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With greater neurotransmitter release,the amplitude of the waves is higher, producing greater depolarization. This results in more action potentials, increasingthe strength of the contraction. Some GI hormones (cholecystokinin [CCK], gastrin) can also depolarize the slow waves,causing contractions. In addition, local mechanoreceptorssensing stretch or chemoreceptors sensing composition ofchyme signal the myenteric plexus to fire excitatory motorneurons, depolarizing the slow waves and causing contractionThe action potentials (AP) generated by depolarizationof the slow waves are caused by the entry of calcium intothe smooth muscle via voltage-gated channels.
The calciumbinds to calmodulin, initiating events that result in smoothmuscle contraction.Table 22.1Slow WavesAreaFrequency ofSlow WavesGeneral Actions whenDepolarized aboveThresholdStomach3/minuteMixingDuodenum12/minutePropulsionIleum10/minutePropulsionProximal Colon3/hourHaustra formation andstorageMass movements andpropulsion11/minuteDistal Colon10/hour17/minuteHaustra formation andstorageMass movements andpropulsion(Fig. 22.3).
The characteristic frequency of the slow waves forany section of the tract determines the maximum rate at whichcontractions could occur, and thus the BER sets the maximalrate of propulsion possible (Table 22.1).There are also inhibitory motor neurons that are stimulatedby the sympathetic nerves and release vasoactive intestinalpeptide (VIP) and nitric oxide (NO), which hyperpolarizethe slow waves (make them more negative), relaxing thesmooth muscle (see Fig. 22.3).
The interplay between excitatory and inhibitory motor neurons results in a variety ofpropulsive and mixing movements through the tract (see“Site-Specific Propulsion”).254Gastrointestinal PhysiologyAUTONOMIC NERVOUS SYSTEMSYMPATHETIC DIVISIONBrainstemPARASYMPATHETIC DIVISIONVagalnucleiPreganglionicfibersLumbarspinal cordSacralspinal cordVagusnervesThoracicspinal cordSympatheticgangliaPostganglionicfibersPelvicnervesENTERIC NERVOUS SYSTEMMyentericplexusSubmucosalplexusSecretorycellsBlood vesselsSmoothmuscleFigure 22.1 Regulation of Enteric Nervous System by Autonomic Nervous System Thisdiagram illustrates basic connections between the enteric nervous system (myenteric and submucosalplexuses) and the autonomic nervous system.
In general, stimulation from the parasympathetic nervesenhances motility and secretions through the ENS, whereas sympathetic stimulation reduces motility andsecretions.INTERDIGESTIVE HOUSEKEEPING: THEMIGRATING MYOELECTRIC COMPLEXFasting, or the interdigestive state, is characterized by longperiods of quiescence, with short periods of waves of contractions. The contractions, called the migrating myoelectriccomplex (MMC), originate in the mid-stomach and continueto the terminal ileum during each cycle and serve to sweep theundigested material and bacteria out of the stomach and smallintestine, into the colon.
This protects the delicate upper GItract (small intestine) from damage and sequesters most ofthe bacteria in the colon, a drier environment less hospitableto bacteria.There are four phases of the MMC that occur in 75- to 120minute cycles during fasting, based on contractile activity.Phases I, II, and IV have little activity.
Phase III is mostMotility through the Gastrointestinal TractGUT LUMENMUCOSAVilli255MUSCULARISAscending(oral) pathwayContractionMechanicalstimulationSensory neuron(mechanical,chemical)Sensoryneuron(stretch)ChemicalstimulationS T R E T C HExcitatorymotor neuron(acetylcholine,substance P)Descending(anal) pathwayInhibitorymotor neuron(VIP, NO)RelaxationFigure 22.2 Local Control of Motility In response to the presence of chyme in the lumen of thesmall intestine, mechanoreceptors and chemoreceptors transduce signals to ascending and descendingneurons.
The ascending (green) pathway leads to excitatory motor neurons behind the bolus of chyme,stimulating depolarization of the slow waves, generation of an action potential, and contraction. At the sametime, the descending (orange) pathway ends in inhibitory motor neurons, which hyperpolarize the slow wavesin the muscle, causing relaxation in front of the bolus. The overall result is a peristaltic contraction, movingthe bolus toward the anus.important of the four, when the hormone motilin is releasedfrom Mo cells of the small intestine into the circulation.Motilin stimulates strong sequential contractions, sweepingbacteria and undigested matter down the tract from midstomach into the colon.
Phase III lasts only 6 to 10 minutesin each cycle. Motilin acts through the enteric and autonomicnerves to stimulate contractions, which proceed for a few feetaborally, and then start up again slightly further down theintestine. This pattern is repeated until the complex reachesthe terminal ileum, and then a new complex is initiated in thestomach.These cycles begin about 3 hours after the last meal and repeatuntil ingestion of food. With ingestion, the normal propulsivepatterns resume.SITE-SPECIFIC PROPULSIONMouth and EsophagusFood is first moved by chewing and then swallowing.
