Anatomy and Physiology of the Enteric Nervous System Gut: First Published As 10.1136/Gut.47.Suppl 4.Iv15 on 1 December 2000

Home , Enteric nervous system, Migrating motor complex, Myenteric plexus, Peristalsis

Gut 2000;(Suppl IV)47:iv15–iv19 iv15 Anatomy and physiology of the enteric nervous system Gut: first published as 10.1136/gut.47.suppl_4.iv15 on 1 December 2000. Downloaded from

M Costa, S J H Brookes, G W Hennig

The enteric nervous system (ENS) is a quasi and in the duodenum. It is on this spontane- autonomous part of the nervous system and ously active muscular apparatus that the includes a number of neural circuits that enteric motor circuits play their roles in control motor functions, local blood flow, shaping diVerent motor patterns. mucosal transport and secretions, and modu- The neural apparatus is composed of a large lates immune and endocrine functions. Al- number of enteric neurones that can be though these functions operate in concert and identified according to their location, neuro- are functionally interlinked, it is useful to con- chemistry, shape, projections, proportions, con- sider the neural circuits involved in each sepa- nections, and function. After intensive research rately.1 This short summary will concentrate from several laboratories over the past two dec- mainly on the neural circuits involved in motor ades, a full description of all functional classes of control.2 The enteric neural circuits are com- enteric neurones has been recently achieved in posed of enteric neurones arranged in networks the guinea pig small intestine (fig 1).6 The strat- of enteric ganglia connected by interganglionic egy included the development of methods com- strands. Most enteric neurones involved in bining immunohistochemistry, electrophysiol- motor functions are located in the myenteric ogy, retrograde tracing, neuronal filling, lesion plexus with some primary aVerent neurones techniques, and pharmacological analysis. located in the submucous plexus. As in all nervous systems involved in sensory-motor control, the ENS comprises primary aVerent The enteric neurones neurones, sensitive to chemical and mechanical PRIMARY AFFERENT NEURONES stimuli, interneurones and motorneurones that Primary aVerent neurones (also termed enteric act on the diVerent eVector cells including primary aVerent neurones (EPANs) or intrin- smooth muscle, pacemaker cells, blood vessels, sic primary aVerent neurones (IPANs)) are mucosal glands, and epithelia, and the distrib- present in both myenteric and submucous gan- uted system of intestinal cells involved in glia. They respond to luminal chemical stimuli, immune responses and endocrine and para- to mechanical deformation of the mucosa, and crine functions. to radial stretch and muscle tension. It is not The digestive tract is unique among internal yet clear whether epithelial cells such as organs because it is exposed to a large variety of enterochromaYn cells release substances, for http://gut.bmj.com/ physicochemical stimuli from the external instance serotonin, in response to chemical or world in the form of ingested food. As a conse- mechanical stimuli, to activate the endings of quence, the intestine has developed a rich rep- the primary aVerent neurones.7 They represent ertoire of coordinated movements of its about 30% of myenteric neurones and 14% of muscular apparatus to ensure the appropriate submucosal neurones, have a distinct Dogiel mixing and propulsion of contents during type II shape and have a long after hyperpolari-

digestion, absorption, and excretion. The sation following action potentials. All of these on October 2, 2021 by guest. Protected copyright. oro-aboral transit of the intestinal contents can neurones project to the villi and branch within be regarded as a form of adaptive locomotion the submucous and myenteric ganglia locally. that occurs over a wide range of spatial and A proportion of these neurones (10% of temporal domains.3 The movements of the primary aVerent neurones) also have long intestine are the result of interaction of the descending projections to aboral myenteric neural apparatus and the muscular apparatus.4 ganglia.8 They receive slow synaptic input The muscular apparatus is organised in (probably mediated by tachykinins) from other muscle layers made up of large collections of primary aVerent neurones to form reciprocally smooth muscle cells interconnected electrically innervated networks. They project circumfer- via gap junctions to operate as larger functional entially to synapse with myenteric ascending mechanical units. The membrane potential of interneurones, descending interneurones, long- smooth muscle is driven to oscillate (slow itudinal muscle motorneurones, excitatory waves) by a syncytial network of pacemaker circular muscle motorneurones, and inhibitory Department of cells (interstitial cells of Cajal) probably also circular muscle motorneurones. It is likely that Physiology and Centre via gap junctions.5 As the action potentials of diVerent subpopulations are connected sepa- of Neuroscience, the smooth muscles, and thus their associated rately, with ascending and descending path- Flinders University, ways. SA Australia muscle contraction, do not appear to propagate M Costa over long distances, the coordination of muscle S J H Brookes activity over long distances is highly dependent G W Hennig on the spatiotemporal patterns of the slow wave Abbreviations used in this paper: ENS, enteric nervous system; EPANs, enteric primary aVerent generated by the pacemaker networks. The Correspondence to: neurones; IPANs, intrinsic primary aVerent neurones; Professor M Costa. myogenic patterns of activity can support pro- ChAT, choline acetyltransferase; VIP, vasoactive marcello.costa@ﬂinders.edu.au pulsive behaviour, for example in the antrum intestinal peptide; NOS, nitric oxide synthase.

