(2) Onset of pain associates with an abnormal frequency of stools (more than three times per day or fewer than three times per week). Rome II criteria define IBS as having abdominal pain/discomfort along with at least two of the following three features. About 70% of IBS patients consulting physicians in Western countries are females. The prevalence of IBS is about 11% to 14% in the general population in North America. However, this signal may exceed the nociceptive threshold and be perceived as painful under the following scenarios. The reason for lack of painful sensation is that the afferent signal generated by a GMC in health is below the nociceptive threshold ( Figure 37A). GMCs occur spontaneously several times a day in healthy subjects and do not produce the sensation of pain.
A recent study showed that the occurrence of a GMC in the small intestine generates a pseudoaffective signal, but small intestinal RPCs do not. Taken together, this means that a GMC will generate a much-higher-intensity afferent signal than an RPC due to its stronger compression of the gut wall and larger mechanosensitive field. These mechanoreceptors respond similarly to compression of the gut wall by a GMC and its distension by an intraluminal balloon, and the two signals are additive. The afferent signal is proportional to the degree of distension. The primary afferent fibers with mechanosensitive nerve endings in the muscularis externa respond to circumferential stretch and send the signal to the CNS via the afferent 1 st- and 2 nd-order splanchnic neurons. (Top diagram) The RPCs do not strongly compress the colon wall and they do not produce mass movements (more.) Differential effects of RPCs and GMCs on the compression of the gut wall, propulsion of luminal contents, and distension of the descending segments.