Pediatric neurogastroenterology


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Pediatric neurogastroenterology

  1. 1. 23C. Faure et al. (eds.), Pediatric Neurogastroenterology: Gastrointestinal Motilityand Functional Disorders in Children, Clinical Gastroenterology,DOI 10.1007/978-1-60761-709-9_3, © Springer Science+Business Media New York 2013IntroductionCoordinated movements of the gastrointestinaltract are crucial for the primary functions of thisorgan: digestion of food, absorption of nutrientsand removal of waste products. Several complexmotor patterns involving coordinated contrac-tions and relaxations of the external muscle lay-ers of the gut have distinct roles in gut motility(see section below). These motility patterns havebeen intensively studied and characterized inadults, but there is far less known about gut motil-ity during development. Here we review the typesof motor patterns that are present in the gut ofdeveloping laboratory animals and humans.We also discuss the mechanisms that regulateintestinal movements during development.Motility Patterns and Their ControlMechanisms in the Mature GutCoordinated movements of the gastrointestinaltract include mixing, propagating motor activitiesand receptive relaxation. These movements areregulated by multiple control systems includingextrinsic neurons, intrinsic neurons (the entericnervous system, ENS), interstitial cells of Cajal(ICC) and myogenic mechanisms, which can alloperate simultaneously [1, 2]. The relative contri-bution of each control system to a particular activ-ity varies between different regions of thegastrointestinal tract [3]. Furthermore, as discussedlater in this article, recent studies in animal modelsalso show that the relative contribution of differentcontrol systems to contractile activity in the intes-tine also varies with developmental age [4]. Thus,the control of gut motility is very complex [2].The primary function of the esophagus is toact as a conduit between the pharynx and thestomach, and the only motor pattern is peristalsis.During the pharyngeal phase of swallowing, theupper esophageal sphincter (UES) relaxes, andthere is then a sequential contraction of esopha-geal muscle from the proximal to the distal end,followed by lower esophageal sphincter (LES)relaxation, so as to allow the bolus to enter theH.M. Young, Ph.D.(*)Department of Anatomy & Neuroscience, Universityof Melbourne, 3010, Parkville, VIC, Australiae-mail: Beckett, Ph.D.Discipline of Physiology, School of Medical Sciences,University of Adelaide, Adelaide, SA, AustraliaJ.C. Bornstein, Ph.D.Department of Physiology, University of Melbourne,Melbourne, VIC, AustraliaS.R. Jadcherla, M.D., F.R.C.P.I., D.C.H., A.G.A.F.Department of Pediatrics, Division of Neonatology,Pediatric Gastroenterology and Nutrition, The OhioState University College of Medicine and Public Health;Section of Neonatology, Nationwide Children’sHospital, Columbus, OH, USA3Development of Gut MotilityHeather M. Young, Elizabeth A. Beckett,Joel C. Bornstein, and Sudarshan R. Jadcherla
  2. 2. 24 H.M. Young et al.stomach. This integrated sequence of reflexesinduced by swallowing constitutes primary peri-stalsis (Fig. 3.1). Peristalsis is also induced byesophageal distension, which is termed second-ary peristalsis. In humans, the upper third of theesophagus, which is striated muscle, is controlledentirely by neurons in the brainstem via the vagusnerves. The lower, smooth muscle regions of theesophagus are controlled by the vagus nerve,intrinsic neurons and myogenic mechanisms [3].Different motor patterns occur in the proximaland distal stomach [3]. In the proximal stomach,receptive relaxation and accommodation occur,which are both mediated by neurons in the brain-stem via vago-vagal reflexes. The distal stomachexhibits different motor patterns in the fed andfasted states. In the fed state, the distal stomachgrinds and mixes. Extrinsic neurons are notessential for this contractile activity, but it can bemodulated by vagal pathways. Migrating motorcomplexes (MMCs) are waves of strong contrac-tions that sweep slowly along the gastrointestinaltract in the fasted state to clear indigestible food,mucous and epithelial debris. In humans, MMCsoccur around once every 2–4 h, and most origi-nate in the distal stomach and propagate alongthe small intestine [3]. The initiation of MMCs ismodulated by vagal input and motilin releasedfrom the duodenum, while the propagation ofMMCs is coordinated by enteric neurons.Fig. 3.1 An example of spontaneous primary esophagealperistalsis in a premature infant evoked upon pharyngealcontraction. Such sequences facilitate swallowing andesophagealclearance.Notethebriefrespiratorymodificationand deglutition apnea during pharyngeal waveform suggest-ing cross-communications between the pharynx and airway
