Autonomic nervous system: Basic and clinical aspects

Abstract

Abstract: The autonomic nervous system (ANS) is an important component of the nervous system consisting of a complex set of neurons and neural pathways that control the function of the various visceral organ systems. The overall function of the ANS is to maintain the body homeostasis and to react adaptively to changes in the external and internal milieu. The ANS innervates the heart, the smooth muscle in all the organs, the abdominal viscera, the exocrine and endocrine glands, and the immune system. Thus, the ANS participates in the regulation of breathing, circulation, digestion, metabolism, and the internal milieu, exocrine and endocrine gland secretion, immune responses, body temperature, and reproduction [1, 2]. Unfortunately, such medical importance tends to be underscored in many books on physiology or neural sciences in which the subject takes up much less space than that accorded, for example, to somatosensory or cognitive functions. The basic structure and operation of the ANS were defined at the beginning of the last century, primarily by Gaskell and Langley, who recognized its two main divisions: the sympathetic and the parasympathetic [3]. Furthermore, Langley designated the enteric nervous system as a third division based on the submucous plexus of Meissner and the myenteric plexus of Auerbach located in the wall of the gastrointestinal tract, albeit controlled by the sympathetic and parasympathetic divisions. Overcoming the classical concept of a purely efferent system, it is presently accepted that the ANS is composed of visceral afferents, integration centers, particularly in the brainstem, hypothalamus, and limbic cortex, and visceral sympathetic and parasympathetic efferents; thus, the ANS extends both into the central nervous system (CNS) and to the periphery. Conceptually, the bio-psycho-social-ecological nature of the individual is truly expressed by the function of his or her ANS. Its name is misleading because none of the components shows “autonomy” in an integrated body. Nor are they solely “passive” or generated “without elaboration by mind.” All body systems are dependent and affected by the action of others in a multicellular organization. These dynamic relationships are the core of homeostasis, a key concept in physiology. “Homeostasis” is used today to define not only the strategies that allow the body’s proper response to changes in the environment (reactive homeostasis), but also the remarkably developed, temporal mechanisms that allow the body to predict the timing of environmental stimuli (predictive homeostasis based on biological rhythms). Autonomic reflexes are mediated by neural pathways in the brainstem and spinal cord and generally regulate organ and system performance very rapidly (in milliseconds). Autonomic control is also mediated by specific brain regions, such as the hypothalamus, which is responsible for medium-term (minutes) and long-term (hours/days) regulation of internal organ systems. Importantly, autonomic reflexes are dynamic, where adaptations can alter rapid homeostatic control over longer time scales [4]. This book discusses the ANS from both an enlarged and a timed perspective. First, it presents how the organization of the ANS is built in four different hierarchical levels. Next, it discusses how the ANS function changes in the three body configurations (wakefulness, slow-wave sleep, and rapid eye movement, REM, sleep) found during a 24-h cycle. Finally, the most important clinical implications for this enlarged and timed vision of the ANS are discussed. The Autonomic Nervous System – Basic and Clinical Aspects is designed as a comprehensive textbook for advanced medical students and health professionals. It primes for a detailed and complete understanding of the neuroscience behind the ANS and a proper clinical applicability of this knowledge. ANS dysfunction and clinical manifestations involve multiple variables, which are often undervalued in clinical practice. However, symptoms and signs of ANS disturbances should always be considered according to their diagnostic implication, their impact on the quality of life of patients, and their prognostic value for life expectancy.