Re: 위장 .. 미주신경 지배 2020년 논문

[문형철]

Ann N Y Acad Sci

. Author manuscript; available in PMC: 2020 Oct 1.

Published in final edited form as: Ann N Y Acad Sci. 2019 Jul 3;1454(1):14–30. doi: 10.1111/nyas.14138

Vagal innervation of the stomach reassessed: brain−gut connectome uses smart terminals

Terry L Powley 1, Deborah M Jaffey 1, Jennifer McAdams 1, Elizabeth A Baronowsky 1, Diana Black 1, Logan Chesney 1, Charlene Evans 1, Robert J Phillips 1

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PMCID: PMC6810743  NIHMSID: NIHMS1030992  PMID: 31268562

The publisher's version of this article is available at Ann N Y Acad Sci

Abstract

Brain−gut neural communications have long been considered limited because of conspicuous numerical mismatches. The vagus, the parasympathetic nerve connecting brain and gut contains thousands of axons, whereas the gastrointestinal (GI) tract contains millions of intrinsic neurons in local plexuses. The numerical paradox was initially recognized in terms of efferent projections, but the number of afferents, which comprise the majority (≈ 80%) of neurites in the vagus, is also relatively small.

The present survey of recent morphological observations suggests that vagal terminals, and more generally autonomic and visceral afferent arbors in the stomach as well as throughout the gut, elaborate arbors that are extensive, regionally specialized, polymorphic, polytopic, and polymodal, commonly with multiplicities of receptors and binding sites—smart terminals. The morphological specializations and dynamic tuning of one-to-many efferent projections and many-to-one convergences of contacts onto afferents create a complex architecture capable of extensive peripheral integration in the brain−gut connectome and offset many of the disparities between axon and target numbers. Appreciating this complex architecture can help in the design of therapies for GI disorders.

요약

뇌와 장의 신경 전달은 

눈에 띄는 수치적 불일치 때문에 

오랫동안 제한적이라고 여겨져 왔습니다. 

뇌와 장을 연결하는 미주신경과 부교감신경은 

수천 개의 축삭을 포함하고 있는 반면, 

위장관(GI)은 국소 신경총에 수백만 개의 고유 신경세포를 포함하고 있습니다. 

숫자적 역설은 처음에는 원심성 돌기에서 인식되었지만, 

미주 신경의 신경돌기의 대부분(≈ 80%)을 구성하고,

구심성 돌기는 상대적으로 small합니다. 

미주신경 구성

80~90% 구심성 섬유

구심성 섬유 대부분 C섬유(직경 0.2~1.5um), 일부 A베타섬유(직경 6~12um), 감마섬유(직경 3~6um)

c섬유(수초화x) 전도속도 1~1.5m/s

10~20% 원심성 섬유

원심성 섬유 대부분 B섬유(직경 3um), 일부 A알파 섬유(직경 13~20um)

b섬유 전도속도 3~15m/sa알파 섬유 전도속도 80!~120m/s

• Aα (A-alpha): 80-120 m/s
• Aβ (A-beta): 33-75 m/s
• Aγ (A-gamma): 4-24 m/s
• Aδ (A-delta): 3-30 m/s

최근의 형태학적 관찰에 대한 현재의 조사에 따르면, 

위와 장 전체에 걸쳐 있는 미주 신경 말단, 

그리고 더 일반적으로 자율 신경과 내장 구심성 신경 말단이 광범위하고, 

지역적으로 전문화되어 있으며, 

다형성, 다위치성, 다중성, 다중 모드인 정교한 말단을 형성하고, 

일반적으로 수용체와 결합 부위가 다중으로 존재하는 스마트 말단이라는 것을 시사합니다. 

extensive, regionally specialized, polymorphic, polytopic, and polymodal, commonly with multiplicities of receptors and binding sites—smart terminals

모양적 전문화와 1대 다수의 심성 돌출부와 구심성 접촉의 다대일 수렴의 동적 조정은 

뇌-장 연결망에서 광범위한 주변 통합을 가능하게 하는 복잡한 구조를 만들어 내고, 

축삭과 표적 수 사이의 많은 불균형을 상쇄합니다. 

이 복잡한 구조를 이해하면 

위장 장애에 대한 치료법을 설계하는 데 도움이 될 수 있습니다.

Keywords: afferent, autonomic, efferent, intestines, parasympathetic, postganglionic, preganglionic, stomach, viscera, visceral afferent, vagus

Graphical abstract

In this short review, we examine and illustrate some of the recent observations that have supported the conclusion that autonomic, and specifically vagal, efferents and visceral afferents from the nodose ganglia establish, respectively, one-to-many and many-to-one connections to create the brain−gut connectome.

이 짧은 리뷰에서는 

자율 신경계, 

특히 미주 신경의 원심성 신경과 장구 신경의 구심성 신경이 

각각 일대다 연결과 다대일 연결을 형성하여 

뇌-장 연결망을 형성한다는 결론을 뒷받침하는 최근의 관찰 결과를 검토하고 설명합니다.

