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The commonly used phrase "gut feeling" is illustrative of the intimate connection long known to exist between gut and brain and speaks to a cultural awareness of how feelings and emotions may be linked with the activities of the gastrointestinal system. Individuals regularly experience changes in gastrointestinal sensation or motor function in association with subjectively stressful situations and emotional arousal. Preliminary scientific observations that the appearance of the gastric mucosa of a patient with a gastric fistula changed in association with emotional distress paved the way for research that has consistently associated emotional arousal with changes in gastrointestinal sensory and motor function, including work employing functional brain imaging (Van Oudenhove et al. 2007).

An established system for communication between gut and brain has been termed the brain-gut axis. Communication is bidirectional and conducted along neural pathways typically considered to be part of the autonomic nervous system, as well as by neuroendocrine and neuroimmune mechanisms (Jones et al. 2006). Primary afferent neurons from the gut project via the vagus to the nucleus of the solitary tract and then to the thalamus, with subsequent projections being directed not only to soma-tosensory areas but also to important brain areas involved in arousal, emotion, and neuroendocrine and behavioral responses to challenge, such as the amygdala, the locus coeruleus, and the periaqueductal gray matter. Descending pathways from brain structures that process information from the gut modulate gut sensation and motility and provide a mechanism linking emotional and cognitive processes with gut functioning.

Familiarity with the structure, function, and vagaries of the human gastrointestinal tract inevitably leads to important questions about the relationship between the gut and its intrinsic nervous system (i.e., the enteric nervous system), the central nervous system, emotion, and the ability to adapt to life adversity and threat. The enteric nervous system, essentially the nervous system of the gastrointestinal tract, is derived from the same embryonic cells as the central nervous system and shares other commonalities with the central nervous system, including interneurons, a myenteric-blood barrier analogous to the blood-brain barrier, glial cell sheaths, and many of the same neurotransmitters and neu-ropeptides (Gershon 1998). The enteric nervous system has as many neurons as the spinal cord, with sensory neurons, interneurons, and motor neurons interconnected via chemical synapses to form an independent nervous system or "minibrain" in the walls of the gut (Wood 2008). Neurotransmitters and peptides that are considered to be of importance in the emotional disorders—such as cortico-tropin-releasing factor, cholecystokinin, neuropep-tide Y, oxytocin, and serotonin—are capable of modulating gastrointestinal transit, motility, and sensation. Such commonalities provide a template for understanding observed connections between gastrointestinal and psychiatric disorders, and the close structural and functional relationships between the gut and the brain make disorders of the gastrointestinal system of special interest in psychosomatic medicine. The remainder of this chapter will focus on three major illnesses relevant to pedi-atric gastroenterology and its day-to-day practice: functional gastrointestinal disorders, inflammatory bowel disease, and hepatitis C. These disorders not only are common and practically important in pedi-atric gastroenterology but also reflect a conceptual spectrum of problems—from the "psychosomatic" to the "somatopsychic"—that often confront specialists in psychosomatic medicine.

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