The colon is the last major organ in the gastrointestinal tract. Therefore, it plays a critical role in regulating the frequency of defecation and consistency of stools. The two primary symptoms of colonic motility dysfunction are altered bowel habits (constipation, diarrhea) and intermittent abdominal cramping. Additional symptoms include straining, urgency, feeling of incomplete evacuation, passage of mucus, bloating or feeling of abdominal distension, and postprandial exacerbation of symptoms. The following sections discuss the symptoms of specific motility disorders affecting the colon, the motor correlates of these symptoms, and the cellular and molecular mechanisms of dysfunction, following a brief discussion of how physiological or pathologic motor activity produces the sensation of abdominal pain.
GMCs and Visceral Pain of Gut Origin
The link between abnormal motility and altered bowel habits is obvious. However, it is not always apparent how a motor event in the gut can be the source of abdominal cramping. The sensation of pain requires three components (Figure 35):
- A noxious mechanical or chemical stimulus to generate an afferent signal
- Afferent sensory neurons to transmit the signal to the CNS, and
- The CNS circuitry, which processes this signal and sends the information to the higher centers for perception of pain.
There are two types of mechanical events in the gut: compression of the gut wall by contractions and distension of the receiving segment during mass movements. Earlier sections showed that the amplitude and duration of a GMC are twofold to threefold greater than those of RPCs. The longer duration of a GMC means that it concurrently contracts a long-segment of the gut (about 20 cm in the colon). In addition, a GMC propagates rapidly over long distances and propels a large bolus ahead of it, which distends the receiving segment (Figure 36). By contrast, an RPC contracts a short segment (about 1 to 2 cm). RPCs move luminal contents back and forth with slow net distal propulsion and do not cause distension of the receiving segment (Figure 36).
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 1st- and 2nd-order splanchnic neurons [245–247]. The afferent signal is proportional to the degree of distension . 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.
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. A recent study showed that the occurrence of a GMC in the small intestine generates a pseudoaffective signal, but small intestinal RPCs do not .
GMCs occur spontaneously several times a day in healthy subjects and do not produce the sensation of pain. 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). However, this signal may exceed the nociceptive threshold and be perceived as painful under the following scenarios.
- The amplitudes of GMCs increase due to neuromuscular dysfunction so that the afferent signal generated by them exceeds the nociceptive threshold (Figure 37B). This scenario is similar to that in which balloon distension in the rectum or sigmoid colon exceeds the nociceptive threshold.
- The development of hypersensitivity in afferent neurons or impairment of CNS processing effectively lowers the nociceptive threshold (Figure 37C) so that GMCs of normal amplitude exceed it and induce painful sensation.
- Due to impairment of descending inhibition, the descending segment generates tone and sends afferent signals to the CNS. The combined afferent signals, due to strong compression by GMC and distension of the receiving segment, exceed the nociceptive threshold to induce the sensation of pain in the CNS (Figure 37D). This scenario develops particularly when the sphincters, such as the anal sphincters, fail to relax. In this case, a GMC attempts to push feces against a closed sphincter, but due to the closure, the intervening segment balloons up. Note that, in normal subjects, the release of NO by descending inhibition prevents the generation of tone in the receiving segment, preventing it from generating the afferent signals .
- GMCs are the essential peripheral stimuli that generate the sensation of pain of gut origin.
- Each GMC from its beginning to its termination lasts for a short period of a few minutes. This explains the sensation of short-lived intermittent abdominal cramping.
- GMCs can start anywhere in the colon and propagate to various distances. This suggests that the pain may localize in any quadrant of the abdomen or migrate among different quadrants.
- Abdominal cramping may occur in the absence of visceral hypersensitivity or impaired CNS processing due to enhancement of GMC amplitudes or impairment of descending inhibition.
- Visceral hypersensitivity by itself does not cause pain. There has to be a stimulus, such as a GMC, that generates an afferent signal for perception of pain.
Irritable Bowel Syndrome
The prevalence of IBS is about 11% to 14% in the general population in North America [248–250]. About 70% of IBS patients consulting physicians in Western countries are females . Rome II criteria define IBS as having abdominal pain/discomfort along with at least two of the following three features. (1) Defecation relieves pain/discomfort. (2) Onset of pain associates with an abnormal frequency of stools (more than three times per day or fewer than three times per week). (3) Onset of pain associates with a change in the form of the stool . Additional supportive symptoms in IBS patients include straining, urgency, feeling of incomplete evacuation, passage of mucus, bloating, feeling of abdominal distension, and postprandial exacerbation of symptoms. Altered colonic motor function may result in constipation (constipation-predominant IBS [IBS-C]), diarrhea (diarrhea predominant IBS [IBS-D]), or alternating constipation and diarrhea IBS (IBS-C/D). In addition, recent studies show that about 10% to 25% of patients develop the symptoms of IBS after an episode of severe or prolonged enteric inflammation (postinfectious IBS [IBS-PI]).
In addition to intermittent abdominal cramping, IBS-D patients have one or more of the following symptoms: (1) more than three bowel movements per day, (2) loose (mushy) or watery stools, and (3) urgency of defecation.
Colonic Motor Dysfunction in IBS-D Patients
IBS-D patients presenting with abdominal pain and diarrhea show a several-fold increase in the frequency and amplitude of spontaneous GMCs, compared with healthy controls . Several of the GMCs in IBS-D patients result in defecation during manometric recordings, indicating urgency. The transit time of radiopaque pellets from the cecum to defecation in these patients was several-fold faster than in controls . About 90% of the GMCs in patients were associated with the sensation of intermittent short-lived abdominal cramping. Diarrhea usually occurred after breakfast, and cramping subsided after defecation. The GMCs in the healthy cohort did not produce the sensation of abdominal cramping. The intensity of RPCs (measured as area under contractions) in IBS-D patients was not different from that in healthy controls, except in the descending colon, where it was greater.
The above findings demonstrate a primary role of motility dysfunction—an increase in the frequency and amplitude of GMCs-in generating the symptoms of diarrhea, urgency and abdominal cramping in IBS-D patients. These findings also show that RPCs may have little role in the induction of these symptoms. As indicated earlier, RPCs are essential in frequent and regular turning over of fecal material, thus allowing uniform and extensive exposure of the fecal material to the mucosa for absorption of water and electrolytes. However, rapid propulsion by increase in the frequency of GMCs deprives the fecal material of adequate exposure to mucosa, resulting in loose stools. IBS-D patients do not have impaired absorptive function.
The above group of patients experienced daily symptoms of abdominal bloating, cramping, urgency, and frequent bowel movements ranging from 4 to more than 15 per day for more than 6 months. Therefore, these patients represent cases of moderate to severe IBS-D. The classification of IBS-D patients by Rome III criteria is liberal . As a result, studies that select patients by strict Rome II or Rome III criteria find an increase in the frequency of GMCs in IBS-D patients, which may not reach statistical significance . The colonic transit in a wider population of IBS-D patients is faster than in controls [254–261] but not as much as in patients of the above study with severe symptoms of IBS-D . However, the association between frequent GMCs and faster propulsion and sensation of abdominal cramping with most GMCs in IBS-D patients is present in almost all studies. The relief in abdominal cramping following a bowel movement is due to the lack of a stimulus in the distal colon to generate GMCs.
As shown in Figure 37B, a significant increase in the amplitude of GMCs in IBS-D patients by itself should be enough to increase compression of the colon wall to above the nociceptive threshold. However, a subset of IBS-D patients show hypersensitivity to colorectal distension by a balloon [247, 262–267], which would increase the susceptibility of inducing abdominal cramping by GMCs (see Figure 37C).
- The frequency and amplitude of GMCs increase significantly in IBS-D patients.
- These increases relate to the severity of symptoms of abdominal cramping and bowel movements per day.
- The increase in the amplitude of GMCs by itself may be sufficient to induce the sensation of abdominal cramping in IBS-D patients. Concurrent visceral hypersensitivity exaggerates this sensation.
- The relief of abdominal cramping may relate to reduction in the incidence of GMCs following defecation.
- However, if a GMC occurs in the absence of feces in the sigmoid colon, it might initiate an urge to defecate, giving the sensation of incomplete evacuation.