Introduction

A century ago, Langley1 highlighted the mismatch between the limited number of vagal preganglionic neurons that project to the GI tract and the extensive network of enteric neurons in the gut wall. He concluded that the limited number of motor axons must contact a circumscribed number of specialized “mother” or “vagal” cells (more recently called “command neurons” by Wood2) in the ENS, and the specialized enteric neurons then coordinated a limited set of effectors. Perhaps because of Langley’s overall influence on physiology and his pivotal role in articulating the features of the ANS, his extrapolation based on the mismatch was codified. His hypothesis was widely, if often implicitly, accepted for much of the last century. Notably, though, in a century of experimentation, command neurons have never been unambiguously identified among gut intrinsic neurons.

None of the earlier observations that shaped the assumptions surrounding the vagal axon:target mismatch attempted to incorporate systematically the prospect that vagal neurites might end in extensive arbors capable of considerable integration locally within the gut wall. The vagal terminals in the gut wall were apparently assumed to be “free nerve endings” and these free nerve endings were assumed to terminate simply. In contrast, however, over the last two or three decades, we and others have reported a variety of experiments that have consistently indicated that both vagal efferents and afferents end in complex, extensive and highly differentiated terminal arbors. These arbor specializations often span different tissue domains, or are “polytopic”. They are sometimes also polymorphic, commonly polymodal, and/or display conspicuous and complex region-specific morphologies. The specialized neurons also regularly express receptors for various signaling molecules.

Contrary to the expectation established by Langley’s early idea, vagal efferent projections to the GI tract develop extensive terminal arbors to establish “one-to-many” networks (without intervening mother cells or command neurons) to create brain-to-gut communication.

Reciprocally, vagal afferent innervation of the GI tract employs complex and highly differentiated terminal arbors in the periphery to establish “many-to-one” dynamic networks to generate gut-to-brain signaling.

In this short review, we examine and illustrate some of the recent observations that have supported the conclusion that autonomic, and specifically vagal, efferents and visceral afferents from the nodose ganglia establish, respectively, one-to-many and many-to-one connections to create the brain−gut connectome. Improvements in neural tracing and labeling have played a large role in characterizing the architecture of the terminals, and we illustrate the arbors using some of these tracing technologies. We also identify recent advances in electrophysiology and other techniques mapping neurochemical phenotypes in the connectome that support the newer perspective on the brain−gut axis.

소개

100년 전, 랭글리1은 위장관에 연결되는 미주신경의 신경절 전구체의 수가 제한되어 있는 것과 장벽에 있는 장신경의 광범위한 네트워크 사이의 불일치를 강조했습니다. 그는 운동 축삭의 제한된 수가 ENS의 특화된 “어머니” 또는 “미주” 세포(최근에는 우드2에 의해 “명령 뉴런”으로 불림)의 제한된 수와 접촉해야 하며, 특화된 장내 뉴런이 제한된 효과기 세트를 조정해야 한다고 결론을 내렸습니다. 아마도 생리학에 대한 랭글리의 전반적인 영향과 ANS의 특징을 명확히 하는 데 있어 그의 중추적인 역할 때문에, 그 불일치에 기반한 그의 추론이 체계화되었습니다. 그의 가설은 지난 세기 동안 암묵적으로 널리 받아들여졌습니다. 그러나 한 세기 동안의 실험에서 명령 뉴런은 장의 고유 뉴런 중에서 명확하게 확인된 적이 없습니다.

미주 신경 축삭과 표적의 불일치에 관한 가정을 형성한 이전의 관찰 중

어느 것도 미주 신경 돌기가 장 벽 내에서 국소적으로 상당한 통합이 가능한

광범위한 축을 형성할 수 있다는 가능성을 체계적으로 통합하려고 시도하지 않았습니다.

장벽에 있는 미주 신경 말단은

“자유 신경 말단”으로 간주되었고,

이 자유 신경 말단은 단순히 종결되는 것으로 간주되었습니다.

그러나

지난 20~30년 동안 우리와 다른 연구자들은

미주 신경의 구심성 신경과 원심성 신경이 복잡하고 광범위하며

고도로 분화된 말단 신경돌기에서 종결된다는 것을 일관되게 보여주는

다양한 실험 결과를 보고했습니다.

이러한

신경돌기 전문화는

종종 다른 조직 영역에 걸쳐 있거나 “다중성”입니다.

때로는 다형성이거나,

일반적으로 다형성이거나,

그리고/또는 뚜렷하고 복잡한 지역 특유의 형태를 나타내기도 합니다.

특수화된 신경세포는

다양한 신호 전달 분자에 대한 수용체를 정기적으로 발현합니다.

랭글리의 초기 아이디어에 의해 확립된 기대와는 달리,

위장관에 대한 미주신경의 구심성 돌기는

광범위한 말단 축삭을 발달시켜 “일대다” 네트워크(중개 모세포나 명령 신경세포 없이)를 구축함으로써

뇌와 장의 소통을 가능하게 합니다.

반대로,

위장관의 미주신경 구심성 신경 분포는

말초에 복잡하고 고도로 분화된 말단 아보어를 사용하여

“다대일” 동적 네트워크를 구축하여

장-뇌 신호를 생성합니다.

이 짧은 리뷰에서는 자율 신경계,

특히 미주 신경의 원심성 신경과 내장 구심성 신경이

각각 일대다 연결과 다대일 연결을 형성하여

뇌-장 연결망을 생성한다는 결론을 뒷받침하는 최근의 관찰 결과를 검토하고 설명합니다.

