Clinics in gastroenterology最新文献

Recent years have seen a number of studies measuring electrical activities of the human colon muscle layers. In vitro studies have enhanced our understanding of myogenic control of colon motility. In vivo studies have suggested a relationship between patterns of electrical activities and the transport of colon contents. This chapter describes the patterns of electrical and motor activity that the human colon can perform depending on the nature and intensity of the stimulus, using recent in vitro and in vivo data. In vitro studies with human tissue have shown differences between the electrical activity of the longitudinal and circular muscles. They have also revealed the unique nature of the electrical control activity of the circular muscle of human colon. The electrical oscillatory activity of this layer is variable in frequency from 1 to 60 cpm, variable in amplitude, and not omnipresent. Furthermore, the activity is sensitive to stretch and markedly altered by excitatory and inhibitory substances. In vivo data, especially spike action potential recordings for 24 h, have revealed patterns of electrical activity related to intake of meals, sleep, and also constipation. The limitations of some intraluminal techniques to record electrical activity are discussed. Further studies are needed to accurately relate in vivo activities to cellular events recorded in vitro, and to relate these to altered patterns of activity in disease. The suggestion is made that a relevant in vivo assessment of the colonic motility of a patient can only be achieved by long-lasting (24-h) studies, because of the large variability in the hour-to-hour colonic activity. Timing of experimental drug intervention is important since colonic motility undergoes diurnal changes. Recent studies into profiles of electrical and motor activity in irritable bowel syndrome (IBS) suggest that there is not a typical IBS myogenic activity. Rather, patterns of electrical activity can be related to the symptoms of IBS: diarrhea and constipation. Recent electrophysiologic data on Hirschsprung's disease reveal absence of intrinsic inhibitory innervation in the aganglionic segment. In vitro studies on tissue from diverticular disease patients show abnormal myogenic activity.

Mucin glycoproteins form a viscoelastic gel which appears as a continuous protective barrier on the colonic mucosal surface. However, their structure and function in health and disease remain unclear. Further work is needed to define the structure of colonic mucin glycoproteins as well as the cellular basis for their heterogeneity. Further elucidation of the factors regulating their synthesis and secretion should provide insight into their function. A more detailed characterization of changes in mucin glycoproteins associated with specific disease processes may shed light on the role of these glycoproteins in the pathogenesis of a wide variety of colonic disorders.

The large intestine is absent from the digestive tract of some mammals and shows a considerable degree of gross structural variation among the other species. The cecum serves as the major site for digesta retention and microbial fermentation in lagomorphs, most rodents, and many herbivorous marsupials, but the proximal colon appears to serve this purpose in most other mammals. The VFA end-products of microbial fermentation play an important role in the normal secretory and absorptive processes of the colon. Among those species that have received the most extensive study, the pig appears to have a colon most similar to that of humans in its gross structural and absorptive characteristics.

Uncomplicated diverticular disease is usually asymptomatic. When abdominal pain or discomfort related to defecation, altered bowel habit, and flatulence occur, they are likely a result of a coexistent irritable bowel. Nonetheless, diverticula are subject to serious complications. Diverticular hemorrhage may be massive and require emergency angiography and segmental resection. Peridiverticulitis occurs when a diverticulum ruptures, perhaps because of inspissated fecal material. This occurs usually in the sigmoid, resulting in a peridiverticular abscess localized by the adjacent fat and mesentery. If the infection extends beyond this, abscess, fistula, or free perforation may occur. These complications require antibiotics, intravenous therapy, and, in the case of uncontrollable abscess or perforation, urgent surgery. Obstruction of the colon, sometimes associated with ileus, may occur and in this case one may find a carcinoma among extensive diverticular disease. Although there is insufficient evidence to justify a high-fiber diet for the general population, the notion of a low-residue diet in the management of uncomplicated diverticular disease should be laid to rest.

The gut is a long convoluted tube, in which food is processed and nutrients, salt and water are absorbed. The degree of absorption depends to a large extent on the degree of contact between the luminal contents and the absorptive epithelium. Motor activity can influence the degree of absorption because it regulates the degree of contact with the epithelium and it may also induce secretion by a reflex mechanism. Many factors that induce diarrhoea are associated with 'abnormal' and highly propagative forms of motor activity that can clear material through the gut, allowing insufficient epithelial contact for absorption. These propulsive motor patterns may be provoked by distension of the gut with fluid, but they can also occur in response to diarrhoeogenic factors when there is minimal distension. Patients who complain of increased frequency, urgency and incontinence but pass normal stool volumes often have an abnormality in the motor activity of the anorectum. Thus, the generation of abnormal or propagated forms of motor activity must be regarded as an important component of the pathogenesis of all types of diarrhoea and an increased stool volume can be regarded as the end result of a vicious spiral (Figure 12) that may start with a primary abnormality in either motor activity or epithelial transport.