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This pattern isorchestrated by the myenteric nerves (with fine-tuning byautonomic nerves and hormones).GallbladderThe gallbladder is a specialized structure for concentrationand storage of bile during the interdigestive state (Fig. 22.7).When chyme first enters the duodenum, cholecystokinin(CCK) is secreted into the blood (in response to the presenceof fats and glucose in the chyme). CCK, in combination withvagal stimulation, causes contraction of the gallbladder,forcing bile out into the cystic duct and the common bile duct.CCK (and vagal stimulation) also relaxes the sphincter ofOddi, allowing the contents of the common bile duct to enterthe duodenum.Large Intestine (Colon)The colon also has specialized forms of propulsion: segmentalpropulsion and mass movements.
The muscle structure of thelarge intestine is different from the small intestine, in thatthere are three bands of longitudinal muscle, called the taeniacoli, which run the length of the organ. When they contract,they form sacs called haustra; this is segmental propulsion(Fig. 22.8). The haustra stay in formation for long periods,which helps “store” the chyme as it is dehydrated and madeinto feces. The ileocecal sphincter regulates the amount ofchyme entering the colon. The sphincter is usually tonicallyclosed, but movement of chyme in the terminal ileum relaxesthe sphincter, allowing small amounts of chyme to enter thecolon.
The ileocecal sphincter is controlled by the entericnerves, as well as hormones (CCK and gastrin) and autonomicnerves.Although segmental propulsion is very slow, mass movementsperiodically occur (usually 1 to 3 times daily). Mass movements are peristaltic in nature, but contractions extend overa longer period of time (seen in Fig. 22.8, descending colon).These strong contractions occur when the distal colon isrelaxed and force the feces to travel quickly through thedescending colon, into the rectum. The mass movements arestimulated by the parasympathetic nerves (vagal in the proxi-Motility through the Gastrointestinal Tract263Segmental PropulsionHaustrationHaustrationReverseperistalsisPendular movementsMass movementsPeristalsisReceptive relaxation (muscle fibersof cecum elongate to accommodatecontents without change in pressure)Red ⫽ propulsive movementsBlack ⫽ nonpropulsive movementsAdaptive relaxation (intraluminal pressureincreased as contents enter bowel segment;later may return to normal as musculaturerelaxes to accommodate contents)Figure 22.8 Motility in the Colon The colon is specialized for storage, and when bands of longitudinal muscle (taenia coli) contract, they formhaustra.
This is called segmental propulsion, which is very slow, and allows time for additional sodium and fluid absorption (colonic salvage). Theslow movement dries out the chyme, producing feces. Several times a day mass movements will occur, creating peristaltic movements that forcethe chyme/feces toward the rectum. Factors that increase intestinal motility can cause diarrhea, by minimizing formation of haustrae and increasingmass movements.mal colon, and pelvic nerves in the distal colon and rectum)as well as the hormones CCK and gastrin. These factors areall active when there is chyme in the upper GI tract (duringactive digestion), and their stimulation of mass movementsserves to clear the lower GI tract of chyme and feces, in preparation for new waste coming down the tract. Conversely,stimulation of the sympathetic nerves (from the superior mesenteric in the proximal colon, and inferior mesenteric andsuperior hypogastric nerves in the distal colon) inhibits colonicmovements, consistent with the overall actions of the sympathetic nervous system (SNS) on the GI system (Fig.
22.9).DEFECATION AND THE RECTOSPHINCTERIC(DEFECATION) REFLEXWhen feces are moved into the rectum, the rectal distensionis sensed by local mechanoreceptors that signal the myentericnerves to relax the internal anal sphincter and initiate peristalsis, further pushing feces into the rectum. This local effectis reinforced by the parasympathetic nervous system (PNS),which initiates stronger contractions via pelvic nerves. At thesame time, an afferent signal is sent centrally, stimulatingthe urge to defecate.
This is the rectosphincteric, or defecation, reflex (Fig. 22.10). This is a rapid reflex, and in responseto the urge to defecate, we voluntarily constrict the externalanal sphincter, to prevent defecation. If defecation cannotoccur, there is some relaxation of the rectum, the internal analsphincter constricts again, and the external anal sphincter canbe relaxed, until another movement pushes feces into therectum and the reflex recurs. At the appropriate time for defecation, the external anal sphincter voluntarily relaxes, intraabdominal pressure increases (Valsalva maneuver), anddefecation occurs.
This reflex takes time to fully develop inchildren, and is usually intact by 2 to 4 years of age.REGULATION OF MOTILITYMotility is regulated by the enteric nerves, autonomic nerves,and hormones. The enteric nerves respond to luminal receptors that sense the presence and composition of the chyme, as264Gastrointestinal Physiologywell as signals from autonomic nerves and hormones.
As previously stated, the PNS primarily promotes motility, whereasthe SNS slows or stops motility. A general rule is that most ofthe GI hormones inhibit gastric motility and/or emptying,thus allowing the duodenum to process the chyme that isreleased (Fig. 22.11). However, gastrin is an exception: it tendsto stimulate gastric motility and activity of the antral cycle,thus increasing gastric emptying.