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EXCITATORY CIRCULAR MUSCLE ASCENDING INTERNEURONES

MOTORNEURONES This small (5%) but most important class of Gut: first published as 10.1136/gut.47.suppl_4.iv15 on 1 December 2000. Downloaded from These represent the ﬁnal motor output to the enteric neurones belongs to the Dogiel type I circular muscle (14%), have a Dogiel type I morphology, and receives fast synaptic inputs shape, receive fast nicotinic and probably slow from other ascending interneurones which synaptic input from local primary aVerent neu- form a chain of ascending excitation. They also rones, and from the only class of cholinergic receive fast nicotinic and slow synaptic inputs ascending interneurones. They also appear to from enteric primary aVerent neurones. They receive excitatory inputs from descending project orally within the myenteric plexus to interneurones. They project to the circular synapse with the ﬁnal excitatory circular mus- muscle where they form a denser arrangement cle motor neurones via fast nicotinic and non- of nerve endings in the deep muscular plexus. cholinergic slow synaptic inputs. They contain They use acetylcholine and tachykinins as not only the enzyme for the synthesis of acetyl- transmitters acting directly on smooth muscle choline but also tachykinins and opioid pep- and possibly indirectly via the network of inter- tides.13 stitial cells in the deep muscular plexus.9–11

DESCENDING INTERNEURONES INHIBITORY CIRCULAR MUSCLE MOTORNEURONES There are several classes of descending in- These Dogiel type I neurones (17%) receive terneurones that comprise about 7% of the fast nicotinic inputs from primary aVerent total.614 Three of these are probably choliner- neurones and non-cholinergic inputs from the gic as they contain the enzyme for the synthe- long descending primary aVerent neurones. sis of acetylcholine, choline acetyltransferase They project to the circular muscle where their (ChAT). Each diVers in their neurochemistry. axons are intimately associated with those of Somatostatin and ChAT containing descend- the excitatory motorneurones in the deep mus- ing interneurones (4%) have a ﬁlamentous cular plexus. They use multiple mechanisms of shape, receive fast and slow synaptic inputs inhibitory transmission including nitric oxide, mainly from non-primary aVerent neurones, adenosine triphosphate, and the peptides and form a chain of interconnected interneu- vasoactive intestinal peptide (VIP) and pitui- rones synapsing with other somatostatin neu- tary activating cyclic AMP peptide acting rones and with other myenteric and submu- directly on smooth muscle or indirectly via cous neurones. Serotonin and ChAT interstitial cells.911 containing neurones (2%) project aborally to other myenteric and submucosal neurones but LONGITUDINAL MUSCLE MOTORNEURONES not to inhibitory motorneurones. Whether This relatively large class (25%) of small these neurones use serotonin in addition to neurones with short projections to the longitu- acetylcholine remains to be conﬁrmed. Serot- dinal muscle receive synaptic inputs from the onin may act via fast ion channel gated recep- enteric primary aVerent neurones and from tors or via slow G protein linked receptors. 12

ascending and descending pathways. Nitric oxide synthase (NOS), VIP, and ChAT http://gut.bmj.com/

LM IN LMMN

DIN on October 2, 2021 by guest. Protected copyright.

MP EPAN

CM EMN IMN

F′ SMP

Mucosa

Oral Anal Figure 1 Classes of myenteric neurones. LM, longitudinal muscle; CM, circular muscle; MP,myenteric plexus; SMP, submucous plexus; AN, ascending neurones; IN, intestinofugal neurones; DIN, descending interneurones; EPAN, enteric primary aVerent neurones; EMN, excitatory motorneurones; IMN, inhibitory motorneurones; LMMN, longitudinal motorneurones. The secretomotor and vasomotor neurones in the myenteric and submucous plexuses are not labelled (modiﬁed from Costa and colleagues6).

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Ascending excitatory Circumferential Descending

pathways pathways pathways Gut: first published as 10.1136/gut.47.suppl_4.iv15 on 1 December 2000. Downloaded from Nicotinic receptors?

