for a long time about the composition and structure of the autonomic nervous system. The proposed reclassification of the autonomic nervous system involves the assignment of the sacral region to its sympathetic, not parasympathetic part. Such a simplified two-component structure offers a new concept of neurophysiology, as well as the evolution and development of the autonomic nervous system. At the same time, a number of researchers give their arguments in favor of rejecting the proposed radical changes and saving the existing classification. In this regard, further studies are highly demanded, which could finally shed light on this issue