  3. 3. 253 Development of Gut MotilityMultiple motor patterns occur in the small andlarge intestines. Segmentation, alternating sta-tionary waves of contraction and relaxation,mixes intestinal contents with digestive enzymesand exposes nutrients to the absorptive epithe-lium (small intestine) or facilitates water extrac-tion (colon). Peristalsis, contraction waves thatmigrate in an anal direction, moves intestinalcontents to new gut regions and is essential forelimination of undigested material. MMCs,which are initiated in the stomach or proximalduodenum, propagate along significant lengths ofthe intestine. In humans, MMCs occur only in thefasted state and only in the small intestine [3]. Inother species, however, MMCs can occur in boththe fed and fasted states and also occur in thecolon. Haustration, the mixing of feces to absorbwater, occurs in sac-like structures called haus-trations of the large intestine of some speciesincluding humans.Studies in animal models have shown that theENS is essential for segmentation in the smallintestine [5]. Peristalsis in the small and largeintestines is controlled by an interplay betweenthe ENS, ICC, and myogenic mechanisms [2].However, the ENS is essential for intestinal peri-stalsis as revealed by the bowel obstructioncaused by the aganglionic region of infants withHirschsprung’s disease. The ENS is also essentialfor the initiation and propagation of the MMC inthe small intestine, although the CNS and hor-mones can modulate MMCs [3]. Studies in therabbit colon have shown that haustral formationand propagation is neurally mediated [6].Furthermore, water and electrolyte secretion isregulated by the ENS, as is the integrationbetween motility and secretion [7].Development of Motility Patternsand Their Control Mechanisms—Studies of Laboratory AnimalsUnlike humans, the mechanisms controllingmotility patterns during development can beexamined in intact segments of gut of laboratoryanimals in vitro or in vivo. Most studies of mam-mals have been performed using segments offetal or postnatal mouse intestine in vitro.However, because larval zebrafish are transpar-ent, propagating contractile activity and transitstudies using fluorescent food have been per-formed in zebrafish in vivo [8–13]. In this sectionwe focus by necessity on the small and largeintestines as there are relatively few studies onthe development of motility patterns and theircontrol mechanisms in the esophagus and stom-ach of laboratory animals.Motility Patterns Presentin the Developing GutAlthough fetal mammals receive nutrition solelyvia the placenta, contractile activity in the gutcommences well before birth. The esophagus ofpreterm piglets (delivered by caesarean section at91% of full gestation) exhibits esophageal con-tractions in response to oral feeding, but comparedto term piglets, the frequency of contractions islower and the contractions propagate at a lowervelocity [14]. In fetal mice, shallow contractionsthat propagate both orally and anally are firstobserved in preparations of small intestine in vitroat embryonic day (E) 13.5 (the gestation period fora mouse is around 19 days) [4]. Moreover, propa-gating contractions are observed in zebrafish lar-vae before the yolk sac is fully absorbed [8–11].The physiological role of prenatal (or pre-yolk sacabsorption) gastrointestinal contractile activity isunclear. Fetal mammals swallow amniotic fluid,which advances along the gut [15–17], and thismeconium progresses towards the distal regions ofbowel during late fetal stages [18]. Although it ishighly likely that the propagating contractile activ-ity that occurs prior to birth contributes to the pro-pulsion of meconium anally prior to birth, this hasyet to be conclusively demonstrated.Development of Enteric Neuronsand Their Role in Motility DuringDevelopmentThe ENS arises from neural crest-derived cellsthat emigrate primarily from the caudal hindbrain