Receptors P2X receptors? Nicotinic Muscarinic Nicotinic Nicotinic receptors receptors ACh ACh ACh/TK ACh/TK

NO Nicotinic Nicotinic Non-cholinergic ACh Non-cholinergic ATP receptors receptors receptors receptors Non-cholinergic VIP ACh TK receptors ACh PACAP TK Circular muscle TK Stretch receptors Circular muscle Circular muscle

Figure 2 Polarised enteric motor pathways involved in control of intestinal motor activity. ACh, acetylcholine; TK, tachykinins; NO, nitric oxide; ATP, adenosine triphosphate; VIP,vasoactive intestinal peptide; PACAP,pituitary activating cyclic AMP peptide; P2X, subclass of purinergic receptors. containing neurones also project aborally to there are very few enteric primary aVerent synapse with other myenteric neurones. Neu- neurones of the Dogiel type II in the stomach. rones with NOS and VIP, but without ChAT, also project to other aboral myenteric and probably submucous ganglia. Whether these Enteric neural circuits non-cholinergic interneurones use other fast The initial steps in the elucidation of the synaptic transmitters such as adenosine tri- enteric neural circuits has been accomplished phosphate or glutamate remains to be estab- by using the classic approach of specific and lished. localised stimuli and recording of the reflex The dual projection of some of these responses. It was the demonstration of polar- interneurones to both myenteric and submu- ised responses to mechanical stimuli by Bayliss cous ganglia represents the likely functional and Starling (1899)15 that started the modern link between motor, secretory, and vasomotor analysis of enteric reflex pathways. They pathways. postulated the existence of short ascending excitatory pathways and longer descending SECRETOMOTOR AND VASOMOTOR NEURONES inhibitory pathways giving rise to the idea of There are two small classes (1% each) of secre- “the law of the intestine”. In recent years, tomotor neurones in the myenteric ganglia. analysis of such pathways has advanced signifi- One are cholinergic and the other non- cantly (fig 2). Thus there are ascending excita- cholinergic containing VIP. They project to the tory pathways that involve the EPANs, a chain mucosa. Neurones with a similar function and of short ascending interneurones, and the final neurochemistry are also present in the submu- excitatory motorneurones to the circular mus- http://gut.bmj.com/ cous ganglia where they represent 32% and cle. The descending inhibitory pathways prob- 42%, respectively. Some of the VIP submucous ably involve a diVerent class of EPANs, with neurones also project to the myenteric ganglia long anal projections connected to the final and may represent the basis for a functional inhibitory motorneurone. There are also cir- connection between secretion and motility. cumferential pathways that are activated by The VIP secretomotor neurones receive inhibi- mechanical stimulation of the EPANs, which

tory synaptic inputs from the extrinsic sympa- synapse with local inhibitory and excitatory on October 2, 2021 by guest. Protected copyright. thetic neurones and from unidentified my- motorneurones. There is also a descending enteric neurones. Most submucous neurones excitatory pathway that involves mechanically receive fast and slow synaptic inputs. A small sensitive EPANs and final excitatory mo- submucous neurone class of submucous torneurones to the circular muscle. Whether cholinergic neurones (12%) project to the there are descending interneurones in these mucosa and to the local blood vessels. pathways remain to be established. The reflex pathways involving motor responses of the long- INTESTINOFUGAL NEURONES itudinal muscle have been poorly investigated. There is a small proportion of cholinergic neu- There are also reflex pathways involving the rones that receive fast synaptic inputs, and rare intestinofugal neurones, located in the project from myenteric ganglia to the preverte- myenteric ganglia, which synapse in the bral ganglia. prevertebral ganglia with postganglionic sym- pathetic neurones projecting to the submucous OTHER GASTROINTESTINAL REGIONS and myenteric ganglia. These represent short The remarkable polarities of enteric motorneu- intestino-intestinal inhibitory reflex pathways rones and interneurones, revealed in the small that, when activated, reduce both motor and intestine, extend also to the oesophagus, stom- secretomotor activity. ach, and large intestine, suggesting that it is a Similar mechanical and chemical stimuli are prominent and preserved feature of the ar- associated with secretomotor and vasomotor rangement of the enteric neural pathways. In responses. The secretomotor and vasomotor the diVerent regions there are also significant neural pathways underlying these responses diVerences in the classes represented and in involve EPANs both in the submucous and their neurochemical coding. For example, myenteric ganglia and cholinergic and non-

www.gutjnl.com iv18 Costa, Brookes, Hennig

Module with local

circumferential Gut: first published as 10.1136/gut.47.suppl_4.iv15 on 1 December 2000. Downloaded from circuit

