Small Intestine Motility

Question 1

Anatomical factors contributing to absorptive surface amplification of small intestine

Answer 1

1. Folds in the intestinal mucosa increase the surface area by a factor of three.
2. A further ten-fold increase in surface area is achieved by the presence of villi (projections) and crypts (depressions) along the surface.
3. At the tip of each villus are the microvilli which increase surface area a further twenty-fold for a total of a six hundred-fold increase.

Question 2

Phases of Migratory Motor Complex (MMC)

Answer 2

Phase I is a period of relative quiescence lasting about 70 minutes (~40-60% of total cycle length).

Phase II is characterized by intermittent motor activity where 1-5 contractions occur with each slow wave alternating with cycles where no contractions occur. This phase lasts 10-20 minutes (~20-30% of cycle length).

Phase III arises from Phase II but is different in that it is composed of regular, propagating contractile activity lasting approximately 5 minutes. Phase III of the MMC clears the small intestine of undigested residue and debris such as mucus and sloughed cells. For this reason this phase is sometimes referred to as “Housekeeper activity”. It is thought that Phase III activity also helps to prevent bacterial overgrowth of the small intestine. As in the stomach, feeding interrupts the MMC in the small intestine.

Question 3

Small intestine contractions when fed

Answer 3

1. Segmentation contractions displace chyme in an oral and aboral (from the Latin word “ab” = away from) direction. The forces for mixing are exerted by the circular muscle layer. Longitudinal muscle does not participate significantly in the phenomenon of segmentation. Segmentation is the most common type of contraction following a meal. The “back and forth” movement enables chyme to be
   thoroughly mixed with the digestive juices and make contact with the absorptive surface of the mucosa. The gradient in BER along the intestine means that there is a higher frequency of segmentation in the proximal intestine resulting in a net aboral movement. As in the stomach, the frequency of the slow waves determines the maximum frequency of contractions.
2. Peristalsis: Peristaltic contractions occur at adjacent segments in an oral to aboral sequence. This is the next most common motility pattern after segmentation. The Law of the Intestine (Bayliss and Starling) states that stimulation (e.g. distension by a bolus) at one locus of the small bowel induces contraction above and relaxation below the point of
   stimulation. This is also called the peristaltic reflex and is mediated by the enteric nervous system. Peristaltic waves are propagated only short distances (3-5 cm) and peristaltic rushes where chyme is propelled long distances are not normally observed.
3. The intestino-intestinal reflex occurs when overdistension of one segment of the intestine reflexly inhibits the contractile activity of other
   segments causing relaxation.

Question 4

Gastroileal reflex

Answer 4

elevated secretory and motor functions of the stomach increase the motility of the terminal part of the ileum and accelerate the movement of material through the ileocecal sphincter.

Question 5

ILEUS

Answer 5

state of no muscle contractility resulting in a partial or complete non-mechanical obstruction. Electrical slow waves are present but there is little or no spike activity. This results from continuous activity of the intrinsic inhibitory neurons and can be caused by peritoneal irritation. May occur post-operatively.

Question 6

SPASM

Answer 6

SPASM of the circular muscle is the opposite of ileus in that there is no activity of the inhibitory neurons. The syncytial properties of the intestinal smooth muscle cells and their inherent contractility then produce maximal contractions.