Constipation-Predominant IBS, Slow-Transit Constipation, Idiopathic Constipation, and Constipation Due to Pelvic Floor Dysfunction
According to the Rome II criteria, IBS-C patients present with one or more of the following symptoms: (1) fewer than three bowel movements per week, (2) hard or lumpy stools, (3) straining during bowel movements, and (4) intermittent short-lived abdominal cramping. The patients with IBS-C differ from those with other subtypes of constipation primarily by the absence of abdominal cramping. However, this discrimination is not absolute: non-IBS-C constipated patients may also have abdominal cramps, albeit less frequently. This section discusses all subtypes of constipation together. This does not imply that constipation in different subtypes has the same etiology or that it is manageable by a common approach. The discussion of the subtypes of constipation together highlights the overlapping motor dysfunctions that lead to straining, hard stools, and fewer than three stools per week.
The severity of constipation and types of symptoms differ widely among patients within the same classification. This heterogeneity has resulted in inconsistent findings among clinical studies, which makes it difficult to formulate hypotheses for mechanistic studies.
Colonic Motor Dysfunction in IBS-C, Slow-Transit Constipation (STC), Idiopathic Constipation, and Pelvic Floor Dysfunction (PFD) Patients
The amplitude and frequency of GMCs in IBS-C, STC, idiopathic constipation, and PFD patients are less than half of those in normal subjects; in severe cases of constipation, GMCs are totally absent or scarce [203, 268–272]. The suppression of GMCs in constipation may be pancolonic or confined to the distal colon. Ambulatory 24-hour recordings from STC patients show depression of the overall contractile activity of the colon throughout the day. These patients also show suppression in normal increase of colonic motor activity after awakening in the morning .
The frequency and amplitude of GMCs in the colon of patients with constipation due to pelvic floor dysfunction do not differ from those in normal subjects . However, the GMCs are noticeably absent or reduced in frequency when patients feel the urge to defecate (Figure 39). The urge to defecate in these patients may come from the accumulation of feces in the sigmoid colon or rectum due to RPCs. In the absence of GMCs propagating to the rectum, the descending inhibition and the propulsive force for defecation are absent in constipated patients. Hard straining may not be enough to push feces against the closed internal and external anal sphincters. Additional structural impairments in the pelvic floor function may exacerbate the difficulty in stool expulsion in some patients.
Pelvic floor dysfunction in constipation may partly be due to impaired motility function of the sigmoid colon. The internal anal sphincter relaxation depends on descending inhibitory signal generated by GMCs propagating up to it. In addition, strong compression by GMCs generates the afferent signal (urge) to relax voluntarily the puborectalis muscle and external anal sphincter. The decrease in the frequency and amplitude of GMCs in constipated patients would compromise both aspects of pelvic floor function. Additional abnormalities in enteric neuromuscular function or in autonomic nerves regulating the puborectalis and external anal sphincter may worsen constipation in these patients. We do not know whether abnormalities in GMCs and impairments in pelvic floor regulation occur independently or whether one leads to the other. Rectal motor complexes are absent in some constipated patients, indicating enteric neuromuscular dysfunction (Figure 40).
The total incidence of RPCs measured as area under contractions in constipation is variable; it depends on the severity of constipation. The area under contractions in patients with normal colonic transit, with moderately slow transit, or in patients with IBS-C is higher than that in healthy controls . Note that these data, obtained by a wireless capsule, have less fidelity than those obtained by manometric tube. Another study, using the manometric method of recording, found that the area under contractions in slow-transit constipation is less than that in healthy subjects throughout the day (Figure 41). It is worth noting that, in the absence of GMCs or a reduction in their frequency, colonic propulsion occurs primarily by propagating RPCs. Therefore, having more RPCs does not necessarily mean that propulsion will be faster. Propulsion is faster only if the incidence of propagating RPCs increases. These data are not available. However, as noted earlier, the contribution of RPCs to colonic propulsion is relatively minor. They may influence the consistency of stools by regulating the intensity of turning over of luminal contents.
- The amplitude and frequency of GMCs decrease significantly in constipated patients.
- The severity of constipation relates to the intensity of suppression of GMCs.
- The suppression of GMCs in the sigmoid colon will impair pelvic floor function, adding to the severity of constipation.
- There is no consistent change in the parameters of propagating or nonpropagating RPCs in any type of constipation.
- Diagnosis of motility disturbances in IBS-D, IBS-C, slow-transit constipation, idiopathic constipation, and obstructed defecation could be made simply by analyzing the frequency of GMCs over 24 hours, their amplitude, duration, distance of propagation, and point of origin in the whole colon. These analyses do not require a computer program. Ambulatory recordings with solid-state transducers would provide more physiological data.
- GMCs are a reliable biomarker of both primary symptoms of IBS: altered bowel habits and abdominal cramping.
- The Rome criteria to subdivide IBS patients into different groups are subjective and symptom based. They have not received universal acceptance after three revisions. A mechanism- based criterion, using 24-hour recordings of GMCs, might be more objective. The inclusion of objective criteria would spur mechanistic studies followed by development of therapeutic agents to normalize dysfunctional proteins.
Alternating Constipation/Diarrhea IBS
The symptoms of patients with IBS-C/D alternate randomly between those of IBS-C and IBS-D. We do not know the precise motor patterns during the two opposite conditions of motility function. However, one can extrapolate from what we know about the motor patterns in IBS-C and IBS-D patients that the frequency of GMCs fluctuates from one extreme to the other in IBS-C/D group: when the frequency is above normal, diarrhea results; when it is below normal, constipation results.
Clinical observations show that a subset (about 10% to 25%) of the subjects exposed to enteric infections in an individual or community setting go on to develop predominantly the symptoms of IBS-D [274–280]. Two major risk factors predispose individuals to developing postinfectious IBS symptoms following an enteric infection: (1) enteritis lasting more than 3 weeks significantly increases the risk for developing IBS-PI over a duration lasting less than 1 week and (2) the presence of comorbid psychiatric disorders or a lifetime history of anxiety and depression at the time of infection increases the risk of developing IBS-PI. The longer duration of enteritis reflects severity of inflammation . The psychosomatic disorders represent dysregulation/impairment of the central nervous system and hypothalamus-pituitary-adrenal (HPA) axis [276, 281]. While motility recordings from these patients are not available, their motility dysfunction is likely similar to that in IBS-D patients.
- Spontaneous variations in the frequency and amplitude of GMCs from one extreme to the other in IBS-C/D patients suggest that environmental conditions (diet, stress, somatic activity) affect their regulatory mechanisms.
- These observations speak against a genetic (mutation, polymorphism) role in motility and sensory dysfunctions in IBS. The genetic dysfunctions are stable.
- The persistence of IBS-D-like symptoms in IBS-PI patients is likely due to epigenetic changes in genes encoding proteins of the regulatory mechanisms. Epigenetic changes in gene expression are sensitive to the cellular microenvironment.
Cellular and Molecular Mechanisms of IBS and Other Types of Constipation
Our understanding of the cellular mechanisms of motility dysfunction in functional bowel disorders (FBD) is limited, largely due to the unavailability of neuromuscular tissues from these patients and the paucity of animal models that mimic salient features of these disorders. However, clues from clinical and animal studies suggest potential cellular mechanisms. The following sections highlight the insights obtained from these studies and from recently available models of IBS-D.
Smooth Muscle and Enteric Neuronal Dysfunction
Impaired Gastrocolonic Response.
Clinical studies show that the increase of motor activity— including the incidence of GMCs—in the sigmoid colon following ingestion of a meal is significantly greater in IBS-D patients than in healthy controls . IBS-D patients also show an exaggerated response to exogenous CCK-8 . The greater increase of GMCs after a meal in IBS-D patients associates with faster postprandial transit than in healthy subjects . By contrast, the increase of postprandial colonic motor activity is significantly less in constipated patients than in healthy controls [269, 271, 282–286]. Antral distension by a balloon and duodenal instillation of lipids mimic the gastrocolonic response by increasing tone in the distal colon . The increase in colonic tone by either stimulus is impaired in patients with slow-transit constipation .
