The autonomic nervous system in functional bowel disorders.
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. As defined, functional disorders comprise 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. A growing number of reports have demonstrated disordered autonomic function in subgroups of functional bowel patients. While a number of different methods were used to assess autonomic function, the reports point to a generally decreased vagal (parasympathetic) outflow or increased sympathetic activity in conditions usually associated with slow or decreased gastrointestinal motility, while other studies found either an increased cholinergic activity or a decreased sympathetic activity in patients with symptoms compatible with an increased motor activity. Under certain conditions, altered autonomic balance (including low vagal tone and increased sympathetic activity) may alter visceral perception. Autonomic dysfunction may also represent the physiological pathway accounting for many of the extraintestinal symptoms seen in irritable bowel syndrome patients and some of the frequent gastrointestinal complaints reported by patients with disorders such as chronic fatigue and fibromyalgia.