A Model of Electromechanical Coupling in the Small Intestine - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 177

the maintenance of life. We now understand much more about the remarkable
mechanisms of digestion and absorption, thanks to over a century of accumulative
research of modern gastrointestinal physiology [ 51 ]. Furthermore, as the focus of
modern healthcare is shifting toward preventative medicine, it is also becoming
clear that maintaining a healthy digestive system is a key strategy in the man-
agement of increasingly prevalent illnesses in the developed and developing
worlds alike, such as diabetes, cardiovascular diseases, and obesity.
Even though the general topology of the gastrointestinal tract is a continuous
long tube, different parts of the tract are unique in both anatomy and function, in
order to specialize in different aspect of digestion and/or absorption. For example,
the stomach is where most of the digestion takes place, with aid of digestive
enzymes and gastric contractions; whereas the small intestine is where most of the
nutrient absorption occurs. Partially digested food, called chyme, is emptied into
the small intestine at a controlled rate from the stomach, through a muscular valve
called the pylorus, and the nutrients in the chyme are absorbed through the blood
vessels embedded in the villi of the intestinal wall via passive diffusion, as the food
travels down the intestine. The adult human small intestine is approximately 6 m
in length and 30 mm in diameter [ 7 ]. In order to ensure that the chyme is con-
tinuously processed through the entire intestine, a series of motor patterns slowly
transport the food content down the small intestine over a period of approximately
8h[ 20 ]. The intestinal content then becomes more compacted in the colon, after
which the content is evacuated from the body. The transit time of food through the
intestine is an important clinical indicator of digestive health. Abnormally quick
transit through the intestine could result in diarrhea, which in severe and/or chronic
cases can lead to malnourishment and dehydration. Conversely, abnormally slow
transit could result in constipation, which is often accompanied with bloating,
pain, and a general reduction in quality of life [ 22 ].
Since the advent of modern medical imaging techniques such as the X-ray,
ultrasound, and magnetic resonance imaging, the motor patterns in the small
intestine have been classified into a variety of modes depending on the physiological
state of the body. This in turn is governed by a cohort of biological systems, such as
the myogenic, neurogenic, and hormonal systems [ 51 ]. Out of the many types of
motor patterns in the intestine, the most studied motor pattern is a series of con-
traction and relaxation movements known as peristalsis. Generally, the term
''peristaltic contraction'' refers to any constriction which travels along the intestine,
while the ''peristaltic reflex'' is a subset of peristaltic contractions, and describes the
neurally-mediated contractile reflex of the intestine to a food bolus [ 31 ].
Peristalsis is partly mediated by a propagating bioelectrical activity in the small
intestine called the intestinal slow wave activity. In 1914, intestinal motor patterns
were first studied in rabbits [ 3 ], and eight years later in 1922, slow wave activity was
recorded for the first time from the intestines in a number of species [ 4 ]. Subsequent
studies have established that the slow wave activity is actively generated by a special
class of pacemaker cells known as the interstitial cells of Cajal. The slow wave
activity then passively conducts to the surrounding smooth muscle cells, driving
their motility. The interstitial cells of Cajal were in fact first described by the Spanish