The parasympathetic nerves mediate the gastrocolonic response to ingestion of a meal. The parasympathetic nerves synapse on nicotinic receptors on the excitatory and inhibitory motor neurons. Accumulating evidence shows that the physiologic stimulation of parasympathetic nerves by ingestion of a meal  or experimental electrical stimulation enhances colonic motor activity by release of ACh from excitatory cholinergic motor neurons in the myenteric plexus [42, 44, 215, 288]. Although the postprandial increase of plasma CCK after ingestion of a normal meal in healthy subjects is not enough to stimulate colonic motor activity , duodenal instillation of lipids and pharmacologic doses of CCK stimulate colonic motor activity [22, 208]. CCK acts on presynaptic interneurons or directly on motor neurons to release ACh and stimulate colonic contractions, while atropine blocks the contractile response to CCK in human colonic circular muscle strips . Exogenous CCK stimulates GMCs in healthy volunteers as well as in IBS-D patients . However, the number of GMCs stimulated by CCK is several-fold greater in IBS-D patients than in healthy controls, indicating greater and prolonged release of ACh from the cholinergic motor neurons in these patients. These findings suggest that the exaggerated motor response in IBS-D patients may be due to enhanced synthesis/release of ACh at the neuroeffector junction, slow hydrolysis of ACh at the neuromuscular junction, or sensitization of excitation-contraction coupling in circular smooth muscle cells (see Figures 10 and 11).
Findings in patients with constipation are just the opposite of those in IBS-D patients. The contractile response to ACh in circular muscle strips from idiopathic chronic constipation patients is significantly less than that in normal strips from patients with normal colon transit . Interestingly, the muscle strips from constipated patients also show smaller contractile responses to electrical field stimulation (EFS). EFS induces in vitro contractions by releasing ACh from the cholinergic motor neurons. These findings suggest that the impaired colonic motor function in constipated patients is due to a reduction in the expression of ChAT, synthesis or release of ACh, or a defect in excitation-contraction coupling in circular smooth muscle cells. A decrease in the evoked release of 3Hcholine confirms the defect in the activity of cholinergic neurons in constipated patients .
An impairment in excitation-contraction coupling in smooth muscle cells follows from the finding that the contractile response to edrophonium chloride—a short acting choline esterase inhibitor—is significantly lower in slow-transit constipation patients than in healthy controls . ACh accumulation at the neuromuscular junction acts directly on muscarinic M3 receptors to stimulate smooth muscle contractions. Smooth muscle dysfunction in idiopathic chronic constipation patients is evident from the inability of cathodal current to generate spikes, which suggests impairment of Cav1.2b (L-type) calcium channels .
About 70% to 90% of patients with different subtypes of IBS report intermittent short-lived abdominal cramping/pain [254, 295]. The perception of pain occurs in the higher centers of the brain when they receive signals from a noxious stimulus from the periphery. Noxious signals reach the higher centers due to an unphysiological condition in the periphery, such as inflammation, amplification of a physiological signal during its transmission to the CNS (visceral hypersensitivity), or impaired supraspinal processing. IBS patients do not have any organic abnormality such as inflammation. Consequently, visceral hypersensitivity and supraspinal processing have received much attention in understanding the etiology of abdominal cramping in IBS patients.
Patients’ recognition of visceral feelings—initial sensation, urge to defecate, and pain—in response to phasic or ramp distensions of a balloon in the colorectal area are used to determine the level of visceral sensitivity. Using this approach, some studies reported that about 90% of IBS patients have visceral hypersensitivity to colorectal distension [263, 296]. These investigators have proposed visceral hypersensitivity as a biomarker of IBS. However, they could not relate most symptoms of IBS to visceral hypersensitivity. They also suggested, without any scientific evidence, that visceral hypersensitivity is the source of motility dysfunction in IBS patients. The concept was that the amplified afferent signals reflexively send aberrant efferent signals to the colon to cause motility dysfunction [297–299]. In proposing these hypotheses, the investigators ignored an important fact: visceral hypersensitivity or impaired central processing does not by itself induce the sensation of pain; a peripheral signal is required.
Numerous other studies show that on average, only about 50% (range, 20% to 80% ) of IBS patients show visceral hypersensitivity [254, 267, 295, 301, 302]. The visceral hypersensitivity hypothesis does not explain abdominal cramping in normosensitive patients [265, 295]. In fact, none of the symptoms of IBS adequately distinguish hypersensitive from normosensitive patients . This hypothesis also does not explain which reflexes alter motility function in response to visceral hypersensitivity, which alterations in colonic contractions they produce, or how the same reflexes cause diarrhea in some patients and constipation in others. This hypothesis ignores the wealth of knowledge we have about the regulation of contractions by smooth muscle cells and enteric neurons, the types of contractions they generate, and the specific functions of those contractions. Several publications have challenged this simplistic hypothesis of visceral hypersensitivity alone as the basis of IBS symptoms [300, 303].
It is noteworthy that the symptom of abdominal cramping usually follows alterations in bowel habits. In addition, repetitive high-pressure mechanical sigmoid stimulation develops hyperalgesia in normosensitive IBS patients [304–306]. GMCs that strongly compress the colon wall send afferent signals similar to those of distension of the wall by a balloon. Therefore, an increase in the frequency of GMCs could be one of the factors inducing visceral hypersensitivity.
A unifying hypothesis, based on accumulated basic science and clinical data, is that GMCs are the source of abdominal cramping. Visceral hypersensitivity, if present, worsens the sensation of abdominal cramping. Figure 37 explains the sensation of abdominal cramping with and without visceral hypersensitivity. First, the afferent signals generated by a GMC in health are below the nociceptive threshold (Figure 37A), so they do not cause the sensation of abdominal cramping. The amplitude of GMCs increases more than twofold in IBS-D patients . The afferent signals they generate are noxious (Figure 37B). Each GMC may induce the sensation of abdominal cramping; however, concurrent visceral hypersensitivity will exaggerate the pain [295, 301]. Figure 37C shows a scenario in which abdominal cramping is entirely due to visceral hypersensitivity. In this case, a GMC of normal or below-normal amplitude will induce the sensation of cramping. The intensity of pain will relate to the degree of hypersensitivity.
There is no evidence that the nociceptive threshold decreases to levels where the afferent signals generated by RPCs become noxious. Were this to happen, patients would feel a continuous sensation of pain, because RPCs are always present somewhere in the colon. By contrast, GMCs occur only a limited number of times per day, even in IBS patients who have more. Therefore, abdominal cramping occurs intermittently and only for the duration of a GMC.
Figure 37D illustrates another scenario in which abdominal cramping may occur with or without visceral hypersensitivity. Impairment of descending inhibition prevents relaxation of the receiving segment ahead of it. Receptive relaxation decreases in some IBS patients . In this situation, the afferent signals due to ballooning of the receiving segment will add to those of the GMCs to become a noxious signal. This is likely to happen when impairment of descending inhibition prevents relaxation of the internal anal sphincter as the GMC is attempting to push feces through for defecation or when voluntary relaxation of the puborectalis muscle and the external sphincter are impaired. Another potential scenario is when a GMC attempts to push fecal material past compacted stool in constipated patients (Figure 42).
- About 90% of IBS patients report intermittent short-lived abdominal cramping. However, only about 50% of these patients have visceral hypersensitivity.
- Visceral hypersensitivity does not correlate well with most symptoms of IBS.
- GMC is the stimulus from the periphery that sends perceptible signals to the CNS. At higher amplitudes of GMCs, these signals are noxious and produce the sensation of abdominal cramping/pain that lasts for the total duration of a GMC.
- Concurrent visceral hypersensitivity and/or failure of descending relaxation enhance the afferent signals generated by a GMC. When this happens, normal-amplitude GMCs produce the sensation of cramping/pain.
- The frequency of GMCs relates to the symptoms of diarrhea and constipation in IBS patients. A significant increase in GMC frequency causes diarrhea, while a significant decrease causes constipation.
- The frequency of GMCs may serve as a biomarker of IBS subtypes.
Stress is an adaptive physiological response of living systems to real or perceived life-threatening situations. This response begins in the CNS. The release of corticotrophin-releasing hormone (CRH) from the paraventricular nucleus of the hypothalamus is an early and essential step in the stress response . The central release of CRH and other mediators, such as arginine vasopressin (AVP), stimulate the neuroendocrine system comprised of autonomic neurons and the HPA axis, which modulate the adaptive and maladaptive responses of peripheral organs in a stress- and cell-type-specific manner. Nontranscriptional mechanisms largely mediate the immediate and short-term effects of acute stress. For example, acute stress releases norepinephrine in the amygdala and hypothalamus to sharpen focus and attention . It also increases the heart rate and blood flow in preparation for the “fight-or-flight” response .