AI AI AI AI

DI DI DI

Oral Anal Figure 3 Modular organisation of the enteric nervous system with repeated and overlapping circumferential, ascending, and descending neural pathways. AI, ascending pathways; DI, descending pathways. cholinergic secretomotor and vasomotor neu- confirmed, that in the colon, activation of the rones in the submucous ganglia. collaterals of the extrinsic primary spinal aVer- ent neurones is essential for peristalsis. The MODULAR ORGANISATION OF THE ENS possible role of such collaterals of spinal All classes of enteric neurones are equally dis- sensory neurones in other motor patterns in tributed along the entire network of ganglia. normal and physiopathology remains to be Thus all enteric pathways described above elucidated. The exact timing of activation of form continuous overlapping networks. As each of these pathways requires further study. small rings of circular muscle can contract However, it appears that the initiation of independently, these rings and the associated peristalsis involves the explosive recruitment of enteric neurones can be regarded as functional enteric excitatory motorneurones reminiscent modules (fig 3). The spatiotemporal coordina- of self-reinforcing neural processes.20 21 The tion of these interconnected modules are the neurones and mechanisms responsible for this determining factor for the generation of the explosive recruitment probably include the rich repertoire of motor patterns.24 primary aVerent neurones and the ascending interneurones. Thus peristalsis can be regarded MOTOR PATTERNS more appropriately as a motor pattern, a type An important question arises about the role of of intestinal locomotion, based on neurome- these reflex pathways in the generation of the

chanical feedback, rather than a simple reﬂex. http://gut.bmj.com/ diVerent motor patterns. In addition to the myogenic mechanisms that can generate rhythmic contractions and in some cases propulsion, MIGRATING MOTOR COMPLEXES other motor patterns can be distinguished One of the most interesting complex neural according to their dependence on intestinal activities outside of the central nervous system contents. is the neurally mediated migrating motor complex. In addition to the motor patterns which

ACCOMMODATION are dependent on the intestinal content activat- on October 2, 2021 by guest. Protected copyright. Local accommodation is well known to occur ing the enteric reflex pathways (as described in the stomach and large intestine. Also, in the above), there are neural circuits within the small intestine during filling, the distension ENS that spontaneously initiate circular mus- activates local pathways, probably a mixture of cle motor activity over extensive portions of the circumferential and descending inhibitory re- intestine.22 The circuits involved in the genera- flex pathways, that relax the circular muscle.16 tion of the migrating motor complex include final excitatory motorneurones, which are acti- NEURAL PERISTALSIS vated by other enteric neurones via nicotinic The propulsion of contents due to the sequen- and non-nicotinic mechanisms. As this neural tial contraction of the circular muscle initiated activity slowly migrates aborally, it is probable by the intestinal content has been described as peristalsis. This pattern occurs when the intra- that some of the descending interneurones are luminal volume reaches a threshold or certain responsible for the aboral migration. Although chemicals are present in the lumen. This in the small intestine this cyclic motor activity pattern of propulsive behaviour consists of a has been recorded only in conscious animals, ring of circular muscle contraction that starts evidence suggests that more localised migrat- orally and is propagated aborally pushing the ing motor activity also occurs in isolated contents forward.17 The neural circuits in- preparations of small and large intestine. It is volved in this stereotyped motor pattern possible that the segmenting motor pattern include activation of the ascending excitatory described by earlier investigators may be due to pathways, descending inhibitory pathways, and more localised, slowly migrating neural activity possibly activation of descending excitatory and its interaction with myogenic rhythmic pathways.18 19 It has been suggested, but not activity.

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Overview 5 Sanders KM. A case for interstitial cells of Cajal as pacemakers and mediators of neurotransmission in the