  4. 4. 26 H.M. Young et al.[19, 20], although sacral level neural crest cellsalso give rise to some enteric neurons, mainly inthe colon and rectum [21, 22]. Neuronal differen-tiation commences early as pan-neuronal mark-ers are first expressed by a subpopulation ofneural crest-derived cells as they are migratingalong the gut in fetal mice and rats [23, 24].In the mature ENS, there are many differentsubtypes of enteric neurons [25]. In the develop-ing mouse gut, cells expressing markers for someenteric neuron subtypes are present shortly afterthe first expression of pan-neuronal proteins [26],but different enteric neuron subtypes develop atdifferent ages [27]. Expression of neuronal nitricoxide synthase (nNOS—the synthetic enzymefor nitric oxide), and choline acetyltransferase(ChAT, the synthetic enzyme for acetylcholine)by developing enteric neurons have been the mostextensively studied. nNOS neurons in the matureENS include interneurons and inhibitory motorneurons to the external muscle layers [28]. ChATneurons include excitatory interneurons andexcitatory motor neurons to the external musclelayers [29]. In both zebrafish and mice, nNOSneurons are one of the first enteric neuron sub-types to appear during development [10, 26, 30,31]. In guinea-pigs, although the total number ofmyenteric neurons in the small intestine increasesbetween neonatal and adult stages, the total num-ber of nNOS neurons in the neonatal guinea-pigis the same as in adults and so the percentage ofmyenteric neurons expressing nNOS declinesduring postnatal development [32]. In zebrafish,the proportion of enteric neurons expressingnNOS does not change between 72 and 120 hpf(hours post-fertilization) [31]. In the rat, however,the proportion of myenteric neurons expressingNOS increases postnatally [33]. ChAT immuno-reactivity is not detected until late in embryonicdevelopment in the mouse [34] although uptakeof 3[H]-choline is detected considerably earlier[35]. Similarly, although ChAT immunoreactiv-ity is detectable in the zebrafish brain duringembryonic development and in the ENS of adultzebrafish, ChAT immunoreactivity is not detect-able in the ENS during zebrafish embryonicdevelopment [31]. In rats, the percentage ofChAT-immunoreactive myenteric neuronsincreases during postnatal development [33].Changes in the proportions of some subtypes ofenteric neurons have also been reported betweenweaning and adulthood in rats and guinea-pig,suggesting that the ENS is not fully mature atweaning [36–38].The development of the innervation of themuscle layers has been examined in a number ofspecies. In the dog, the plexuses of nerve fibers inthe small intestine and colon are immature atbirth [39]. In the guinea-pig ileum, the density ofcholinergic nerve fibers in myenteric ganglia andin the tertiary plexus is higher at neonatal stagesthan in adults [32], whereas in the mouse colonthe density of cholinergic nerve fibers in the cir-cular muscle layer increases during early postna-tal stages [40]. These differences might reflectthe fact that mice are born at a developmentallyearlier age than guinea-pigs.Although enteric neuron differentiation com-mences prior to the presence of propagating con-tractile activity, studies in mice and zebrafishusing pharmacological inhibitors of neural activ-ity or mutants lacking enteric neurons haveshown that the first motility patterns are not neu-rally mediated [4, 11]. There is therefore asignificant delay between when enteric neuronsfirst develop and when neurally mediated motil-ity patterns are observed. This very likely reflectsthe fact that the neural circuitry mediating motil-ity patterns involves at least three different typesof neurons [41], which must develop and thenform the appropriate synaptic connections witheach other and with target cells. In mice, neurallymediated motility patterns are not observed untilshortly before birth in the duodenum [4], and aweek after birth in the colon [40]. In longitudinalmuscle strips from postnatal rats, electrical fieldstimulation-induced contractions are reduced bya muscarinic acetylcholine receptor antagoniststarting at postnatal day (P) 14, whereas inhibi-tion of nNOS caused a significant increase in thecontractile response only from P36 [33]. Thus,cholinergic neuromuscular transmission to thelongitudinal muscle in the rat colon does notdevelop until postnatal stages and precedes thedevelopment of nitric oxide-mediated transmis-sion. In the mouse small intestine, cholinergic
