The autonomic nervous system: a balancing act.

The overarching theme of the present chapter is the importance of the interaction between brain and body in order to maintain homeostasis - an interaction, rather than a mere top-down or reflex regulation, as signals from the organs may influence the functioning of the brain. For example, the reflex regulation of blood pressure and heart rate is not only subject to modulation by ascending information from the body, but also by descending information from several areas in hypothalamus and cortex. The central nervous system (CNS) has the capacity to control its output via the autonomic nervous system (ANS) using an amazing differentiation. For example, not only do the biological clock and prefrontal cortex contain neurons which influence the parasympathetic or sympathetic motor neurons, they also contain different neurons that project to diverse body compartments. In the end this leads to integrated responses whereby visceral sensory information reaches higher centers in the CNS via vagal or spinal sensory pathways, causing a reaction which takes into account factors such as the time of day, the season, the reproductive status, mood. Based on all this information, the brain sets the balance of the different parts of the ANS, causing its output to change its emphasis according to the situation. A disturbed balance, either as a result of behavior or of disease of any of the organs, may lead to pathology affecting the functioning of the entire individual.