Physiological Pharmaceutics

Small intestine 121Figure 6.5 Segmental contractions (left) and propulsive contractions (right) of the smallintestineof the food determines the number of contractions; for example, twice as many contractionsoccur when solid food is ingested than when an equicalorific liquid is consumed 2 .Carbohydrates stimulate the largest number of contractions, followed by proteins andlipids. The fed pattern of motility consists of segmental and peristaltic contractions, thesegmental contractions being the most frequent (Figure 6.5). Initially a segment of thebowel, less than 2 cm in length, contracts while the adjacent segments are relaxed; theprocedure is then reversed, as the contracted segment relaxes and vice versa. This type ofmotility mixes chyme by continually moving it in the lumen and increasing contact with theabsorbing surface, but since there are less frequent contractions aborally than orally, thereis a net movement of chyme towards the large bowel. This movement is enhanced byperistaltic contractions which occur less frequently than segmental contractions, and eachmove the chyme a few centimetres. The continuous movement shears the chyme resultingin effective mixing (Figure 6.6).The interdigestive migrating myoelectric complex (Chapter 5) continues from thestomach to the small intestine. Phase I is a period of no activity, Phase II is characterized byrandom activity and Phase III is a period of intense activity which is associated with theaboral movement of the intestinal contents. The migrating myoelectric complex occursevery 140 to 150 minutes, and as one complex reaches the ileum, another starts at theduodenum. The velocity of the contractile wave decreases as it approaches the ileum andonly rarely does it reach the terminal ileum 10 .Motility in the small intestine, as in all parts of the digestive tube, is controlledpredominantly by excitatory and inhibitory signals from the enteric nervous system. Theselocal nervous signals are modulated by inputs from the central nervous system, and to somedegree by a number of gastrointestinal hormones.Stagnation at the ileocaecal junctionThe ileocaecal junction divides the terminal small intestine from the caecum. The junctionor sphincter appears to formed by papillary protrusions into the lumen of the caecum, ratherthan two flat lips of a valve 11 . Its function seems to be to retain chyme in the small intestineuntil digestion is largely complete and then to empty its contents into the large bowel. Theileocaecal junction also serves to prevent the spread of the colonic bacteria into the smallintestine 12 . Contraction of the ileocolonic sphincter is produced by a-adrenergic agonistsincluding phenylephrine, adrenaline and noradrenaline, and by cholinergic agonists such asbethanechol, whereas pure b-adrenergic agonists, such as isoprenaline, cause relaxation 13 .