The small intestine, like the stomach, displays two distinct patterns of motility. The fed pattern is characterized by random motor activity, in groups of 1 to 3 sequential contractions, separated by 5 to 40 seconds of inactivity. The physical and chemical nature
Figure 6.5 Segmental contractions (left) and propulsive contractions (right) of the small intestine
Figure 6.5 Segmental contractions (left) and propulsive contractions (right) of the small intestine of the food determines the number of contractions; for example, twice as many contractions occur when solid food is ingested than when an equicalorific liquid is consumed2. Carbohydrates stimulate the largest number of contractions, followed by proteins and lipids. The fed pattern of motility consists of segmental and peristaltic contractions, the segmental contractions being the most frequent (Figure 6.5). Initially a segment of the bowel, less than 2 cm in length, contracts while the adjacent segments are relaxed; the procedure is then reversed, as the contracted segment relaxes and vice versa. This type of motility mixes chyme by continually moving it in the lumen and increasing contact with the absorbing surface, but since there are less frequent contractions aborally than orally, there is a net movement of chyme towards the large bowel. This movement is enhanced by peristaltic contractions which occur less frequently than segmental contractions, and each move the chyme a few centimetres. The continuous movement shears the chyme resulting in effective mixing (Figure 6.6).
The interdigestive migrating myoelectric complex (Chapter 5) continues from the stomach to the small intestine. Phase I is a period of no activity, Phase II is characterized by random activity and Phase III is a period of intense activity which is associated with the aboral movement of the intestinal contents. The migrating myoelectric complex occurs every 140 to 150 minutes, and as one complex reaches the ileum, another starts at the duodenum. The velocity of the contractile wave decreases as it approaches the ileum and only rarely does it reach the terminal ileum10.
Motility in the small intestine, as in all parts of the digestive tube, is controlled predominantly by excitatory and inhibitory signals from the enteric nervous system. These local nervous signals are modulated by inputs from the central nervous system, and to some degree by a number of gastrointestinal hormones.
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If you weaken the center of any freestanding structure it becomes unstable. Eventually, everyday wear-and-tear takes its toll, causing the structure to buckle under pressure. This is exactly what happens when the core muscles are weak – it compromises your body’s ability to support the frame properly. In recent years, there has been a lot of buzz about the importance of a strong core – and there is a valid reason for this. The core is where all of the powerful movements in the body originate – so it can essentially be thought of as your “center of power.”