believed to be due to the abnormal function (dysfunction) of the
muscles of the organs of the gastrointestinal tract or the nerves
controlling the organs. The nervous control of the gastrointestinal
tract, however, is complex. A system of nerves runs the entire length
of the gastrointestinal tract from the esophagus to the anus in the
muscular walls of the organs. These nerves communicate with other
nerves that travel to and from the spinal cord. Nerves within the
spinal cord, in turn, travel to and from the brain. (The
gastrointestinal tract is exceeded in the numbers of nerves it
contains only by the spinal cord and brain.) Thus, the abnormal
function of the nervous system in IBS may occur in a gastrointestinal
muscular organ, the spinal cord, or the brain.
The nervous system that controls the gastrointestinal organs, as with
most other organs, contains both sensory and motor nerves. The sensory
nerves continuously sense what is happening within the organ and relay
this information to nerves in the organ's wall. From there,
information can be relayed to the spinal cord and brain. The
information is received and processed in the organ's wall, the spinal
cord, or the brain. Then, based on this sensory input and the way the
input is processed, commands (responses) are sent to the organ over
the motor nerves. Two of the most common motor responses in the
intestine are contraction or relaxation of the muscle of the organ and
secretion of fluid and/or mucus into the organ.
As already mentioned, abnormal function of the nerves of the
gastrointestinal organs, at least theoretically, might occur in the
organ, spinal cord, or brain. Moreover, the abnormalities might occur
in the sensory nerves, the motor nerves, or at processing centers in
the intestine, spinal cord, or brain. Some researchers argue that the
cause of functional diseases is abnormalities in the function of the
sensory nerves. For example, normal activities, such as stretching of
the small intestine by food, may give rise to abnormal sensory signals
that are sent to the spinal cord and brain, where they are perceived
as pain.
Other researchers argue that the cause of functional
diseases is abnormalities in the function of the motor nerves. For
example, abnormal commands through the motor nerves might produce a
painful spasm (contraction) of the muscles. Still others argue that
abnormally functioning processing centers are responsible for
functional diseases because they misinterpret normal sensations or
send abnormal commands to the organ. In fact, some functional diseases
may be due to sensory dysfunction, motor dysfunction, or both sensory
and motor dysfunction. Still others may be due to abnormalities within
the processing centers One area that is receiving a great deal of
scientific attention is the potential role of gas produced by
intestinal bacteria in patients with IBS. Studies have demonstrated
that patients with IBS produce larger amounts of gas than individuals
without IBS, and the gas may be retained longer in the small
intestine. Among patients with IBS, abdominal size increases over the
day, reaching a maximum in the evening and returning to baseline by
the following morning. In individuals without IBS, there is no
increase in abdominal size during the day.
There has been a great deal of controversy over the role that poor
digestion and/or absorption of dietary sugars may play in aggravating
the symptoms of IBS. Poor digestion of lactose, the sugar in milk, is
very common as is poor absorption of fructose, a sweetener found in
many processed foods. Poor digestion or absorption of these sugars
could aggravate the symptoms of IBS since unabsorbed sugars often
cause increased formation of gas.
Although these abnormalities in production and transport of gas could
give rise to some of the symptoms of IBS, much more work will need to
be done before the role of intestinal gas in IBS is clear.
Dietary fat in healthy individuals causes food as well as gas to move
more slowly through the stomach and small intestine. Some patients with
IBS may even respond to dietary fat in an exaggerated fashion with
greater slowing. Thus, dietary fat could--and probably does--aggravate
the symptoms of IBS.