  5. 5. 273 Development of Gut Motilityneuromuscular transmission commences at latefetal stages [42]. In contrast, cholinergic neuro-muscular transmission in the guinea-pig taeniaand in the frog gut commences after inhibitory ornitric oxide-mediated transmission [43, 44]. Inthe longitudinal muscle of human and guinea-pigintestine, nitric oxide-mediated transmission isrelatively more prominent at postnatal stages thanin adults [32, 45].In summary, although enteric neurons developearly, the first gastrointestinal motility patterns arenot neurally mediated. However, neurally medi-ated contractile activity is prominent by birth, andis essential for propulsive activity as shown by thebowel obstruction that occurs proximal to theaganglionic region in infants with Hirschsprung’sdisease. The first subtype of enteric neuron todevelop is the nNOS neurons, and although thereare some exceptions, nitric oxide-mediated trans-mission develops earlier and/or is more prominentduring pre- and postnatal development than inadults. As the relative importance of different neu-rotransmitters to gastrointestinal contractile activ-ity changes significantly during development,drugs that successfully treat motility disorders inadults will not necessarily have similar effects ininfants and children.Development of Interstitial Cellsof Cajal (ICC) and Their Rolein Motility During DevelopmentIn the adult gut, there are several different sub-populations of ICC, most of which are in closeassociation with enteric neurons [46]. Differentsubpopulations of ICC play different roles. Forexample, ICC at the level of the myenteric plexus(ICC-MY) mediate slow waves, the electricalevents that time the occurrence of phasic contrac-tions [47–50], and ICC within the circular muscleact as intermediaries in neuromuscular transmis-sion [51, 52].Unlike enteric neurons and glia, ICC do notarise from the neural crest during embryologicaldevelopment as ICC develop in explants of avianand mammalian embryonic gut which have beenremoved prior to the arrival of neural crest cells inthat region [53, 54]. Furthermore, ICC are distrib-uted normally and slow wave activity is generatedin the bowel of mutant mice lacking enteric neu-rons [55, 56]. Hence ICC development and main-tenance is independent of crest-derived cells inmice. In an infant with intestinal aganglionosisextending into the jejunum, abundant ICC werepresent in the myenteric region, but degeneratingICC were observed in the circular muscle of theaganglionic region [57]. Thus, in humans, ICCalso arise independently of neurons, althoughsome subpopulations of ICC may directly or indi-rectly require neurons for their long term survival.Developmental studies in mice suggest thatsmooth muscle cells and ICC arise from a com-mon mesenchymal precursor [58, 59] and that dif-ferentiation to the ICC phenotype duringembryogenesis is dependent upon cellular signal-ing via the tyrosine kinase receptor, Kit [42, 59–61].The natural ligand for the Kit receptor is stem cellfactor (SCF or steel), which is expressed in bothenteric neurons and smooth muscle cells [55, 56, 62].Mutations leading to deficiency of Kit in W/Wvmice or membrane bound SCF in Sl/Sldmice resultin disruptions of particular ICC populations andaberrant gastrointestinal motility [47–49]. Bothmigrating motor complexes and higher frequencyphasic contractions can be recorded from the smallintestine of W/Wvmice, which lack intestinalICC-MY [63], but the phasic contractions arecharacteristically abrupt and uncoordinated [64].Treatment of embryonic jejunal explants with Kit-neutralizing antibodies prior to the emergence ofcells with the ultrastructural characteristics of ICCprevents the development of ICC and slow waveactivity [42]. The postnatal maintenance of ICCalso appears dependent upon Kit-signaling asinjection of Kit neutralizing antibodies resulted inloss of ICC and lethal paralytic ileus in neonatalmice [61]. Loss of ICC due to Kit blockade isaccompanied by a loss of electrical slow waveactivity in the small intestine and reduced neuralresponses in the small bowel and colon [65]. In theabsence of Kit-signaling, ICC appear to differenti-ate to a smooth muscle phenotype, but appear toretain, at least in the short term, the ability toregenerate the ICC phenotype if Kit signaling isrestored [60].
