The gut-brain axis refers to the network of nerve pathways that connect the myenteric plexus, the veritable "gut brain," to the central nervous system. Nearly 80% of these neurons are sensory neurons, and the afferent pathways that transmit information from the digestive tract to the central nervous system play a crucial role in the physiological regulation of digestive functions, as well as in certain pathological conditions. A large majority of these sensations remain unconscious and give rise to reflex responses. Only those requiring a conscious response reach the level of awareness in a normal state (hunger, thirst, the urge to defecate). In pathological situations, the same is true for painful sensations of digestive origin. Functional bowel disorders are a frequent reason for consultation. Their pathophysiology is now based on a model integrating the various etiological factors around the brain-gut axis. These patients frequently present with visceral hypersensitivity, which manifests as an increased perception of digestive sensations, notably the onset of pain in response to stimuli that are not painful in normal subjects. Recognizing the role of visceral hypersensitivity has made it possible to explain the mechanism of action of medications used to treat functional bowel disorders and paves the way for the development of new molecules acting on digestive afferents. In this book, we will describe the anatomical and physiological basis for understanding the concept of visceral sensitivity and the role of digestive afferents in the pathophysiology of acute and chronic abdominal pain, particularly irritable bowel syndrome.