The autonomic nervous system

The neural regulation of cardiac function is mainly determined, on its efferent side, by the interaction of sympathetic and vagal mechanisms. In most physiological conditions, the activation of either the sympathetic or vagal outflow is accompanied by the inhibition of the other suggesting the concept of sympatho-vagal balance as a horizontal beam pivoted at its center. This reciprocal organization, alluding to a synergistic design, seems instrumental to the fact that sympathetic excitation and simultaneous vagal inhibition, or vice versa, are both presumed to contribute to the increase or decrease in cardiac performance required for the various behaviors. The balance oscillates from states of quiescence, when homeostatic negative feedback reflexes predominate, to states of excitation, such as those due to emotion or physical exercise. However, this neural activity is not limited to peripheral regulation but it is able to deeply modify cortical circuitry related to stress responses and sleep homeostasis, as well as the immune system. As described in this chapter, the assessment of sympatho-vagal balance is a useful noninvasive tool to infer information about the state of the autonomic nervous system modulating the viscera and stress coping systems.