These hormones worktogether to regulate the exit of chyme from the stomach. Therelaxation of the pylorus by gastrin and the PNS is moderatedby the effects of gastric inhibitory peptide (GIP), CCK, andsecretin, which increase pyloric tone.KEYSympathetic efferentsParasympathetic efferentsSomatic efferentsAfferents(and CNS connections)Indefinite pathsThalamusHypothalamus(red—sympatheticblue—parasympathetic)MedullaoblongataDorsalnucleusof vagusDorsal rootganglionT9Vagus nerve (X)Sympathetic trunkGray ramus communicansWhite ramuscommunicansCeliac gangliaGreater thoracicsplanchnic nerveAorticorenalganglionLesser thoracicsplanchnic nerveCeliac trunkSuperior mesentericganglionT10T11T12ThoracolumbarcordLeast thoracicsplanchnicnerveSuperior mesentericarteryIntermesentericnervesInferior mesentericganglionL1L2L3L4LumbarsplanchnicnervesInferior mesentericarterySuperior hypogastricplexusL5Hypogastric nervesS1SacralcordS2S3Sacral(sympathetic)splanchnicnervesInferior hypogastric(pelvic) plexusS4Pelvic splanchnic nerves (nervi erigentes)Pudendal nerveFigure 22.9 Autonomic Innervation of the Colon The small intestines and colon are innervated by both sympathetic and parasympatheticnerves.
Sympathetic fibers from the spinal cord lead to the celiac, superior mesenteric ganglia, and inferior mesenteric ganglia. Parasympatheticfibers arise from the vagus (innervate through transverse colon) and pelvic (innervate descending colon, sigmoid, and rectum) nerves. Sympatheticstimulation reduces motility and secretions, whereas parasympathetic stimulation increases motility, secretions, and relaxation of the internal analsphincter.Motility through the Gastrointestinal TractIn addition, CCK and gastrin have stimulatory effects on motility in the lower small intestine and colon.
They initiate massmovements in the colon, facilitating removal of the undigestedwaste matter. Motilin has the specific function of stimulatingcontractions during fasting (the MMC) (see Fig. 22.11).Box 22.1■■■VOMITING AND DIARRHEA■Vomiting is a reflex action that is under central regulationby the vomiting center in the medulla oblongata. Stimuli forvomiting also initiate salivation, and usually retching precedesvomiting. During retching, intestinal and gastric contents riseinto the esophagus, but not into the mouth, due to a closedupper esophageal sphincter.
The reflex can be stimulated byfactors within the GI tract, as well as centrally through thechemoreceptor trigger zone (Box 22.1).The events induced by all stimuli are consistent. Reverse peristalsis occurs (with aboral constriction, and relaxation behindthe bolus) from the middle of the small intestine to the pylorus,Examples of Stimuli InitiatingVomiting ReflexIrritant in stomach or small intestine, enteric virus, or bacteria (local effect through enteric chemoreceptors)Systemic irritant sensed by chemoreceptor trigger zone infourth ventricle of brain (near area postrema)Head injury (concussion) (central effect)Abnormal stimulation of vestibular organs (central effect viaautonomic nervous system [ANS] to chemoreceptor triggerzone, then to vomiting center)which relaxes to allow chyme to enter the stomach.
Thestomach fills, and strong abdominal muscle contractions forcegastric contents into the esophagus, causing retching. Withfurther stimulus, the upper esophageal sphincter relaxes,allowing contents to rise into the mouth. Because the vomitusincludes intestinal and stomach contents, acid, bile, and electrolytes are excreted. Chronic vomiting can result in esophageal damage and metabolic disorders.T12 to L1T9 to T10L1 and L2T10 to T12T10 to T12Superior rectal arteryInferior rectal nerveFigure 22.9cont’d.265L1 and L2Gastrointestinal PhysiologyV266agusvenerCertain physiologic events, such asarising (orthocolic reflex) and ingestionof food (gastrocolic and gastroilealreflexes), may initiate a massmovement propelling fecal bolusinto rectum.VaguTransmission ofperistaltic waveby intrinsic nervess nerveGastroilealreflex may bemediated viavagus orintrinsic nerves,or both.Pelvic splanchnic nervesGastrocolic reflex may bemediated via pelvic splanchnicnerves or intrinsic nerves ascontinuation of gastroilealreflex, or both.ⴙ“Awareness” of urge, correlated withvisual and auditory stimuli, plusmemory and habit, cause individualto seek out toilet and make otherappropriate preparations andsimultaneously cause…Local autonomic reflexes(via pelvic splanchnicnerves) cause contractionof rectal musculature andrelaxation of internalsphincter in effort toexpel feces.Stimulation of rectal stretchreceptors sends afferent impulsesto spinal cord (for local reflexes)and thence to brain (forawareness of urge).ⴙⴙⴙⴙⴙPelvic splanchnic nervesPudendal and levator ani nervesVoluntary contraction of external sphincter andlevator ani muscles (via pudendal and levator aninerves) to retain feces until suitable conditionsprevail.ⴙⴙⴙⴙ ⴚⴙⴙⴚ ⴙⴙLevator ani muscleInternal sphincterExternal sphincterFigure 22.10 Defecation The sequence of events leading to defecation are detailed in this illustration.The defecation (or rectosphincteric) reflex is triggered by distension of the rectum by feces.
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