The HPA axis and the sympathetic nervous system show subtle alterations in IBS patients in the resting state and after stressors [310–321]. Acute psychological as well as physical stress modestly stimulate colonic motor activity. Animal studies show that hypothalamic release of CRH and vagal nerves mediate the stimulation of colonic motor function by acute stress [322–324].
Acute stress modestly reduces the thresholds to colorectal distension in IBS patients relative to normal subjects, presumably due to baseline alterations in the HPA axis and the autonomic nervous system [310, 325, 326]. However, we do not know the cause-and-effect relationship between individual mediators of stress and transient sensitization of visceral afferents.
The effects of acute stress are transient, lasting more or less for the duration of the stressor. Clinical studies show that chronic stress, as opposed to acute stress, precipitates/relapses or exacerbates the symptoms of IBS [327, 328]. This is not surprising, because stress targets some of the same physiological functions already impaired in IBS patients, i.e., altered motor function and visceral hypersensitivity to motor events in the colon.
The mechanisms by which chronic stress relapses or exaggerates the symptoms of IBS are not investigable in patients due to ethical considerations and lack of availability of neuromuscular tissues. Animal studies show that heterotypic or homotypic intermittent chronic stress (HeICS and HoICS, respectively) induces visceral hypersensitivity in rats that persists after the stress is over by the following mechanisms [329, 330] (Figure 43).
- Chronic stress releases CRH and angiotensin vasopressin from the paraventricular nucleus in the hypothalamus.
- CRH and arginine vasopressin stimulate the locus ceruleus/norepinephrine system. In parallel, CRH releases adrenocorticotropic hormone from the pituitary, which releases corticosterone from the adrenal cortex.
- Norepinephrine enhances the expression of NGF in the colon wall.
- NGF complexes with trkA receptors, and the complex transports retrograde to the thoracolumbar DRG .
- NGF/trkA complex in the DRG sensitizes the ion channels.
- Hypersensitization of these ion channels amplifies the afferent signals in response to colonic distension/compression to increase perception. This sensitization occurs in the absence of a detectable inflammatory response in the muscularis externa or in the mucosa/submucosa.
Based on the topology and phenotypes of afferent nerve endings in the colon wall [245–247, 335, 336], an increase in NGF in the muscularis externa mediates the induction of visceral hypersensitivity by HeICS, whereas an increase in NGF in the mucosa/submucosa mediates an altered physiological response to digesta in the lumen.
The systemic upregulation of norepinephrine by HeICS also enhances the reactivity of colonic circular smooth muscle cells to ACh in muscle strips as well as in single isolated cells, resulting in an increase in colonic transit and pellet defecation, producing diarrhealike conditions in rats  (Figure 44). Adrenalectomy, but not the depletion of sympathetic neurons by guanethidine, blocks these effects. Corticosterone, CRH, or vagal nerves do not mediate these effects.
Norepinephrine enhances expression of the pore-forming α1C1b subunit of Cav1.2b channels in circular smooth muscle cells, which increases Ca2+ influx in response to ACh  (see Figure 11) to enhance the amplitude of contractions and hence colonic transit. These effects peak at about 8 hours after the last stressor and return to baseline by 24 hours . These findings show that prolonged upregulation of plasma norepinephrine by chronic stress remodels the cellular regulatory mechanisms, resulting in organ dysfunction. Similar remodeling occurs in CNS neurons and cardiac muscle cells [337–340]. Acute chronic stress does not induce these effects. By contrast, HoICS induces hyperalgesia in rats that lasts up to 40 days . The prolonged effects of chronic stress in animal models is consistent with clinical observations that the symptoms of IBS improve with the resolution of major life stressors.
- Chronic, rather than acute, stress in animal models produces prolonged motor dysfunction and visceral hypersensitivity.
- Chronic stress precipitates/exaggerates the symptoms of IBS.
- Sustained increase in plasma norepinephrine following chronic stress makes a major contribution to the development of visceral hypersensitivity and altered motor function.
- Increase in the expression of NGF in the colonic muscularis externa mediates the induction of visceral hypersensitivity by norepinephrine.
- The retrograde transport of NGF/trkA complex sensitizes the colon-specific DRG neurons.
Early-Life Trauma and IBS
Retrospective studies show that prenatal, infant, or childhood trauma predisposes to developing the symptoms of IBS at an early age, which continue in adulthood [341–350]. Animal models of neonatal trauma support the hypothesis that early-life trauma results in visceral hypersensitivity to colorectal distension and/or motility dysfunction in adulthood.
Mechanical or chemical irritation in neonates results in persistent sensitization of the spinal afferents and visceral hypersensitivity to colorectal distension in adulthood . Maternal separation of neonatal rats induces allodynia and hyperalgesia in adulthood by enhancing expression of NGF in the colon wall [352, 353]. In this model, the proliferation and degranulation of mast cells increase the expression of NGF, which mediates hypersensitivity to colorectal distension. The maternally separated rats also show heightened response to acute water avoidance stress.
A randomized double-blind placebo-controlled study found little improvement in symptoms of IBS- PI patients by prednisone treatment . Therefore, it is not clear whether neonatal maternal separation represents a model of IBS or IBD. Regardless, we do not know yet the epigenetic mechanisms, described later, that underlie colonic motor dysfunction and visceral hypersensitivity in response to adverse early life experiences.
Neonatal inflammation on postnatal day 10 (PND 10) significantly enhances the mRNA and protein expression of the α1C-subunit of Cav1.2 (L-type) calcium channels, Gαq, and the regulatory myosin light chain kinase (RLC20) in adulthood . The enhanced expression of each of these cell-signaling proteins favors increased reactivity to ACh (see Figure 11). As a result, the contractile responses of single smooth muscle cells and of circular smooth muscle strips from affected rats are greater than those from control rats. The faster colonic transit and greater pellet output in these rats simulate the diarrhealike conditions of IBS-D patients.
The neonatal insult in these rats also enhances the VIP content of muscularis externa and plasma concentrations of norepinephrine. The motility dysfunction in adult rats who received neonatal inflammatory insult occurs in the absence of any inflammation or structural damage. Of note, a similar inflammatory insult in adult rats does not result in enhancement of smooth muscle reactivity to ACh or faster colonic transit .
Note that there is seldom a perfect animal model of human disease. However, these models closely mimic specific features of IBS and their regulatory mechanisms. They are indispensable in identifying the underlying mechanisms of organ dysfunction, allowing for testing of hypotheses in humans and development of therapeutic agents.
- Neonatal psychological and inflammatory insults induce visceral hypersensitivity and motor dysfunction in adulthood.
- The maladaptive effects of chronic stress on gut function—altered motor function and visceral hypersensitivity to motor events in the colon—are the same as those that characterize IBS patients. However, the mechanisms by which chronic stress exaggerates these effects in IBS patients may be different from those that underlie abnormal functions without stress.
Impaired Enteric Reflexes
Balloon distension in in vitro experiments in the intact human colon stimulates contractions above and relaxation below it . The ascending stimulation—mediated by the release of ACh—is blunted in the colon of slow-transit patients , which agrees with other findings that the synthesis and/or release of ACh and the excitation-contraction coupling are impaired in constipated patients. However, the descending relaxation—mediated by NO—is not different between slow-transit constipation patients and healthy controls, suggesting a normal function of inhibitory motor neurons. In vitro findings in muscle strips from patients with idiopathic chronic constipation support the notion that their nitrergic neurons are functioning near normal . The normality of inhibitory neuronal function, however, may not be universal in constipation. One study found enhanced NO-induced and ATP-induced relaxation in a group of idiopathic chronic constipation patients .
Impaired release of ACh proximal to the site of balloon distension confirms the defects in the synthesis/release of ACh and/or excitation-contraction coupling in smooth muscle cells in slow-transit patients.