신경 추적과 라벨링의 개선은 말초 신경 말단의 구조를 특성화하는 데 큰 역할을 해왔으며,

우리는 이러한 추적 기술 중 일부를 사용하여 말초 신경 말단을 설명합니다.

또한,

우리는 뇌-장 축에 대한 새로운 관점을 뒷받침하는

커넥토메의 신경화학적 표현형을 매핑하는

전기 생리학 및 기타 기술의 최근 발전을 확인합니다.

Vagal efferents

Vagal preganglionic efferents extensively innervate the ganglia of the myenteric plexus in the stomach smooth muscle wall. Such profiles have long been observed,3 but with older, nonspecific staining protocols the source or origin and extent of individual varicose fibers remained controversial. With the introduction of neural tracers, it became possible to (a) establish a source through the use of local injections (e.g., the dorsal motor nucleus of the vagus or dmnX; see Berthoud et al.4) of tracers, (b) anterogradely label terminals with high definition5, 6 (see Fig. 1), and (c) reconstruct arborization patterns and selectively labeled individual fibers throughout an organ (see Figs. 2 and 3).

미주 원심성 신경

미주 신경의 원심성 신경은

위 평활근벽에 있는 장간막 신경총의 신경절에 광범위하게 분포되어 있습니다.

이러한 프로필은 오랫동안 관찰되어 왔지만, 오

래된 비특이적 염색 프로토콜을 사용하면 개별 정맥류의 출처나 기원,

범위가 논란의 대상이었습니다.

신경 추적제의 도입으로

(a) 국소 주사를 통해 근원을 파악할 수 있게 되었습니다(예: 미주신경의 등쪽 운동핵 또는 dmnX; Berthoud et al.4 참조). 센서,

(b) 고해상도5, 6(그림 1 참조)로 말단부위에 선행적으로 라벨을 붙이고,

(c) 기관 전체에 걸쳐 분지화 패턴을 재구성하고 개별 섬유를 선택적으로 라벨링하는 것(그림 2 및 3 참조).

Figure 1.

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Preganglionic projections to myenteric ganglia.

(A) Simultaneous labeling of multiple motor projections (brown fibers: PHA-l labeling): large dorsal motor nucleus of vagus injections label efferent contacts on essentially all cells in the individual myenteric ganglia in the stomach (myenteric postganglionic neurons counterstained with Cuprolinic Blue). (B) Selective labeling of limited numbers of vagal preganglionic efferents with small injections (brown fibers: dextran-biotin labeling) reveal contacts around individual myenteric postganglionic neurons (individual postganglionic stained immunohistochemically for nNOS: steel gray secondary). (C) One of three nNOS-positive cells (steel gray secondary) selectively encircled by two preganglionic branches (brown fibers: dextran-biotin labeling). (D) Myenteric ganglia with immunohistochemistry for nNOS (steel gray positive postganglionic neurons) illustrate that some vagal preganglionic terminals selectively encircle the unstained (nNOS-negative) cells of the ganglion. Other preganglionic fibers (not shown; cf. panels B and C) preferentially contact nNOS-positive neurons within the myenteric plexus. Scale bars in plates = 30 μm (panel A), 16 μm (panel B), 25 μm (panel C), and 100 μm (panel D). Panel A reproduced by permission of John Wiley & Sons from Holst et al.5 Panel B reproduced by permission of Elsevier from Walter et al.6

미주신경절에 대한 교차상 신경돌기.

(A) 여러 개의 운동 신경돌기에 대한 동시 표지(갈색 섬유: PHA-l 표지): 미주신경의 큰 등쪽 운동핵은 위 내장신경절의 개별 미주신경절에 있는 거의 모든 세포의 원심성 접촉을 표지합니다(미주신경절 후부 신경세포는 큐프로린 블루로 염색).

(B) 소량의 주사를 통해 선택적으로 제한된 수의 미주 신경절 전방 원심성 신경에 라벨을 붙이면(갈색 섬유: 덱스트란-비오틴 라벨링), 개별 장간막 신경절 후부 신경 주변의 접촉이 드러납니다(개별 신경절은 nNOS에 대한 면역조직화학 염색: 스틸 그레이 2차).

(C) 세 개의 nNOS 양성 세포 중 하나(강회색 2차)가 두 개의 신경절 가지(갈색 섬유: 덱스트란-비오틴 표지)에 의해 선택적으로 둘러싸여 있습니다.

(D) nNOS(강글리오신 후부 신경절 양성 신경세포)에 대한 면역조직화학적 검사를 실시한 장간막 신경절은 일부 미주 신경절 전방 신경절 말단이 염색되지 않은(nNOS 음성) 신경절 세포를 선택적으로 둘러싸고 있음을 보여줍니다. 다른 전방 신경절 섬유(표시되지 않음, 패널 B와 C 참조)는 장간막 신경총 내의 nNOS 양성 신경세포와 우선적으로 접촉합니다. 그림의 눈금 막대 = 30μm(A 패널), 16μm(B 패널), 25μm(C 패널), 100μm(D 패널). A 패널은 John Wiley & Sons의 허가를 받아 Holst et al.의 그림을 재현했습니다.5 B 패널은 Elsevier의 허가를 받아 Walter et al.의 그림을 재현했습니다.6

Figure 2.