  6. 6. 28 H.M. Young et al.During embryogenesis there is a rostral-to-caudal development of ICC along the gastro-intestinal tract. In embryonic mice, the circularmuscle layer differentiates prior to the longitudi-nal muscle layer. Nearly all of the mesenchymalcells between the serosa and the newly formedcircular muscle layer, consisting of precursors ofboth longitudinal muscle and ICC, initiallyexpress Kit [42, 65]. As embryonic developmentprogresses a subpopulation of these mesenchy-mal precursors lose expression of Kit and differ-entiate into longitudinal smooth muscle [59]. TheKit-positive cells on the circular muscle side ofthis newly formed longitudinal muscle layerdevelop into the anastomosing network termedICC-MY.Motility patterns of the stomach during devel-opment have not been extensively researchedusing laboratory animals. In mouse, 2 days priorto birth, ICC-MY and slow wave activity arepresent in the gastric antrum whilst spindleshaped intramuscular ICC (ICC-IM) are evidentand neurally mediated responses can be recordedfrom the gastric fundus [66].Intramuscular ICC (ICC-IM) are closely asso-ciated with the varicose terminals of both excit-atory and inhibitory motor nerves and withoutICC-IM neural transmission from enteric motorneurons is significantly compromised [51, 52].Despite this close anatomical arrangementbetween nerves and ICC-IM, the outgrowth ofmotor nerve processes does not appear to bedependent upon the presence of ICC as the distri-bution of both excitatory and inhibitory nerveprocesses is normal in W/Wvfundus musclesdevoid of ICC-IM. In contrast, the terminal pro-cesses of vagal intramuscular arrays do not ram-ify within the circular muscle layer of the stomachin the absence of ICC-IM [67, 68].Electrical rhythmicity can be recorded fromsegments of mouse small intestine 3 days prior tobirth [42, 60]. However, the first propagatingcontractions in mouse intestine are evident in themid stages of embryonic development (embry-onic day 13), prior to the emergence of a Kitpositive ICC network and slow wave activity atembryonic day 18 [4]. The frequency of these ini-tial contractions is similar in wildtype mice andin mutant (W/Wv) mice lacking ICC-MY, providingfurther evidence that these contractile patternsare myogenic1rather than ICC-mediated. Closerto the time of birth, after anastomosing networksof ICC-MY have established, slow waves andphasic contractions occur at a similar frequencysuggesting that myogenic contractions becomeentrained by ICC-MY [4]. Around 5 days afterbirth, a second layer of Kit-positive cells, termedICC-DMP, are evident in the region of the deepmuscular plexus of the rodent small intestine [59,65, 69, 70]. Development of neuromuscularresponses to stimulation is concomitant with thedevelopment of ICC-DMP and blockade of ICC-DMP development with Kit neutralizing antibod-ies has been shown to lead to a severe attenuationof postjunctional responses to nerve stimulation[71] suggesting their role as mediators of neu-rotransmission in the intestine.Role of Myogenic Mechanisms inIntestinal Motility During DevelopmentStudies in embryonic mice and zebrafish haveshown that the first intestinal motility patterns toappear during development, spontaneous con-tractions that propagate anally and orally, are notmediated by neurons or ICC [4, 11]. Hence thecontractions must be myogenic, that is, generatedby the smooth muscle cells themselves. Motilitypatterns that are not mediated by either neuronsor ICC are present in the intestine of mature ani-mals, but under normal conditions are not veryprominent [72, 73]. However, propagating con-tractions in other organs of mature animals,including the upper urinary tract, vas deferensand uterus are entirely myogenic in origin [74].In the duodenum and colon of fetal mice, themyogenic contractions require the entry of extra-cellular calcium [4], but it is unknown how theyare initiated or propagated.1In the field of gastrointestinal motility, the term “myo-genic” has been used to describe contractile activity gen-erated by ICC as well as muscle cells, but here we usethe term myogenic to refer to contractions specificallyoriginating from the muscle cells themselves.