Impaired Smooth Muscle Excitation-Contraction Coupling in Slow-Transit Constipation
The prevalence and severity of slow-transit constipation are higher in females than in males [356, 357]. Alterations in cell-signaling proteins of excitation-contraction coupling in smooth muscle cells in response to progesterone partly explain this disparity. Progesterone acting on its nuclear receptors regulates the expression of some G proteins (Gαq and Gαi3) negatively and others (Gαs) positively [358–360]. Progesterone levels in females with slow-transit constipation are normal. However, due to genetic/epigenetic abnormality, these patients overexpress progesterone B (PGR-B) receptors on colonic smooth muscle cells. As a result, transcription and protein expression of Gαq decrease, while those of Gαs increase. The suppression of Gαq reduces the binding of ligands such as ACh and CCK to their respective receptors coupled with this G protein, resulting in reduction in smooth muscle contractility in response to ligands (Figure 45). The contractile response to diacylglycerol and KCl, which bypass the Gαq protein, remains intact, indicating normality of the rest of excitation-contraction coupling (see Figure 11). Incidentally, progesterone concurrently suppresses the expression of COX-1 and enhances that of COX-2 , decreasing the generation of thromboxane A2 (TxA2) and prostaglandin F2α (PGF2α) and increasing the expression of prostaglandin E2 (PGE2). PGF2α and TxA2 contract smooth muscle cells, while PGE2 inhibits these contractions. However, the contribution of these pathways in spontaneous colonic motor function is unknown.
Upregulation of progesterone B receptors in smooth muscle cells in the human colon explain the higher incidence of slow-transit constipation in female patients.
Role of ICCs
Some reports found a deficiency in the volume of ICC throughout the colon of patients with slow-transit constipation [99, 361–364]. However, these publications did not establish a cause-and-effect relationship between the reduction in the volume of ICC and patient symptoms, disorders in colonic motor activity, or its regulatory mechanisms. According to numerous clinical studies cited above, the slow transit in these patients is primarily due to the reduction in GMCs. There is no evidence that ICC regulate GMCs, which occur independently of slow waves. A recent study has demonstrated that ICC do not mediate the neuronal input to smooth muscle cells . The normal function of inhibitory nitrergic motor neurons in descending inhibition is additional evidence that reduction in the volume of ICC in constipated patients does not mediate neuronal input to smooth muscle cells [79, 365]. The RPCs regulated by slow waves play a relatively smaller role in slow-transit constipation. However, the slow waves do not show a defect in in vitro recordings from the colonic smooth muscle cells of slow-transit patients . The slow-wave frequency and its spatial organization are not different between IBS patients and healthy controls under resting conditions and after stimulation with a meal or neostigmine .
- The volume of ICC is decreased in the colon of slow-transit constipation patients. However, there is no evidence that this decrease causes motility dysfunction or visceral hypersensitivity.
- Both the frequency of slow waves and nitrergic neuronal function are normal in these patients.
Role of Alterations in the Expression of Neuropeptides in the Myenteric Plexus and Structural Damage to Enteric Neurons and Smooth Muscle Cells
Several immunohistochemical, radioimmunoassay, and ultrastructural studies have identified abnormalities in enteric neurons and smooth muscle cells in tissue from IBS patients [367–374]. Most of these studies are on tissues obtained from severely constipated patients undergoing colonic resection. Disappointingly, these findings are often divergent; some show a positive change, some show a negative one, and others find no change in the same parameter, such as damage to neurons containing a certain neurotransmitter or global damage to neurons . This is partly due to the qualitative nature of analysis in these methods and the heterogeneity of tissues and observations at the microscopic level. Another major limitation is the absence of efforts to establish a cause-and-effect relationship between the findings and functional impairment. These approaches have been very helpful in identifying the cause of a disease when the defect is simple and confined to one type of cell, such as aganglionosis in Hirschsprung’s disease. However, these approaches seem to be of limited use in complex diseases like IBS.
Over the past three decades, discoveries of gene mutations that cause or contribute to simple Mendelian diseases, such as sickle cell anemia, hemophilia, and cystic fibrosis have been reported [375–377]. However, the search for gene mutation that causes complex diseases, such as diabetes, most cancers, asthma, inflammatory bowel disease, and functional bowel disorders has largely been unsuccessful. Complex diseases exhibit an inheritable component but do not follow Mendel’s laws. For example, discordance of monozygotic twins reaches 30%–50% in diabetes, 70% in multiple sclerosis and rheumatoid arthritis, and 80% in breast cancer . Differential environmental factors during fetal and neonatal development usually account for discordance of monozygotic twins. The simple diseases following Mendel’s laws begin predominantly before puberty , whereas complex diseases tend to appear later in life and may exhibit more than one peak of increased risk of onset . The simple diseases progressively worsen after onset, whereas complex diseases, such as major psychosis, inflammatory bowel disease, functional bowel disorders, and rheumatoid arthritis, exhibit relapses and remissions. Epigenetics plays a prominent role in cancer and autoimmune and inflammatory diseases [381–386].
The inherited genetic code is identical in all cell types in an organism, with the exception of a few, such as the gametes . During ontogeny, epigenetic mechanisms set the transcription rates of each gene in the genome ranging from complete silence to full activation, imparting phenotype to each cell. The transcription rates of different genes are set for survival of the fetus and the neonate as well as for optimal responses of the cells to their microenvironment of hormones, neurotransmitters, growth factors, and inflammatory mediators in adulthood. However, if the fetus (indirectly through the mother) or the neonate is exposed to psychological or inflammatory stress, the transcription rates of genes vulnerable at the time of insult may be set at abnormal levels, ensuring current survival but leading to abnormal cell function in adulthood, causing a complex disease. This is known as Barker’s hypothesis  or neonatal/fetal programming.
Epigenetic regulation during neonatal inflammatory or psychological stress can modify gene expression by post-transcriptional histone modifications and by DNA methylation.
Posttranslational Histone Modifications.
DNA is packaged tightly into a highly organized and dynamic protein-DNA complex called chromatin. The basic subunit of chromatin is the nucleosome, which contains about 146 bp of DNA wrapped twice around an octomer core of four histones (two molecules each of histones H2A, H2B, H3, and H4) in a 1.65 left-handed superhelical turn [389–392] (Figure 46).
Normally, the histone proteins are positively charged and form tight electrostatic associations with negatively charged DNA, which results in tight compaction of chromatin and inaccessibility of the DNA to transcription factors and transcriptional machinery. The N-terminal tails are the main sites of posttranslational modifications including acetylation, methylation, phosphorylation, citrullination, sumoylation, ubiquitination, and ADP-ribosylation by enzymes, and this affects their function in gene regulation . Acetylation, one of the most widespread modifications of histone proteins, including H2B, H3, and H4, occurs on lysine residues in the N-terminal tail and on the surface of the nucleosome core as part of gene regulation . The addition of an acetyl group to histone proteins reduces their positive charge to form a more relaxed configuration with DNA, which allows the transcription factors and transcriptional machinery access to their recognition sites on the promoters of specific genes to induce transcription.
The opposing actions of histone acetyltransferases (HATs) and histone deacetylases (HDACs) control the acetyl group turnover. The HATs are present as part of large protein complexes and act as transcriptional coactivators. The deacetylases (HDACs) are recruited to target genes via their direct association with transcriptional activators and repressors, as well as their incorporation into large multiprotein transcriptional complexes . Together, these two classes of enzymes account for the coordinated changes in chromatin structure that carry out its functions [395, 396]. The balance between the actions of these enzymes is a key regulatory mechanism for gene expression and governs numerous developmental processes and disease states . Lysine acetylation is associated with active gene expression and open chromatin. H3K9ac and H4K16ac are two histone modifications often associated with euchromatin. Chromatin immunoprecipitation (ChIP) assay shows that neonatal colonic inflammation significantly increases the association of RNA polymerase II (RNAP II) with the core promoter region of the Cacna1c gene in adulthood, which would increase the transcription rate of this gene (Figure 47).
Methylation of lysine and arginine residues can occur in histones H3 and H4, in the mono-, di-, or tri-methylated form . Depending on the site and type of histone, the methylation pattern will result in a different transcriptional outcome. Methylation of H3K9, H3K27, and H4K20 links generally to heterochromatin formation, whereas methylation of H3K4 and H3K36 associates with transcriptionally active regions. Di- and tri-methylation of histone H3 lysine 4 (H3K4me2 and H3K4me3) are hallmarks of chromatin at active genes .