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Digitization of individual vagal efferents establishes that single preganglionics link multiple neighboring myenteric ganglia and selectively innervate different phenotypes of myenteric postganglionic neurons.

(A) Neurolucida® (MBF Bioscience, Williston, VT) reconstruction of whole mount and fiber location (upper left) and preganglionic arbor contacting multiple myenteric neurons in multiple ganglia (blue ganglia) within a larger field of ganglia un-innervated by the fiber (pale orange ganglia). Insets

(B) and (C) illustrate the digitized vagal preganglionic fiber (dextran-biotin labeled) and its selectivity for certain myenteric neurons within the ganglia (counterstained with Cuprolinic blue).

(D and E) In other specimens, immunohistochemistry used to selectively label nNOS neurons (steel gray chromogen) illustrates that some vagal preganglionic efferents (brown dextran-biotin labeled fibers) preferentially or selectively ring nNOS-negative or unstained (i.e., presumably cholinergic) profiles (e.g., panel D) whereas other efferents preferentially encircle nNOS-positive (presumably nitrergic) postganglionic neurons (e.g., panel E). Scale bars in plates = 1000 μm (panel A) and 25 μm (panels B−E).

개별 미주 원심성 신경의 디지털화는 단일 신경절이 여러 개의 인접한 장간막 신경절에 연결되어 있고, 장간막 신경절 후부 신경세포의 다양한 표현형을 선택적으로 자극한다는 것을 보여줍니다.
(A) Neurolucida® (MBF Bioscience, Williston, VT) 전체 마운트 및 섬유 위치(왼쪽 위)와 섬유에 의해 자극되지 않는 더 큰 신경절 영역(옅은 주황색 신경절) 내의 여러 신경절(파란색 신경절)에서 여러 신경절성 신경과 접촉하는 신경절 전 신경절의 재구성. 삽입
(B)와 (C)는 디지털화된 미주신경 신경절 전 섬유(덱스트란-비오틴 표지)와 신경절 내의 특정 장신경 세포에 대한 선택성(큐프로린 블루로 염색)을 보여줍니다.
(D와 E) 다른 표본에서, nNOS 뉴런을 선택적으로 표지하는 데 사용되는 면역조직화학(스틸 그레이 발색제)은 일부 미주신경절 원심성 원심성 신경(갈색 덱스트란-비오틴 표지 섬유)이 nNOS 음성성을 우선적으로 또는 선택적으로 둥글게 나타냄을 보여줍니다. 또는 염색되지 않은(즉, 아마도 콜린성) 프로파일을 선호하는 반면(예: 패널 D), 다른 원심성 신경은 nNOS 양성(아마도 니트레신성) 신경절 후부 뉴런을 우선적으로 둘러싸는 경향이 있습니다(예: 패널 E). 플레이트의 눈금 막대 = 1000μm(패널 A) 및 25μm(패널 B-E).

Figure 3.

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Individual intraganglionic laminar endings (IGLEs) issued by a single afferent innervate multiple neighboring myenteric ganglia (red regions), apparently generating large receptive fields, but (unlike vagal efferents) IGLEs do not obviously discriminate or selectively innervate either nNOS-positive or -negative neurons within their receptive fields. (A) A Neurolucida reconstruction of a gastric whole mount and an IGLE afferent within it (right side of image) as well as an enlargement of the IGLE afferent (left part of figure). (B) A photomicrograph of a region of the IGLE afferent illustrated in (A), which reproduces several of the IGLE plates issued by the afferent as well as the myenteric ganglia (stained with a polyneuronal Cuprolinic blue protocol) that the afferent contacts. ( C, D, and E) Examples of IGLE plates from other specimens that illustrate the terminal patterns of IGLE afferents and display how they appear to contact local ensembles of adjacent neurons, whether they are nNOS-positive or -negative (panels C−E are counterstained by nNOS immunohistochemistry). Scale bars in plates = 500 μm (panel A), 250 μm (panel B), and 25 μm (panels C−E).

At the level of the myenteric plexus, individual tracer-labeled vagal preganglionic projections commonly enter ganglia and form rings of contacts surrounding many of the neurons within the plexus (Fig. 1A and 1C). The rings routinely make conspicuous, highly varicose presumptive contacts on the postganglionic neurons. Ultrastructural observations7 substantiate that such varicosities are synaptic contacts with myenteric ganglionic neurons and commonly contain round and translucent, presumably cholinergic, vesicles massed in the region of the presynaptic site of the cell membrane. In addition to cholinergic vesicles, some presynaptic varicosities of vagal efferents also contain (presumptively) dopaminergic vesicles. The vast majority, possibly all, of the myenteric ganglion neurons in the plexus of the stomach wall appear to receive such terminal projections of vagal preganglionics (see Fig. 1A, 1B, and 1C).