  7. 7. 293 Development of Gut MotilityEnvironmental Influences on MotilityPatterns During DevelopmentThe composition of gut contents changes dra-matically immediately after birth and then atweaning. There is evidence from piglets thatthe introduction of solid food at weaninginduces changes in some of the properties ofMMCs [75], but it is unknown whether changesin luminal contents immediately followingbirth also induce changes in motility patterns.Dietary components have recently been shownto affect motility and gene expression in theENS of mature rats; in particular, long termexposure to resistant starch diet enhancedcolonic propulsive motility and increased thenumber of ChAT immunoreactive myentericneurons [76]. Furthermore, piglets treated witha probiotic showed increases in the expressionof some neurotransmitters in submucosal, butnot myenteric, neurons [77]. Additional stud-ies are required to determine whether thechanges in motility patterns that occur duringpostnatal development are induced by, or coin-cident with, dietary changes.Motility is likely to be altered during fetal hypoxicstress as the transit of fluoescein-labeled luminalcontents along the small intestine in fetal rabbits isdecreased after a 1-h hypoxic episode [17].Motility in Human Neonatesand ChildrenIn human infants, gastrointestinal motility is verycomplex, and as in laboratory animals, is almostcertainly influenced by maturational changes inthe CNS and ENS, gut muscle and ICC, as wellas diet and changing anatomical postures duringinfancy. Furthermore, in the vulnerable high riskinfant in intensive care units, hypoxia,inflammation, sepsis and other comorbidity con-ditions can complicate the feeding process andgastrointestinal transit.Immunohistochemical studies of humanfetuses have shown that neurons, muscle andICC differentiate from proximal-to-distal andthat the longitudinal and circular muscle layersand myenteric and submucosal plexuses have amature appearance by week 14 [78, 79]. As inlaboratory mammals, many subtypes of entericneurons develop prior to birth [27]. Kit-expressing ICC-MY first appear around weeks7–9 [78, 79]. In the stomach, ICC-MY, ICC-IM(intramuscular), and ICC-SEP (ICC locatedwithin connective tissue septa separating mus-cle bundles) are all present by the end of thefourth month of development [80].The simple physiological functions of theneonatal foregut, midgut, and hindgut, respec-tively, are to facilitate (1) safe feeding by steeringingested material away from the airway, (2) gas-trointestinal transit and mixing of luminal con-tents to permit absorption and propulsion, and(3) evacuation of excreta to modify the intestinalmilieu. In this section on human neonates, wereview the developmental aspects of (1) phar-yngo-esophageal motility, (2) gastric motility, (3)small intestinal motility, and (4) colonic motility.Developmental Pharyngo-EsophagealMotility in Human NeonatesSwallowing Prior to BirthNumerous studies have shown that the humanfetus swallows amniotic fluid [15, 81]. By 11weeks gestation, the ability to swallow hasdeveloped and by 18–20 weeks sucking move-ments appear. There is an increase in the vol-ume swallowed with gestational age, and bynear term, the human fetus swallows around500 ml of amniotic fluid per day [81]. Studiesusing a sheep model have shown that, as inadults, swallowing in near-term fetusesinvolves central cholinergic mechanisms [82].Upper and Lower Esophageal SphincterFunctions and Esophageal Peristalsisin Human NeonatesUsing micromanometry methods, upper esopha-geal sphincter (UES), esophageal body and loweresophageal sphincter (LES) functions have beencharacterized in neonates [83–85]. The restingUES tone increases with maturation from around
  8. 8. 30 H.M. Young et al.18 mmHg in 33 week preterm infants, to 26mmHg in full term born neonates compared to 53mmHg in adults. In contrast, the motor eventsassociated with LES relaxation in healthy pre-term infants 33 weeks and older have similarcharacteristics to adults [86].In 33 weeks preterm infants, primary esopha-geal peristalsis occurs, but considerable matura-tion occurs pre- and postnatally [83, 85]. Forexample, evaluation of consecutive spontaneoussolitary swallows in preterm infants at 33 weeks,preterm infants at 36 weeks, full term infants andadults showed significant age-dependent changesin the amplitude and velocity of the peristalticcontractions [84].During anterograde movement of a bolus fol-lowing swallowing or during retrograde move-ment of a bolus during gastroesophageal refluxevents, the bolus comes in close proximity to theairway. Peristalsis is the single most importantfunction that ensures clearance of luminal con-tents