Covalent addition of methyl groups, catalyzed by enzymes known as DNA methyltransferases (DNMTs), modifies DNA to alter gene transcription. DNA methylation occurs at specific dinucleotide sites along the genome, cytosines 5' of guanines (CpG sites). About 40% to 50% of the protein-coding genes have GC-rich sequences in their promoter regions, known as CpG islands, and about 70% to 80% of all CpG dinucleotides in the genome are methylated . DNA methylation affects the correct temporal and spatial silencing of gene expression during development and during disease processes such as tumor progression . The methylation of CpG islands restricts the access of transcription factors to the promoter region, thereby suppressing transcription of the targeted genes .
Four members of DNA methylation transferases (DNMTs) regulate DNA methylation in mammals. DNMT1 has a high affinity for the hemimethylated form of DNA, maintaining the constitutive methylation status of the DNA . DNMT2 does not have a DNA-binding domain, and its role in DNA methylation is unknown . By contrast, the roles of DNMT3a and DNMT3b in regulating DNA methylation in oncogenesis and in response to stressors are well established .
Functional bowel disorders do not have the traits of genetic diseases. Genetic alterations (mutations and polymorphisms) inherited from parents or mutations due to environmental factors once acquired are irreversible. Mutations in a gene may produce a wrong protein or no protein at all; polymorphisms may produce a variant protein. The functional effects of mutations and polymorphisms are stable.
By contrast, the severity and types of symptoms in functional bowel disorders vary, arguing against a genetic component [404, 405]. The symptoms of altered bowel function in IBS-C/D patients switch from one extreme to the other. Acute events such as stress precipitate/exaggerate the symptoms of functional bowel disorders . All these characteristics of functional bowel disorders suggest fluctuating expression of proteins causing dysfunction, a result of epigenetic regulation rather than genetic variance. Epigenetic mechanisms, discussed above, can alter the expression of target proteins in target cells, such as smooth muscle cells and afferent neurons, in response to changes in their microenvironment.
- Epigenetic regulation modifies the expression of selective genes in cells following changes in their microenvironment.
- If the changes in microenvironment occur during the vulnerable stages of fetal and neonatal development, the changes in expressions of selective genes may persist into adulthood to cause complex diseases, such as IBS.
- The relapsing/recurring changes in symptoms of IBS do not make them candidates for genetic mutations/polymorphisms.
Inflammatory Bowel Disease
Inflammatory bowel disease (IBD), comprised of ulcerative colitis and Crohn’s disease, is a chronic, idiopathic, and relapsing inflammation of the gut. Ulcerative colitis usually begins in the rectum/distal colon and progresses orally. Crohn’s disease usually begins in the terminal ileum but may extend to other areas of the gastrointestinal tract, especially the colon (Crohn’s colitis). The two types of IBD are clinically, immunologically, and morphologically distinct. In spite of differing etiologies, the primary symptoms of both types of IBD (diarrhea, abdominal cramping, and urgency of defecation) are strikingly similar. Stools of ulcerative colitis patients are bloody and contain mucus.
IBD patients present with motor diarrhea (diarrhee motrice), frequent nonwatery stools . The daily frequency of unformed stools is about five times per day in mild to moderate pancolitis and four times per day in mild to moderate distal colitis. These numbers increase with severity of colitis. About 80% to 90% of pancolitis patients show urgency and nocturnal defecation, and 30% have incontinence . About 80% of these patients report incomplete evacuation. Paradoxically, about 20% to 30% of pancolitis and distal colitis patients pass hard stools . The total gut transit in ulcerative colitis patients is not different from that in healthy controls . However, the proximal colon shows stasis while the rectosigmoid colon shows rapid propulsion, which counteract each other to produce normal whole colon transit [409–412].
Motility Dysfunction in Colonic Inflammation
Limited data are available from manometric recordings in ulcerative colitis patients due to the risk of perforation; much less is available from Crohn’s colitis patients. However, the disturbances in small intestinal motor activity in Crohn’s disease are similar to those seen in the colon of ulcerative colitis patients . Much of our understanding of motility dysfunction in both types of IBD has come from animal models of inflammation.
Studies in IBD patients and in experimental models show that inflammation suppresses RPCs and tonic contractions, at the same time enhancing the frequency of GMCs [19, 166, 409, 414–416]. The degree of suppression of RPCs and increase in the frequency of GMCs are independent variables, but each correlates with the intensity of inflammation and clinical symptoms [166, 409]. The stimulation of GMCs and suppression of RPCs are most intense in the inflamed part of the colon. However, inflammation in one part of a gut organ can reflexively alter motility function at distal locations , which means that colitis in the distal colon may suppress RPCs in the middle and the proximal colon.
The above motility dysfunctions explain most of the observed clinical symptoms in IBD patients.
Note that most studies of ulcerative colitis have recruited patients with mild to moderate colitis. Patients with severe colitis are likely to have more intense motility dysfunction, as judged by inflammation in experimental models.
In one group of patients with moderate colitis, the frequency of GMCs increased about twofold over that in healthy controls . The increased frequency of GMCs produces frequent mass movements. The concurrent suppression of RPCs facilitates distal propulsion of luminal contents. The GMCs that propagate up to the rectum or the distal sigmoid colon stimulate afferent signals to generate urges to defecate as well as causing descending relaxation of the internal anal sphincter in preparation for defecation. A strong GMC propagating to the rectum can result in involuntary defecation (fecal incontinence). It is noteworthy that even though the frequency of GMCs increases in colonic inflammation, it still occurs no more than 10 to 15 times per day in moderate colitis. The frequent rapid propulsion by GMCs reduces the contact time of fecal material with the inflamed mucosa to reduce absorption of water and electrolytes. In addition, the concurrent suppression of RPCs reduces the mixing and turning over of fecal material to reduce its total exposure to the mucosa. Together, these two factors result in unformed, but not watery, stools. Note that the degree of stool softness depends on the intensity of inflammation, which stimulates GMCs and suppresses RPCs.
The GMCs compress the colon wall very strongly because of their large amplitudes (>100 mm Hg). Excessive occurrence of GMCs causes hemorrhages, thick mucus secretion, and mucosal erosions in experimental models . These lesions explain the bloody stools with mucus characteristic of ulcerative colitis. While the GMCs are also the driving force for diarrhea in IBS-D patients, their mucosa is not inflamed and fragile as in ulcerative colitis patients. So while IBS-D patients have diarrhea, they do not have bloody stools. The higher frequency of GMCs propagating up to the rectum in the inflamed colon induces frequent bowel movements in ulcerative colitis patients (motor diarrhea).
In a canine model of moderately severe acute pancolitis, the frequency of GMCs increased more than 10-fold . About half of these GMCs propagated to the sigmoid colon, resulting in uncontrollable defecation (urgency). The rest occasionally expelled gas and caused tenesmus, which may result if a GMC generates the urge to defecate in the absence of any stool in the rectum. The false urges caused by GMCs in an empty distal colon may also generate the sensation of incomplete evacuation. These symptoms and abnormal motility cease on recovery from inflammation.
About 20% of ulcerative colitis patients pass hard stools , giving the perception that they are constipated. The ascending colon in colitis patients shows stasis, while the sigmoid colon shows rapid transit . Concurrent manometric recordings from the ascending and sigmoid colons of these patients are not available. However, on a speculative note, stasis in the ascending may result if inflammation in the sigmoid colon reflexively suppresses both RPCs and GMCs in the proximal colon, thus prolonging stool transit and forming hard stools. However, when these hard stools reach the inflamed sigmoid colon, the frequently occurring GMCs propel them rapidly, so that the passing of hard stools gives the impression of constipation.
One study in patients with inactive Crohn’s ileitis reported suppression of small intestinal RPCs and stimulation of GMCs . These effects are similar to the colonic motor dysfunction seen in ulcerative colitis patients. Animal models of ileal inflammation confirm these findings . Ileal inflammation suppresses RPCs in the ileum as well as proximal to it, extending up to the stomach. Many of the GMCs stimulated by ileal inflammation propagate up to the terminal ileum. The animals are visibly uncomfortable during the passage of an ileal GMC. The frequency of bowel movements increases several-fold due to ileal inflammation . Spontaneous GMCs in the ileum occur primarily in the interdigestive state . However, in ileal inflammation, they also occur after a meal, resulting in rapid emptying of undigested food and bile from the ileum into the colon.