When the terminal arbors of vagal efferents are traced and digitally reconstructed, they display a consistent pattern. Individual efferents entering their terminal projection fields form arbors that ramify so as to contact myenteric ganglion cells of multiple neighboring ganglia in a particular regional field within the myenteric plexus. Figure 2A illustrates a digitized and flattened 3D tracing of a representative vagal arbor. As Figure 2A also illustrates, the terminal arbors issued by an individual preganglionic vagal motor axon innervate a collection or string of essentially all neighboring ganglia within a circumscribed field and limit their terminal contacts to those ganglia. Preganglionic arbors do not regularly issue meandering branches that “stray” far beyond the circumscribed and coherent terminal field. The pattern suggests that adjacent terminal fields supplied by neighboring preganglionics composed of “plates of neighboring ganglia,” collectively girdle the stomach with an extensive, potentially complete, tessellated network of projections.

Further, the local or regional networks formed by individual preganglionics are apparently cross-coupled in a pattern that may produce dimensionally larger ensembles organized for more extensive regional coordination. Individual preganglionics or “motor tiles”, such as the one in Figure 2A, apparently overlap insofar as multiple preganglionics seem to converge on individual ganglia. The pattern is conspicuous when one compares the extrinsic projections to a single ganglion in animals with multiple (or large) injections into the dmnX (e.g., Fig. 1A) versus the efferent projection to a ganglion when one uses single (or smaller) injections into the motor nucleus (e.g., Figs. 1B, 1C, 2B, and 2C). In the case of larger injections, virtually all of the postganglionic cells appear innervated, whereas in the case of the smaller injections, a single preganglionic fiber appears to innervate only a fraction of all of the neurons in a ganglion.

Additional features suggest that the vagal preganglionics generate a terminal architecture to support complex motor programming of the stomach. In brief, one feature stems from considering the pattern of peristaltic waves that moves chyme through the stomach and intestines; a second feature is the cholinergic and nitrergic ganglionic organization in the myenteric plexus that generates peristalsis.

Peristaltic waves that move food more distally in the GI tract, including the stomach, consist of patterns of leading smooth muscle relaxation reducing intraluminal pressure immediately aboral to a bolus of food or chyme timed to coordinate with a muscular contraction immediately oral to the bolus. This choreography or coordination of reciprocating relaxation and contraction must travel along the GI tract in the appropriate phase relationships to move nutrient material along the alimentary canal. Though in its most elemental pattern, peristalsis can be generated by the enteric nervous system without extrinsic input,8, 9 optimally timed and most efficient wave patterns are coordinated with extrinsic nervous system modulation. The enteric nervous system, of course, has a rich and varied collection of neurochemical phenotypes;10 however, nearly all myenteric neurons in the stomach wall are also either nitrergic, producing and using nitric oxide as a transmitter, or cholinergic, expressing and using acetylcholine as a transmitter. The nitrergic neurons typically inhibit smooth muscle contractions, whereas the cholinergic neurons excite smooth muscle activity. In terms of peristaltic activity, the relaxing distal leading edge of the peristaltic wave seems to reflect nitrergic activity, whereas the contracting and more proximal trailing edge of the wave reflects cholinergic potentiation of contraction.

A recent and ongoing analysis of vagal efferent projections suggests one integrative function of the vagal efferent arbors. Jaffey and coworkers (unpublished) have eval‎uated whether individual vagal preganglionic arbors selectively contact nitrergic (or NOS-positive) ganglion cells, selectively contact cholinergic (NOS-negative) ganglion cells, or both. Throughout the stomach, two distinct preganglionic phenotypes occur. One phenotype of efferent preferentially contacts NOS-positive, presumably “inhibitory,” ganglion cells in multiple neighboring myenteric ganglia (Fig. 2E). The other phenotype selectively contacts NOS-negative (Fig. 2D), presumably cholinergic myenteric neurons that are “excitatory” and cause contractions, in multiple neighboring myenteric ganglia. (A third phenotype is also observed and appears to crosslink NOS-positive and -negative postganglionic neurons.) The common dichotomized or “bipolar” architectural pattern of preganglionics suggests that one population of efferents crosslinks NOS-positive neurons throughout its respective projection fields to coordinate relaxation, whereas a second population crosslinks NOS-negative cholinergic neurons through its target fields to coordinate contraction. If so, cross coupling of the two phenotypes could then program and phase peristaltic activity. Thus, many vagal preganglionics apparently are organized in a push-pull architectural pattern that might well be responsible for pacing and coordinating aspects of traveling peristaltic waves generated in the stomach.

미주신경총 수준에서,

개별 추적자 표지된 미주신경의 신경절 전 가지가

일반적으로 신경절에 들어가서 신경총 내의 많은 뉴런을 둘러싸는 접촉 고리를 형성합니다(그림 1A 및 1C).

이 고리는 흔히 신경절 후부 신경세포에 뚜렷하고

매우 다발성인 추정 접촉을 형성합니다.

초구조적 관찰7은

이러한 다발성이 신경절 신경세포와의 시냅스 접촉이며,

일반적으로 세포막의 시냅스 전 영역에 둥글고 반투명하며,

아마도 콜린성 소포가 밀집되어 있음을 입증합니다.

콜린성 소포체 외에도,

미주 원심성 신경의 일부 시냅스 전 정맥류에는 (추정되지만)

도파민성 소포체가 포함되어 있습니다.

위벽 신경총에 있는 장간막 신경절 뉴런의 대부분,

아마도 전부는 미주 신경절 신경의 이러한 말단 돌출부를 받는 것으로 보입니다(그림 1A, 1B, 1C 참조).