away from the airway. During primaryesophageal peristalsis, there is a respiratorypause called deglutition apnea that occurs duringthe pharyngeal phase of swallow (see Fig. 3.1).This brief inhibition in respiration is due to abreak in respiratory cycle (inspiratory or expira-tory) and is a normal reflex. On the other hand,during esophageal provocation events (for exam-ple, infusion via a manometry catheter, or gas-troesophageal reflux) proximal esophagealcontraction and distal esophageal relaxationresult in secondary peristalsis, which occursindependent of central swallowing mechanisms(Fig. 3.2) [87–89]. These reflexes prevent theascending spread of the bolus and promotedescending propulsion to ensure esophagealclearance.Secondary esophageal and UES contractilereflexes have been compared in 33 weeks and 36weeks mean post menstrual age premature infants[90]. The occurrence of secondary peristalsis wasvolume dependent, and the characteristicsmatured with age. Furthermore, as the prematureinfant grew older, the occurrence of secondaryperistalsis increased significantly with incrementin dose volumes of air or liquids. Thus, it appearsthat vago-vagal protective reflex mechanismsthat facilitate esophageal clearance are present inhealthy premature neonates, but these mecha-nisms mature with age.Esophageal provocation can also result in anincrease in UES pressure [87, 88]. This reflex isthe esophago-UES-contractile reflex, and ismediated by the vagus. The UES contractilereflex has been studied in premature infants, andlike secondary peristalsis, the occurrence of UEScontractile reflex is volume dependent, and thereflex matures during prenatal stages. This reflexmay provide protection to the airways by limitingthe proximal extent of the refluxate during spon-taneous gastroesophageal reflux events.Gastric Motility in Human NeonatesScant information is available about receptiverelaxation in the fundus in human neonates.Ultrasound studies of the fetal stomach detectedgastric emptying as early as 13 weeks of gesta-tion [91], and the length of gastric emptyingcycles in fetuses increases just prior to birth [92].The rate of gastric emptying is not influenced bynonnutritive sucking, but is influenced by calorificvalue and stress: Calorically denser formulaaccelerates gastric emptying and extreme stress,such as the presence of systemic illness, delaysgastric emptying [93].Small Intestinal Motility in HumanNeonatesIn 28–37 weeks of gestation preterm infants, themajority of the contractile activity in the smallintestine consists of clusters of low amplitudecontractions that propagate for a short distance ornot at all [94]. Propagating, cyclical MMCs withclearly defined phases develop between 37 weeksand term [95].In adults, motilin, which is released frommucosal cells in the duodenum, is an importantregulator of MMCs, and initiation of phase III ofthe MMC (intense rhythmic contractions) in the
  9. 9. 313 Development of Gut Motilityantrum is correlated with an increase in plasmaconcentrations of motilin [3]. In human neonates,the fasting plasma concentrations of motilin aresimilar to those in adults, but there are no detect-able increases in motilin levels coincident withthe initiation of MMCs [96]. The antibiotic,erythromycin, is also a motilin receptor agonistand accelerates gastric emptying in adults [97].Erythromycin triggers initiation of the MMC inpreterm infants whose gestational ages exceed 32weeks [98]. Administration of erythromycin failsto trigger MMCs in infants younger than 32weeks, suggesting immaturity of the neuronalcircuitry mediating MMCs or that the motilinreceptor cannot be activated by erythromycin atthese ages.Developmental Colonic Motilityin Human NeonatesThere is a marked lack of data on colonic motilityin neonatal humans owing to technical limita-tions and ethical concerns.Mechanisms Controlling Motilityin Human Infants and ChildrenAs in laboratory animals, enteric neurons andICC appear to be essential for normal motility inhuman infants and children. An essential role forenteric neurons in gut motility after birth is bestdemonstrated by Hirschsprung’s disease, whereFig. 3.2 Swallow independent secondary esophagealperistalsis in a premature infant in response to a mid-esophageal infusion. Such sequences are evoked duringesophageal provocations and contribute to esophageal andairway protection by facilitating clearance
