Three distinct types of contractions perform colonic motility
functions. Rhythmic phasic contractions (RPCs) cause slow net
distal propulsion with extensive mixing/turning over. Infrequently
occurring giant migrating contractions (GMCs) produce mass
movements. Tonic contractions aid RPCs in their motor function. The
spatiotemporal patterns of these contractions differ markedly. The
amplitude and distance of propagation of a GMC are several-fold
larger than those of an RPC. The enteric neurons and smooth muscle
cells are the core regulators of all three types of contractions.
The regulation of contractions by these mechanisms is modifiable by
extrinsic factors: CNS, autonomic neurons, hormones, inflammatory
mediators, and stress mediators. Only the GMCs produce descending
inhibition, which accommodates the large bolus being propelled
without increasing muscle tone. The strong compression of the colon
wall generates afferent signals that are below nociceptive
threshold in healthy subjects. However, these signals become
nociceptive; if the amplitudes of GMCs increase, afferent nerves
become hypersensitive, or descending inhibition is impaired. The
GMCs also provide the force for rapid propulsion of feces and
descending inhibition to relax the internal anal sphincter during
defecation. The dysregulation of GMCs is a major factor in colonic
motility disorders: irritable bowel syndrome (IBS), inflammatory
bowel disease (IBD), and diverticular disease (DD). Frequent mass
movements by GMCs cause diarrhea in diarrhea predominant IBS, IBD,
and DD, while a decrease in the frequency of GMCs causes
constipation. The GMCs generate the afferent signals for
intermittent short-lived episodes of abdominal cramping in these
disorders. Epigenetic dysregulation due to adverse events in early
life is one of the major factors in generating the symptoms of IBS
in adulthood. Table of Contents: Introduction / Regulatory
Mechanisms / Colonic Motility in Health / Colonic Motility
Dysfunction / References
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