The autonomic nervous system (ANS) is the part of the nervous system that controls the visceral functions of the body, which are totally or largely independent of voluntary control of the individual. This part of the nervous system consists of autonomic regions in the central nervous system and of peripheral nerves. According to anatomical and functional characteristics, the ANS is classically divided into two main sections: the sympathetic and the parasympathetic systems. The former division promotes a so-called "fight-or-flight" response, while the parasympathetic autonomic system promotes a "rest and digest" response of the organism. The heart receives nerve fibers from both the sympathetic and the parasympathetic divisions, which variably contribute to the control of heart rate (chronotropism), contractile strength of the heart (inotropism), conductivity (dromotropism) and excitability (bathmotropism) of myocardial cells, as well as of coronary vascular tone and myocardial blood flow. The sympathetic system promotes an increase in heart rate and a positive inotropic response in order to increase cardiac output. On the contrary, the parasympathetic (vagal) system induces bradycardia and reduces myocardial contractile strength, thus resulting in decreased cardiac output.
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