Thereare three stages of swallowing.256Gastrointestinal PhysiologyAction potentialElectricalthreshold(–40 mV)Threshold forcontractionSlow wavesContractileforceFigure 22.3 Slow Waves Slow waves are present from mid-stomach through the rectum and are theresting membrane potential of the GI smooth muscle. Depolarization of the slow waves above −40 mVstimulates action potentials, which causes contraction of the smooth muscle.In the voluntary oral stage, the bolus of food is moved tothe back of the mouth into the pharyngeal region, stimulating touch receptors and initiating the swallowing reflex(Table 22.2).
In the pharyngeal stage, the bolus of food movesto the back of the pharynx, the larynx moves toward the epiglottis to prevent food entering the trachea, and the upperesophageal sphincter relaxes. The pharyngeal muscles contract,propelling the bolus through the upper esophageal sphincterinto the esophagus. During the pharyngeal phase, the swallowing reflex prevents respiration. The last is the esophageal stage.When the bolus enters the esophagus, voluntary control of themovement is lost. A wave of primary esophageal peristalsis isgenerated by the swallowing center in the medulla (efferentvagal nerves terminate on the myenteric plexus → excitatorysignal → depolarized slow waves → contraction).
The alternating contraction and relaxation helps carry the bolus of foodthrough the esophagus to the stomach.During swallowing, if the bolus is dry and does not movequickly through the esophagus, secondary esophageal peristalsis is initiated by local mechanoreceptors sensing stretch.The lower esophageal sphincter has a high resting muscletone, to prevent reflux of gastric acid. The basal tone is maintained by enteric nerves, hormones, and vagal cholinergicfibers: The tone can be increased by sympathetic stimulation.As the bolus nears the lower esophageal sphincter, noncholinergic vagal fibers acting on inhibitory interneurons releaseVIP and NO, causing relaxation of the sphincter (Fig.
22.4).The bolus then passes into the stomach. If the lower esophageal sphincter is unable to relax, pathology can occur (see“Smooth Muscle Disorders: Achalasia and Hirschsprung’sDisease” Clinical Correlate). In addition, the esophagus, likethe rest of the tract, is highly vascularized, and damage to theesophageal capillaries (acid reflux, portal hypertension) canresult in esophageal bleeding, which can be life-threatening(see “Portal Hypertension and Esophageal Varices in Obstructive Liver Disease” Clinical Correlate in Chapter 24).StomachAs the bolus enters the stomach, receptive relaxation increasesthe stomach size, allowing accommodation of the meal.
Thisis vagally-mediated through the release of VIP. As the stomachfills, the stretch and chemical contents stimulate contractions,which help mix the food with the gastric secretions, formingchyme. As the chyme mixes, the contents separate, with carbohydrates and readily digested substances in the lower part(antrum), large chunks in the body of the stomach, and mostfats floating at the top. The transit time through the stomachdepends on the amount and type of food ingested. Small,easily digested meals (high in carbohydrates such as pasta andsugar) move through the stomach quickly (30 to 60 minutes).Meals with more solid foods (meats) and high fat content(fried foods) take much longer (3 to 4 hours) to clear thestomach.Figure 22.5 illustrates the waves of contractions that beginmid-stomach and move the chyme toward the antrum andpylorus (which acts as a sphincter).
Regulation of the tone ofthe pyloric sphincter occurs through both neural and hormonal pathways (Table 22.3).For the most part, pyloric tone is high, so as the stomach isfilling and contractions are starting, most of the acidic chymemoving into the antrum undergoes retropulsion away fromthe sphincter (see Fig. 22.5B, 1 and 2). As the waves ofMotility through the Gastrointestinal TractCLINICAL CORRELATEBariatric SurgeryReceptive relaxation is crucial for the storage function of thestomach, which allows the contents to be well mixed with gastricacid and enzymes and some digestion to occur.
If vagal innervation to the fundus is lost, the accommodation will not occur, andstomach pressures will quickly rise, imparting a feeling of “fullness,” and the inability to physically get more into the stomach.Reducing the storage ability of the stomach is the rationale behindgastric bypass and stomach stapling in severely obese persons.■■Adjustable gastric banding restricts the size of the pylorus, sothat only small amounts of food can comfortably enter thestomach. This restricts food intake because of discomfort, butdoes allow food to enter the intact stomach.Vertical banded gastroplasty was a common procedure involving both stomach stapling and banding, to create a small pouch■while leaving the rest of the tract intact.
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