  10. 10. 32 H.M. Young et al.the segment lacking enteric neurons is unable topropel gut contents. Genetic alterations of Kit,and reduced ICC density, have recently beendirectly linked to a severe case of idiopathic con-stipation and megacolon in a 14 year old child[99], demonstrating the critical relationshipbetween Kit function, ICC development andfunctional gastrointestinal motility patterns in thehuman intestine. Other studies have reportedalterations in ICC networks in Hirschsprung’sdisease, chronic idiopathic intestinal pseudoob-struction, and pediatric constipation [100–106],but these defects may be an indirect consequence,rather than the cause, of the gut dysfunction.However, it is important to remember that motil-ity disorders in children are not necessarily due todefects in neurons or ICC. For example, studiesin mice have shown that defects in the gut musclecan also result in motility defects [107].References1. Sanders KM. Regulation of smooth muscle excita-tion and contraction. Neurogastroenterol Motil.2008;20 Suppl 1:39–53.2. Huizinga JD, Lammers WJ. Gut peristalsis is gov-erned by a multitude of cooperating mechanisms.Am J Physiol Gastrointest Liver Physiol.2009;296(1):G1–8.3. Hasler WL. Motility of the small intestine and colon.In: Yamada T, editor. Textbook of gastroenterology,vol. 1. 5th ed. Philadelphia: Wiley-Blackwell; 2009.p. 231–63.4. Roberts RR, Ellis M, Gwynne RM, et al. The firstintestinal motility patterns in fetal mice are not medi-ated by neurons or interstitial cells of Cajal. J Physiol.2010;588(Pt 7):1153–69.5. Gwynne RM, Thomas EA, Goh SM, Sjovall H,Bornstein JC. Segmentation induced by intraluminalfatty acid in isolated guinea-pig duodenum and jeju-num. J Physiol. 2004;556(Pt 2):557–69.6. Lentle RG, Janssen PW, Asvarujanon P, Chambers P,Stafford KJ, Hemar Y. High-definition spatiotempo-ral mapping of contractile activity in the isolatedproximal colon of the rabbit. J Comp Physiol B.2008;178(3):257–68.7. Wood JD. Enteric nervous system: sensory physiol-ogy, diarrhea and constipation. Curr OpinGastroenterol. 2010;26(2):102–8.8. Holmberg A, Schwerte T, Fritsche R, Pelster B,Holmgren S. Ontogeny of intestinal motility in cor-relation to neuronal development in zebrafishembryos and larvae. J Fish Biol. 2003;63:318–31.9. Holmberg A, Schwerte T, Pelster B, Holmgren S.Ontogeny of the gut motility control system inzebrafish Danio rerio embryos and larvae. J ExpBiol. 2004;207(Pt 23):4085–94.10. Holmberg A, Olsson C, Holmgren S. The effects ofendogenous and exogenous nitric oxide on gut motil-ity in zebrafish Danio rerio embryos and larvae.J Exp Biol. 2006;209(Pt 13):2472–9.11. Holmberg A, Olsson C, Hennig GW. TTX-sensitiveand TTX-insensitive control of spontaneous gutmotility in the developing zebrafish (Danio rerio)larvae. J Exp Biol. 2007;210(Pt 6):1084–91.12. Kuhlman J, Eisen JS. Genetic screen for muta-tions affecting development and function of theenteric nervous system. Dev Dyn. 2007;236(1):118–27.13. Field HA, Kelley KA, Martell L, Goldstein AM,Serluca FC. Analysis of gastrointestinal phys-iologyusing a novel intestinal transit assayin zebrafish.Neurogastroenterol Motil. 2009;21(3):304–12.14. Rasch S, Sangild PT, Gregersen H, Schmidt M,Omari T, Lau C. The preterm piglet—a model in thestudy of oesophageal development in preterm neo-nates. Acta Paediatr. 2010;99(2):201–8.15. McLain Jr CR. Amniography studies of the gastroin-testinal motility of the human fetus. Am J ObstetGynecol. 1963;86:1079–87.16. Sase M, Lee JJ, Park JY, Thakur A, Ross MG,Buchmiller-Crair TL. Ontogeny of fetal rabbit uppergastrointestinal motility. J Surg Res. 2001;101(1):68–72.17. Sase M, Lee JJ, Ross MG, Buchmiller-Crair TL.Effect of hypoxia on fetal rabbit gastrointestinalmotility. J Surg Res. 2001;99(2):347–51.18. Anderson RB, Enomoto H, Bornstein JC, YoungHM. The enteric nervous system is not essential forthe propulsion of gut contents in fetal mice. Gut.2004;53(10):1546–7.19. Yntema CL, Hammond WS. The origin of intrinsicganglia of trunk viscera from vagal neural crest in thechick embryo. J Comp Neurol. 1954;101:515–41.20. Le Douarin NM, Teillet MA. The migration of neu-ral crest cells to the wall of the digestive tract inavian embryo. J Embryol Exp Morphol.1973;30(1):31–48.21. Burns AJ, Le Douarin NM. The sacral neural crestcontributes neurons and glia to the post- umbilicalgut: spatiotemporal analysis of the development ofthe enteric nervous system. Development.1998;125(21):4335–47.22. Kapur RP. Colonization of the murine hindgut bysacral crest-derived neural precursors: experimentalsupport for an evolutionarily conserved model. DevBiol. 2000;227(1):146–55.23. Baetge G, Pintar JE, Gershon MD. Transiently cate-cholaminergic (TC) cells in the bowel of the fetal rat:precursors of noncatecholaminergic enteric neurons.Dev Biol. 1990;141(2):353–80.24. BaetgeG,SchneiderKA,GershonMD.Developmentand persistence of catecholaminergic neurons in
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