The increase in the incidence of GMCs in the ileum, by itself, cannot induce frequent defecation. Colon involvement is necessary. Animal studies show that many GMCs originating in the ileum propagate to the colon, causing uncontrollable defecation if they propagate to the sigmoid colon . Furthermore, postprandial GMCs occurring during ileal inflammation rapidly transfer undigested chyme into the colon, which increases its osmotic load to suppress RPCs and stimulate colonic GMCs . In an animal model of ileal inflammation, a collection cannula located distal to the inflamed segment of the ileum collected copious discharge of mucus with fresh blood . These data indicate that an excessively high incidence of GMCs in the inflamed ileum likely causes the severe hemorrhage seen in some Crohn’s disease patients [421, 422].
- Increase in the frequency of GMCs and suppression of RPCs characterize colonic motor dysfunction in IBD patients.
- Frequent mass movements by GMCs cause diarrhea and urgency.
- A difference between IBS-D and IBD patients is that RPCs are suppressed in IBD patients but not in IBS-D patients.
- Strong compression of the colon wall with inflamed mucosa by GMCs causes hemorrhage. Hemorrhage does not occur in IBS-D patients because their mucosa is not fragile.
Visceral Hypersensitivity in IBD
The sensation of pain in IBD patients is generally located in the lower abdomen and rectal areas. Most information on visceral hypersensitivity in these patients comes from distension studies in the rectum. There are two schools of thought regarding rectal hypersensitivity in IBD patients. One is that the rectum is hypersensitive to balloon distension in patients with moderate colitis, when compared with healthy subjects or patients in remission [408, 414, 423]. These patients present with diarrhea, urgency, feeling of incomplete evacuation, tenesmus, incontinence, and intermittent lower abdominal pain. The rectum in patients with active colitis is less compliant than in controls or in quiescent colitis. The other school of thought is that the rectum is hyposensitive in mild or inactive ulcerative colitis or when active inflammation is in the ileum (Crohn’s disease) [424, 425]. Data from distension studies in the sigmoid colon are not available.
The visceral hypersensitivity that accompanies inflammation is due to the upregulation of neurotrophin growth factor (NGF) in response to the enhanced production of inflammatory mediators in the colon wall [426, 427]. Animal models of inflammation show consistent visceral hypersensitivity, which subsides after inflammation is over [428–432].
Rectal hypersensitivity in moderate to severe inflammation explains the frequent urge to defecate in response to the arrival of smaller volumes of feces in the rectum. The descending inhibition of the internal anal sphincter in response to rectal distension remains intact in colitis patients , suggesting that the internal anal sphincter does not obstruct the mass propulsion by a propagating GMC preceding defecation. The perception of pain in these patients is therefore entirely due to strong compression of the colon wall and sensitization of the afferent splanchnic neurons. Colitis patients in remission are relatively free of symptoms because the events precipitating them—excessive frequency of GMCs—are absent. This may happen regardless of whether the afferent sensitization normalizes during remission.
Strong compression of the sigmoid colon along with visceral hypersensitivity causes the sensation of intermittent short-lived pain in IBD patients.
Increased expression of NGF in the colon wall mediates visceral hypersensitivity in animal models of colonic inflammation.
Cellular and Molecular Mechanisms
A great deal of our understanding of the cellular mechanisms of motility dysfunction in colonic inflammation has come from animal models of inflammation . The animal models of IBD fairly well replicate the acute inflammatory component of human disease; however, they lack the remission and relapse features. While the animal models have these limitations, they have the advantage of having more or less similar lesions within the study group, and they are free of disease modification by medications. In many cases, the animals serve as their own controls.
Smooth Muscle Dysfunction
Organ bath studies show that circular smooth muscle tissues from human ulcerative colitis [434, 435], Crohn’s disease , and their animal models [12, 17, 437–439] are less reactive to ACh than the tissue from respective controls. ACh acts directly on muscarinic M3 receptors on smooth muscle cells to stimulate contractions. Therefore, the suppression of contractility in inflammation is due, in part, to a defect in the excitation-contraction coupling in smooth muscle cells. Studies in human tissue from ulcerative colitis patients  show little change in the characteristics of slow waves. The nitrergic nerves also seem to function normally in tissue from ulcerative colitis patients, which concurs with normal relaxation of the anal sphincter in response to rectal distension .
A major abnormality contributing to the suppression of contractility by inflammation seems to be in the excitation-contraction coupling in smooth muscle cells. TNFα and IL-1β, prominent inflammatory mediators, significantly suppress expression of the pore-forming α1C-subunit of Cav1.2b (L-type) calcium channels in human and animal colonic circular smooth muscle cells [62, 177, 441–443]. These inflammatory mediators activate NF-κB, which translocates to the nucleus to suppress transcription of the gene encoding the α1C-subunit. The suppression of the α1C-subunit reduces the number of calcium channels in smooth muscle membrane and the calcium influx/inward calcium current moving through them [62, 173]. The inhibition of NF-κB activation, in vivo or in vitro, blocks the suppression of Cav1.2 channels to restore cell contractility.
The conventional perception is that inflammation in Crohn’s disease is transmural, while that in ulcerative colitis is limited to the mucosa. This concept might have developed from morphological observations of significant infiltration of white blood cells in the muscle layers of Crohn’s disease but not in those of ulcerative colitis. However, this concept is not consistent with the fact that inflammation in both types of IBD similarly suppresses circular muscle contractility [413, 434–436]. There is no known mechanism by which inflammation confined to the mucosa impairs smooth muscle function, since smooth muscle impairment in inflammation requires local release of inflammatory mediators in the muscularis externa.
Studies on the experimental models of Crohn’s colitis and ulcerative colitis—trinitrobenzene sulfonic (TNBS) acid- and dextran sodium (DSS)-induced colonic inflammations, respectively [433, 444, 445]—show that the inflammatory mediators and their genetic targets to suppress circular smooth muscle contractility differ markedly between the two types of colonic inflammation.
Recent studies in animal models of the two forms of IBD and accumulating clinical findings [446, 447] suggest that inflammation is transmural in both forms of IBD. Crohn’s colitis–like inflammation is due to transmural generation of oxidative stress and peptide inflammatory mediators. The ulcerative colitis–like inflammation is primarily due to transmural generation of oxidative stress. Peptide inflammatory mediators play a minor role in ulcerative colitis–like inflammation. Oxidative stress (H2O2) suppresses the Gαq protein of the excitation-contraction coupling in smooth muscle cells to suppress their contractility. By contrast, cytokines, such as IL-1β, suppress the α1C-subunit and CPI-17 proteins of the excitation-contraction coupling to suppress circular smooth muscle reactivity to ACh .
Both types of inflammation begin with a breakdown of the mucosal barrier, exposing the sterile interior of the colon wall to a pathogenic luminal environment. The breakdown of the mucosal barrier by TNBS results in the translocation of luminal bacteria across the colon wall within 24 hours . TNBS impairs the epithelial barrier function by necrosis. By contrast, Toll-like receptor 4 (TLR4) signaling, which limits bacterial translocation, mediates DSS inflammatory response [449, 450]. DSS arrests the epithelial cell cycle, resulting in apoptosis, impaired proliferation, and weak release of peptide inflammatory mediators [451–453]. DSS inflammation can occur in germ-free or severely-combined-immunodeficiency (SCID) mice [454, 455]. Consequently, bacterial translocation is marginal and confined to the mucosa, indicating its lesser role in DSS inflammation than in TNBS inflammation.
Taken together, aggressive bacterial translocation in TNBS inflammation may underlie the transmural infiltration of immune cells and release of cytokines/chemokines. On the other hand, limited bacterial translocation results in much smaller infiltration of immune cells and release of cytokines/chemokines in the mucosa/submucosa of DSS inflammation. It is noteworthy that TNBS inflammation in the absence of intestinal flora is also primarily mucosal . The differences in the nature of the damage to the epithelium (e.g., apoptosis and necrosis) may underlie the two strikingly different types of inflammatory responses in TNBS and DSS.
Enteric Neuronal Dysfunction
Together with smooth muscle cells, the enteric neurons play an essential role in regulating motility function. They are in the same hostile inflammatory environment as the smooth muscle cells, but their precise role in impaired motility dysfunction in colonic inflammation remains ambiguous. This is largely due to the lack of availability of neuronal cultures until recently , the multiple types of neurons containing more than one neurotransmitter, and our limited ability to correlate neuronal abnormality with motor dysfunction. Immunohistochemical studies on inflamed and normal tissues have yielded mixed results [457–461].