The mechanisms of motor activity include gastrointestinal tract. Gastroenterology 1996;11:492–515. Gut: first published as 10.1136/gut.47.suppl_4.iv15 on 1 December 2000. Downloaded from myogenic rhythmic activity, with its own 6 Costa M, Brookes SJH, Steele PA, et al. Neurochemical classification of myenteric neurons in the guinea-pig ileum. spatiotemporal patterns, the content depend- Neuroscience 1996;75:949–67. ent neural mechanisms, namely accommoda- 7 Furness JB, Kunze WAA, Bertrand PP, et al. Intrinsic tion and propulsion, and the spontaneous primary aVerent neurons of the intestine. Prog Neurobiol 1998,54:1–18. cyclical migrating motor complexes. These 8 Brookes SJH, Song Z-M, Ramsay G, et al. Long aboral pro- mechanisms interact to produce the rich reper- jections of Dogiel type II, AH neurons within the myenteric plexus of the guinea-pig small intestine. J Neurosci toire of intestinal movements. The way in 1995;15:4013–22. which the relative importance of each of these 9 Brookes SJH, Steele PA, Costa M. Identification and immunohistochemistry of cholinergic and non-cholinergic circu- mechanisms shifts continuously, thus provid- lar muscle motor neurones in the guinea pig small ing an adaptive switching between diVerent intestine. Neuroscience 1991;42:863–78. 10 Steele P, Brookes S, Costa M. Immunohistochemical identi- motor patterns, is still far from being under- fication of cholinergic neurons in the myenteric plexus of stood. Yet, studies of this kind will provide the guinea-pig small intestine. Neuroscience 1991;45:227–39. basis for understanding of the physiopathology 11 Brookes SJH, Costa M. Enteric motor neurones. In: Tache Y, Wingate DL, Burks TF, eds. Innervation of the gut. Patho- so often associated with gastrointestinal func- physiological implications. Boca Raton, Florida: CRC Press, tions. The muscular and neuronal mechanisms 1994:237–48. 12 Brookes SJH, Song Z-M, Steele PA, et al. Identification of described above are based on rather robust motor neurones to the longitudinal muscle of the processes. However, there is a wealth of guinea-pig ileum. Gastroenterology 1992;103:961–73. 13 Brookes SJH, Meedeniya ACB, Jobling P, et al. Orally substances contained in the enteric neurones projecting interneurones in the guinea-pig small intestine. J capable of modulating the activities of the Physiol 1997;505.2:473–91. 14 Song ZM, Brookes SJH, Costa M. Projections of specific enteric neural circuits and of the muscular morphological types of neurons within the myenteric apparatus.23 Every mechanism described is plexus of the small intestine in the guinea-pig. Cell Tissue Res 1996;285:149–56. potentially subject to intrinsic physiopathologi- 15 Bayliss WM, Starling EH. The movements and innervation cal modulation and is thus a potential target for of the small intestine. J Physiol Lond 1899;24:99–143. 16 Waterman SA, Tonini M, Costa M. Accommodation medi- pharmacological intervention. This opens an ated by enteric inhibitory reflexes in the isolated guinea-pig enormous new field of research both in gastro- small intestine. J Physiol 1994;474:539–46. 17 Hennig GW, Costa M, Chen BN, et al. Quantitative analysis intestinal physiology and in the pharmaceutical of peristalsis in the guinea-pig small intestine using spatio- development of therapies to correct deficien- temporal maps. J Physiol 1999;517:575–90. cies and abnormalities. 18 Waterman SA, Costa M. The role of enteric inhibitory motoneurons in peristalsis in the isolated guinea-pig small intestine. J Physiol 1994;477.3:459–68. 1 Furness JB, Costa M. The enteric nervous system. Edinburgh: 19 Waterman SA, Tonini M, Costa M. The role of ascending Churchill Livingstone, 1987. excitatory and descending inhibitory pathways in peristalsis 2 Costa M, Brookes SJ. The enteric nervous system. Am J in the isolated guinea-pig small intestine. J Physiol Gastroenterol 1994;89:129–37. 1994;481:223–32. 3 Costa M, Hennig GW, Brookes SJH. Intestinal peristalsis: A 20 Tonini M, Costa M, Brookes SJH, et al. Dissociation of the mammalian motor pattern controlled by enteric neural cir- ascending excitatory reflex from peristalsis in the guinea- cuits. In: Keihn RMH-WO, Jordan LM, Hultborn H, et al, pig small intestine. Neuroscience 1996;73:287–97. eds. Neuronal mechanisms for generating locomotor activity. 21 Brookes SJH, Chen BN, Costa M, et al. Initiation of New York: New York Academy of Sciences 1998;860:464– peristalsis by circumferential stretch of flat sheets of 6. guinea-pig ileum. J Physiol 1999;516:525–38. 4 Costa M, Brookes SJH, Song Z-M. From enteric neurons to 22 Sarna SK. Cyclic motor activity; migrating motor complex.

peristalsis. In: Fuxe K, Hockfelt T, Olson L, et al,eds. Gastroenterology 1985;89:894–913. http://gut.bmj.com/ Molecular mechanisms of neuronal communication. Wenner- 23 Furness JB, Bornstein JC, Murphy R, et al. Roles of peptides Gren International Series, vol. 68. Amsterdam: Elsevier, in transmission in the enteric nervous system. Trends Neu- 1996: 89–110. rosci 1992;15:66–71. on October 2, 2021 by guest. Protected copyright.