The autonomic nervous system in functional bowel disorders

Abstract

Communications along the brain-gut axis involve neural pathways as well as immune and endocrine mechanisms. The two branches of the autonomic nervous system are integrated anatomically and functionally with visceral sensory pathways, and are responsible for the homeostatic regulation of gut function. The autonomic nervous system is also a major mediator of the visceral response to central influences such as psychological stress and other central factors. As presently defined, functional disorders represent a constellation of symptoms, some of which suggest the presence of altered perception while other symptoms point to disordered gastrointestinal function as the cause of the symptoms. There have been a growing number of reports demonstrating disordered autonomic function in subgroups of functional bowel patients. While a number of different methods were used to assess autonomic function, they generally point to decreased vagal (parasympathetic) outflow or increased sympathetic activity in conditions usually associated with slow or decreased gastrointestinal motility. Other studies found either increased cholinergic activity or decreased sympathetic activity in patients with symptoms compatible with increased motor activity. Under certain conditions, altered autonomic balance (including low vagal tone and increased sympathetic activity) may also alter visceral perception. Autonomic dysfunction could also represent the physiological pathway accounting for many of the extraintestinal symptoms seen in irritable bowel syndrome (IBS) patients, and for some of the frequent gastrointestinal complaints reported by patients with disorders such as chronic fatigue and fibromyalgia.