Morphological data show that inflammation does not alter the density of neurons innervating circular smooth muscle cells . However, it may impair the packaging, storage, and release of neurotransmitters from the nerve endings of motor and sympathetic neurons [463–465]. Impairment in the synthesis/release of ACh will suppress in vivo motor activity by reduced stimulation of excitation-contraction coupling in smooth muscle cells. Electrophysiological studies show that inflammation in guinea pig colon enhances the excitability of AH neurons and facilitates synaptic transmission in S neurons [466, 467]. However, we do not know yet how these changes relate to the suppression of neurotransmitter release, suppression of RPCs, and the stimulation of GMCs during inflammation.
The number of ICC-MP in the affected areas of Crohn’s disease does not differ from that in controls, whereas the number of ICC-IM decreases and that of ICC-DMP increases . However, recent publications have discounted any role of ICC in regulating motility function [79, 365, 468]. In spite of the changes found in the number of ICCs or damage to their processes, the slow waves and nitrergic inhibition seem to be normal in inflammation, as discussed above.
- Impairment of excitation-contraction coupling in smooth muscle cells due to suppression of key cell-signaling proteins by inflammatory mediators contributes to the suppression of RPCs and tone in the colons of human IBD patients and in animal models of inflammation.
- Inflammation impairs the release of neurotransmitters from enteric motor neurons.
- We do not know the cellular mechanisms by which colonic inflammation enhances the frequency of GMCs.
Diverticular disease is prevalent in up to 30% of the population over sixty years of age—about 15% of these patients go on to develop clinical symptoms [469–473]. Clinically, these patients are divided into three categories: asymptomatic diverticular disease, symptomatic uncomplicated diverticular disease, and symptomatic complicated diverticular disease. Some complications of diverticular disease—perforation, fistula, or bowel obstruction—relate to the severity and duration of colitis. The following discussion focuses primarily on asymptomatic and symptomatic diverticular disease patients.
The symptoms of diverticular disease include recurrent abdominal pain in the lower left quadrant and altered bowel habits: diarrhea, constipation, or alternating diarrhea (loose stools) and constipation (hard stools). Additional secondary symptoms are bloating, straining, urgency, incontinence, and mucus and blood in stools [404, 473–475]. These symptoms generally develop in patients over the age of 50 years. Low fiber in the diet is a likely contributor to its higher prevalence in Western counties. However, there is no hard evidence for it. The diverticula form primarily in the sigmoid colon. The severity of the symptoms relates to the degree of diverticulitis .
The symptoms of diverticulitis overlap with those of IBD and IBS-D, i.e., abdominal cramping accompanied by altered bowel habits. However, the etiologies of the two conditions may differ to some degree. In IBS-D, inflammation plays little role in the induction of these symptoms. A randomized double-blind placebo-controlled study found little improvement in IBS symptoms in IBS-PI patients by prednisone treatment . We do not fully understand the events leading up to inflammation in IBD patients. However, in IBD, inflammation evenly covers the affected segment. Prednisone treatment is a major therapy in IBD patients. In diverticulitis, the inflammation starts by the translocation of pathogenic fecal material into the diverticula, causing abscess formation. Therefore, in diverticular disease, inflammation occurs in pockets centered on diverticula, and it may be unevenly distributed through the muscle layer. The circular muscle layer in diverticulitis shows hypertrophy and hyperplasia [473, 477, 478].
Colonic Motor Dysfunction in Diverticular Disease Patients
Manometric recordings show a higher incidence of GMCs in the sigmoid colon (and distal to it) in symptomatic diverticular disease patients than in asymptomatic patients or healthy controls. Overall motor activity, quantified as total duration of contractions, is also higher in symptomatic (complicated or uncomplicated) diverticular disease patients than in asymptomatic patients or in normal healthy subjects [476, 479, 480].
- Although, diverticular disease involves inflammation in the diverticula, its symptoms in symptomatic patients are similar to those of IBS-D, i.e., abdominal cramping, diarrhea with loose stools, and alternating diarrhea and constipation.
- Diarrhea with loose stools is a result of an increase in the frequency of GMCs.
Cellular and Molecular Mechanisms
Pioneering studies in diverticular disease patients proposed that the diverticula form by high outward pressures generated in the lumen [481, 482]. These studies did not identify the source of the pressure. Our current understanding of risk factors for the formation of diverticula are:
- Strong compression of the colon wall by GMCs can generate high outward pressure.
- The sigmoid colon wall near the entry of blood vessels is weaker than at other regions.
Based on these understandings, a potential sequence of events leading to diverticulitis is:
- A low-fiber diet results in lesser retention of water in feces, causing them to harden.
- Colonic GMCs occur spontaneously up to about 10 times a day in normal subjects. When a GMC strongly squeezes over a hardened stool pellet, it generates a bulge.
- Weaker tensile strength of the colon wall increases the risk that the bulge will herniate the colon wall to form a diverticulum. Note that it might take repeated incidents to form a diverticulum.
- Thereafter, colonic contractions, especially the GMCs, push the pathogenic fecal material into the diverticula.
- The diverticula do not generate contractions to expel the fecal material.
- The trapped fecal material starts infection, resulting in an inflammatory response and abscess formation.
- The continuity of the muscle layers between the diverticula and the unaffected colon spreads inflammation to neighboring smooth muscle cells.
- Inflammation in muscularis externa causes enteric neuronal and smooth muscle dysfunction to increase the frequency of GMCs as well as induce visceral hypersensitivity.
- The inflammation that begins in the diverticula becomes transmural .
Note that the formation of diverticula by itself does not generate the symptoms of pain and altered bowel habits. The symptoms of intermittent abdominal cramping and altered bowel habits result primarily from the increase in the frequency of GMCs at the site of the inflamed diverticula. The GMCs that propagate to the rectum induce urgency and frequent defecation. As noted earlier [419, 421, 422], frequent GMCs rupture the mucosal barrier in the inflamed colon segment to cause bleeding and exudation of mucus, both expelled with the stool. The stool is loose because frequent mass movements by GMCs reduce its contact time with the mucosa in the sigmoid colon.
Diverticulitis patients show the same phenomena as IBD patients, sometimes passing loose stools, sometimes hard stools. Manometric data during the two conditions are not available. The frequency of GMCs likely fluctuates above and below normal levels to produce alternating diarrhea and constipation.
The amplitude of GMCs in diverticulitis—110 to 120 mmHg —is about the same as that in normal subjects: 115 mmHg [28, 195, 197]. Therefore, pain in these patients is likely due to inflammation-induced visceral hypersensitivity to colorectal distension by a balloon or its compression by a GMC (see Figure 37C). The sigmoid colon bearing the diverticula and the rectum are hypersensitivity to luminal distension without a change in their compliance .
Diverticular disease patients tend to have raised scores on the Hospital Anxiety and Depression scale . Whether a cause-and-effect relationship exists between an increase in anxiety/depression and colonic pain is unknown. However, it seems likely that these patients develop higher anxiety/depression scores following the development of frequent and debilitating colonic pain.
A recent animal study shows that bacterial translocation to the muscularis externa enhances the expression of insulin growth factor-1 (IGF-1) and transformation growth factor-β (TGF-β) in the muscularis externa, causing hypertrophy and hyperplasia, thus thickening the muscle layers . No data are available on changes in the expressions of these growth factors in symptomatic diverticular disease patients. One may speculate, however, that similar changes might induce thickening of muscle layers in diverticular disease patients [477, 478].
Immunofluorescence findings show that inflammation in diverticular disease alters the expressions of several endogenous peptides, including substance P (SP), galanin, neuropeptides K (NPK), pituitary adenylate cyclase–activating peptide (PACAP), and vasoactive intestinal polypeptide (VIP). However, the cause-and-effect relationship between these changes and the symptoms of diverticular disease are unknown. Recent studies in human colonic smooth muscle cells show that VIP regulates transcription of the α1C-subunit of Cav1.2b (L-type) calcium channels in circular smooth muscle cells . The influx of calcium, essential for smooth muscle contraction, occurs through these channels. Therefore, an increase in the expression of these channels enhances calcium influx, the amplitude of contractions, and colonic transit [174, 355]. The twofold increase of VIP in the circular muscle layer of the colon of symptomatic diverticular disease patients might be a contributing factor in the increased frequency of GMCs in diverticulitis.