미주 신경 원심성 신경의 말단 돌기가 추적되고

디지털 방식으로 재구성되면,

일관된 패턴을 보여줍니다.

개별 원심성 신경이 말단 돌기 영역에 들어가면,

미주 신경총 내의 특정 지역 영역에 있는

여러 인접 신경절의 미주 신경절 세포와 접촉할 수 있도록

가지가 뻗어나가는 돌기를 형성합니다.

그림 2A는 대표적인 미주 신경 가지의 디지털화된 평면화된 3D 추적을 보여줍니다. 그림 2A에서 볼 수 있듯이, 개별 신경절 전미주 신경 운동 축삭에서 나오는 말단 가지들은 한정된 영역 내에 있는 본질적으로 모든 인접 신경절의 집합체 또는 끈을 자극하고, 말단 접촉을 그 신경절들로 제한합니다.

신경절 전 신경절 가지들은

일정한 경계를 이루고 일관된 말단 영역을 훨씬 넘어 “이탈”하는

구불구불한 가지를 정기적으로 분지하지 않습니다.

이 패턴은

인접한 신경절 전 신경절들이 공급하는 인접한 말단 영역이 “인접한 신경절의 판”으로 구성되어,

잠재적으로 완전한 광범위한 돌출부 네트워크로 위를 둘러싸고 있음을 시사합니다.

또한,

개별 신경절에 의해 형성된 국소적 또는 지역적 네트워크는 분명히 교차 결합되어,

더 광범위한 지역적 협응력을 위해 조직된

더 큰 차원의 앙상블을 만들어 낼 수 있는 패턴을 형성합니다.

그림 2A에 있는 것과 같은 개별 신경절 또는 “운동 타일”은 여러 개의 신경절이 개별 신경절에 수렴하는 것처럼 보이는 한, 분명히 겹쳐져 있습니다. 이 패턴은 dmnX에 다중(또는 큰) 주사를 한 동물의 외생성 돌기(예: 그림 1A)와 운동핵에 단일(또는 작은) 주사를 한 동물의 원심성 돌기(예: 그림 1B, 그림 1C, 2B, 2C)를 비교해 보면 뚜렷하게 드러납니다. 더 큰 주사를 놓는 경우, 사실상 모든 신경절 후부 세포가 신경 분포된 것처럼 보이지만, 더 작은 주사를 놓는 경우, 단일 신경절 전방 섬유만이 신경절에 있는 모든 뉴런의 일부만 신경 분포하는 것처럼 보입니다.

추가적인 특징은

미주 신경의 신경절 전섬유들이

위장의 복잡한 운동 프로그램화를 지원하는 말단 구조를 생성한다는 것을 시사합니다.

간단히 말해서, 한 가지 특징은

위와 장을 통해 소화액을 이동시키는 연동 파동의 패턴을 고려할 때 발생합니다.

두 번째 특징은

연동 운동을 생성하는 장간막 신경총의 콜린성 및 니트로글리세린성 신경절 조직입니다.

위장을 포함한 위장관 내의 음식을 더 먼 곳으로 이동시키는 연동파는

음식 덩어리 또는 소화액의 바로 앞쪽에서

근육 수축과 동시에 시작되어 바로 뒤쪽에서 끝나는 평활근 이완의 패턴으로 구성되어 있습니다.

이러한 연동파의 안무 또는 수축과 이완의 협응력은

영양소를 소화관으로 이동시키기 위해

적절한 위상 관계로 위장관을 따라 이동해야 합니다.

가장 기본적인 패턴의 경우,

장내 신경계에 의해 외부 입력 없이 연동 운동이 생성될 수 있지만,8, 9

최적의 타이밍과 가장 효율적인 파동 패턴은

외부 신경계 조절과 함께 이루어집니다.

장 신경계는 물론,

풍부하고 다양한 신경화학 표현형을 가지고 있습니다10.

그러나

위벽에 있는 거의 모든 장 신경세포는

또한 산화질소를 전달 물질로 생성하고 사용하는 질산 신경세포이거나,

아세틸콜린을 전달 물질로 표현하고 사용하는 콜린 신경세포입니다.

The enteric nervous system, of course, has a rich and varied collection of neurochemical phenotypes;10 however, nearly all myenteric neurons in the stomach wall are also either nitrergic, producing and using nitric oxide as a transmitter, or cholinergic, expressing and using acetylcholine as a transmitter. The nitrergic neurons typically inhibit smooth muscle contractions, whereas the cholinergic neurons excite smooth muscle activity. In terms of peristaltic activity, the relaxing distal leading edge of the peristaltic wave seems to reflect nitrergic activity, whereas the contracting and more proximal trailing edge of the wave reflects cholinergic potentiation of contraction.

니트로겐성 신경세포는

일반적으로 평활근 수축을 억제하는 반면,

콜린성 신경세포는 평활근 활동을 자극합니다.

연동 활동의 관점에서 볼 때,

연동 파동의 이완된 원위 선단은 니트로겐성 활동을 반영하는 것으로 보이며,

수축하고 더 근위인 후미는 콜린성 수축의 강화 작용을 반영합니다.

최근 진행 중인 미주신경 원심성 돌기의 분석은

미주신경 원심성 돌기의 통합적 기능을 제시합니다.

