Defining the gut-brain axis

The gut-brain axis (Figure 1) comprises the enteric nervous system (ENS), the CNS, the gut wall in the periphery, and the hypothalamo-pituitary-adrenal (HPA) axis (37). The communication between the gut and the CNS is bi-directional and is centered upon the neural, endocrine, and neuroimmune pathways (38). At the physiological level, the GI tract sends signals that affect the brain, resulting in alterations in immune function, secretion, and motility (36). Thus, this axis serves as a key communication hub in the regulation of food intake, digestion, and the sensation of adequate control of gut and bowel movements. Structural and functional disruptions of the gut-brain axis alter the nervous system’s reflexive and perceptual responses, which can potentially instigate GI disorders, such as IBS (39).

The gut-brain axis. The vagus nerve provides a link between the gut and the brain. This connection involves the ENS, the CNS, the gut wall at the periphery, and the HPA axis. Alterations in the gut microbiota can influence mood, behavior, stress, anxiety, and neurotransmitters. Imbalance of the gut microbiota affects the signals sent by the gut to the brain, resulting in alterations in secretion, motility, nutrient delivery, microbial balance, and immune function. Together, these disruptions contribute to IBS symptoms. ENS, enteric nervous system; CNS, central nervous system; HPA, hypothalamo-pituitary-adrenal; IBS, irritable bowel syndrome.