# Bioelectrical Signals for the Diagnosis and Therapy of Functional Gastrointestinal Disorders
## 1 Introduction  
The gastrointestinal (GI) tract is responsible for the digestion and nutrient absorption of food we ingest. The GI tract is composed of multiple organs, including the orophnx, esophagus, stomach, and small and large intestine, each with its own specialized anatomy, properties and function, as illustrated in Figure 1. The process of digestion starts in the mouth, where food particles are propelled through the pharynx into the stomach via the esophagus. The stomach mechanically and chemically breaks and mixes the food with the help of contractions, gastric acids, and enzymes. The small intestine breaks the food into even smaller components, absorbs the nutrients, and transfers the remaining contents to the large intestine. In the large intestine, water is absorbed, and the contents are prepared for expulsion through the rectum [1]. The digestive process is reliant on coordinated contractions and motility patterns unique to each organ and its respective function. These motor activities are coordinated by bioelectrical events, sensory and motor nerves, and hormones [2; 3]. The patterned pharyngeal response and proximal esophageal contractions can be volitionally modulated, but are mainly regulated by the central and enteric nervous system [3]. In contrast, the motility patterns in the rest of the GI tract are involuntary and coordinated by the enteric nervous system and bioelectrical events known as slow waves (SWs) that are generated by a specialized network of pacemaker cells known as interstitial cells of Cajal (ICC) [4]. Functional GI disorders (FGIDs) are motility problems within the GI organs which do not stem from an anatomical or structural abnormality. In general, the dysfunction is caused by impaired neuromuscular activity where the underlying pathophysiological factors are largely unknown [5]. FGIDs are common, with a recent global-scale study reporting more than 40% of the population have at least one FGID [6]. FGIDs are also reported to result in a direct healthcare cost of USD 5000-8000 annually per patient in the US [7]. Despite their high prevalence rate and impact on the health system, the diagnosis of FGIDs is challenging and remains largely exclusionary. Figure 1 lists the most prevalent FGIDs specific to each GI organ together with current clinical functional tests. Conventional therapies primarily target symptoms, as the etiologies of many FGIDs have not been completely elucidated. However, there is a substantial research on the brain-gut axis and the bidirectional communication between the enteric nervous system and the central nervous system [8; 9]. Bio-behavioral interventions such as cognitive-behavioral tests and heart rate variability biofeedback have shown to be effective in reducing the symptoms of certain FGIDs and enhancing the quality of life [10; 11]. These cognitive behavioral-based interventions are hypothesized to target the underlying biopsychological factors of FGIDs and reduce symptoms via autonomic regulation of the GI tract [10]. Another therapeutic approach that has garnered significant interest in the clinical field is the use of electroveticals. Figure 1: The overview of functional disorders within the gastrointestinal (GI) tract together with conventional motility tests and bioelectromagnetic diagnostic methods and therapies. Electroceuticals is a term for bioelectrical therapies to treat diseases and have recently gained a significant interest. New electroceutical approaches are being investigated for disorders ranging from cardiovascular disease to rheumatoid arthritis and cancer [12]. Similarly, electroceuticals could potentially provide therapeutic benefit to FGIDs, as there is an increasing level of research and clinical evidence suggesting a link between FGIDs and impaired bioelectrical activity [13], and bioelectrical therapy can influence and modify GI tract function [14].