Jaffey와 동료 연구원(미발표)은

개별 미주신경 신경절 전돌기가

선택적으로 질소산화물(또는 NOS 양성) 신경절 세포에 접촉하는지,

선택적으로 콜린성(NOS 음성) 신경절 세포에 접촉하는지,

아니면 둘 다 접촉하는지 평가했습니다.

위 전체에 걸쳐

두 가지 뚜렷한 신경절 전 표현형이 발생합니다.

원심성 표현형 중 하나는

우선적으로 NOS 양성,

아마도 “억제성” 신경절 세포와 여러 개의 인접한 장간막 신경절에 접촉합니다(그림 2E).

다른 표현형은

우선적으로 NOS 음성(그림 2D), 아마도 “흥분성”이고 수축을 일으키는

콜린성 장간막 신경세포와 여러 개의 인접한 장간막 신경절에 접촉합니다.

(세 번째 표현형도 관찰되며, NOS 양성 및 음성 신경절 후 신경세포를 교차 연결하는 것으로 보입니다.)

신경절 전 신경세포의 일반적인 이분법적 또는 “양극성” 구조 패턴은,

하나의 원심성 집단이 이완을 조정하기 위해

각각의 투영 영역 전체에 걸쳐 NOS 양성 신경세포를 교차 연결하는 반면,

두 번째 집단은 수축을 조정하기 위해 표적 영역을 통해

NOS 음성 콜린성 신경세포를 교차 연결한다는 것을 시사합니다.

그렇다면,

두 가지 표현형의 교차 결합은

연동 운동을 프로그램화하고 단계적으로 진행할 수 있습니다.

따라서,

많은 미주 신경 신경절은

위에서 생성된 연동 운동 파동의 속도를 조절하고 협응하는 역할을 하는

푸시풀 구조 패턴 push-pull architectural pattern 으로 구성되어 있는 것으로 보입니다.

Vagal afferents

Three broad phenotypic classes of vagal afferents (intraganglionic laminar endings, intramuscular arrays, and mucosal arbors—see below) innervate the stomach wall. The distinctive, defining architecture of each of the classes suggests that these afferent projections also support complex and extensive integrative capacities in the brain−gut connectome. Moreover, the topography of afferents suggests that much pre-processing, or peripheral integration, in the stomach wall offsets the limited number of “transmission lines” for afferent neurite relays in the vagus nerve.

Further, though there is some tendency in the experimental literature to assume that the different individual afferent inputs to the central nervous system (CNS) operate separately and need to be considered separately (perhaps because they commonly need to be dissected individually in most experimental protocols), if one thinks of the visceral afferent stream to the CNS as representing a cross-fiber pattern representation, then the combinatorial possibilities become enormous. The different phenotypes, regional locations, intensities of responses, patterns of responses, and “hormonal/paracrine tone,” all may well be factored into continuous cross-fiber real-time representations of GI conditions.

미주신경 구심성 신경세포

미주신경 구심성 신경세포의 세 가지 광범위한 표현형

(신경절 내 층상 말단, 근육 내 배열, 점막 기둥 - 아래 참조)은 

위벽을 자극합니다. 

각 표현형의 독특한 특징은 

이 구심성 신경세포가 

뇌-장 연결망의 복잡하고 

광범위한 통합 능력을 지원한다는 것을 의미합니다. 

또한, 

구심성 신경의 위벽에서의 위상학적 구조는 

미주신경의 구심성 신경돌기의 “전송 라인”의 제한된 수를 

위벽에서의 많은 사전 처리 또는 주변 통합이 상쇄한다는 것을 시사합니다.

또한, 실험 문헌에서는 

중추신경계(CNS)에 대한 개별 구심성 입력들이 개별적으로 작용하며 

개별적으로 고려되어야 한다고 가정하는 경향이 있습니다

(아마도 대부분의 실험 프로토콜에서 개별적으로 분석해야 하기 때문일 것입니다). 

그러나, 

중추신경계에 대한 내장 구심성 흐름을 교차 섬유 패턴 표현으로 간주한다면, 

조합의 가능성은 엄청나게 많아집니다. 

The different phenotypes, regional locations, intensities of responses, patterns of responses, and “hormonal/paracrine tone,” all may well be factored into continuous cross-fiber real-time representations of GI conditions.

다양한 표현형, 

지역적 위치, 

반응의 강도, 

반응의 패턴, 

그리고 “호르몬/파라크린 톤”은 

모두 위장 상태의 지속적인 교차 섬유 실시간 표현에 포함될 수 있습니다.

Intraganglionic laminar endings (IGLEs)

Of the three general phenotypes of gastric afferents, intraganglionic laminar endings or IGLEs were the first to be recognized morphologically,11 established as vagal by vagotomy,12 and named.13 IGLEs are found throughout the GI tract including the esophagus,14 stomach,15 small intestine,16, 17 cecum, and colon.15

IGLEs are associated with the ganglia of the myenteric plexus. Conventionally, an IGLE consists of a plate of lamelliform terminal puncta apparently in contact with a ganglion. The plate is typically situated superficial to the ganglion, effectively lying in association with laminae between the ganglion and either the longitudinal muscle above the ganglion or the circular muscle below the ganglion. When an IGLE afferent fiber reaches its target field in the stomach wall, it arborizes extensively, producing IGLE plates in association with a number of neighboring ganglia (Fig. 3).1

Individual IGLE plates (Figs. 3B, 3C, 3D, 3E and 4A’) commonly produce a web of flattened puncta that tend to variously over- or under-lay the ganglion (though the former pattern predominates) and appear to establish close approximations or contacts with many of the ganglion cells. The puncta commonly appear lamelliform in appearance, and these lamellar puncta often issue secondary and tertiary puncta emerging from an initial punctum, not the parent neurite itself. The lamelliform puncta of the IGLE plates, when well stained, often appear to issue spiny protrusions or fingers seemingly analogous to dendritic spines seen in the cerebral cortex (Figs. 3C, 3D, 3E, and 4A’). IGLE plates also often produce a neurite “bar” along the surface of the ganglion or its immediate connectives; such “bars” often appear to be coalesced by or formed from the lamelliform puncta, rather than directly from the neurite branch itself (Fig. 3B, 3C, and 3D). The IGLE plates have been the subject of only limited ultrastructural work, but available EM observations on IGLEs in the esophagus and gastric cardia18 and in the forestomach19 indicate that IGLEs, perhaps via their “fingers” or spines and similar protrusions which appear to extend into the parenchyma around ganglion cells, do make contacts with the ganglion cells and do contain translucent vesicles and limited numbers of opaque vesicles.

Figure 4.

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The conventional IGLE phenotype exhibits regional specializations. IGLE branches, particularly in the distal stomach, often have more varicose, less lamelliform specializations of puncta. (A’) Inset illustrates common IGLE puncta morphology of flattened lamelliform puncta, frequently displaying spinous extensions, routinely observed in the proximal stomach. (A, B, and C) In the distal stomach, IGLE afferents often issue some—or even a majority of—branches that terminate in rounded or beaded varicosities, rather than flattened and lamelliform puncta. These IGLE specializations can consist of branches with a mixture of varicosities and smaller lamelliform puncta (e.g., panel A), branches issuing almost exclusively simpler varicosities (e.g., panel B), or branches terminating in dense aggregations of simple and varicose puncta (e.g., panel C). Scale bars = 10 μm (panel A’), 25 μm (panels A and B), and 50 μm (panel C).

IGLE plates appear to function (though perhaps not exclusively) as tension receptors in the smooth muscle wall. In an early assessment of the structural features of IGLE plates, Neuhuber and Clerc20 suggested that the IGLE plates would be distorted by shearing when tension created shearing forces and distorted the laminar interface—including the myenteric plexus tissue--between the longitudinal and circular muscle layers. Based on several lines of indirect evidence, including morphology and distributions, Phillips and Powley21 compared IGLEs and intramuscular arrays (IMAs, described below) and concluded that IGLEs might indeed serve as tension receptors in the GI tract, whereas IMAs might function as stretch or length detectors in the wall of the GI tract (and though the analogy still seems apt, it is, of course, imperfect—see Timmermans and Adriaensen22). In our analysis, we also emphasized that “tension” and “stretch” were regularly confounded and confused in early electrophysiological experiments. More recently, Zagorodnyuk, Brookes et al.23, 24 were able to record from neurites associated with tracer-identified vagal afferent terminals and concluded that IGLEs were tension receptors.

Individual vagal IGLE afferents elaborate these individual ganglionic plates in an extensively branching terminal arbor (cf. Fig. 3A). Typically, the afferent neurite travels some distance in the myenteric plexus and then, on reaching the specific region of gastric wall that it innervates, the neurite arborizes into a complex of branches ending in IGLE plates on a field of adjacent ganglia. Much like the local concentration of ganglia innervated by an individual efferent (see above), IGLE afferents arborize in local concentrations of ganglia, each receiving a plate of puncta from the afferent in question. Also much like the local concentration of ganglia innervated by an individual efferent, the pattern of a concentrated field of IGLE plates issued by a single common IGLE afferent suggests that the IGLE plates may be in some sense integrating the activity or information distributed within a terminal “receptive field.”

Other observations also suggest that the architecture of the IGLE afferent arbors invests the fibers with local integrative potential: Individual afferent plates and entire individual afferent arbors commonly seem to establish lamelliform contacts with both inhibitory (NOS-positive) and presumptive excitatory (or NOS-negative/cholinergic) postganglionic neurons within individual ganglia (Figs. 3C, 3D, 3E, and 4A; Jaffey et al., unpublished). Potentially, IGLEs as tension receptors are well positioned to, and may track waves of inhibitory and excitatory responses associated with peristalsis and provide feedback about motility as extrinsic efferent inflows pace or recalibrate and coordinate forces of tension with motor scores.

Vagal afferents, and specifically vagal IGLE afferents, also appear to be polymodal and to integrate additional information and presumably modify afferent traffic (and vagovagal reflex activity) used by the CNS to generate both physiology and behavior involved in nutrient handling and energy balance. Substantial evidence establishes that vagal afferents innervating the GI tract have receptors, and are affected by both endocrine and paracrine release, for myriad GI hormones secreted by enteroendocrine cells (EEC) and specialized glandular tissues,25, 26 also, see below).

The evidence for CCK, for example, is particularly strong. Binding studies, in situ hybridization experiments, electrophysiological effects, deployment of blockers, and denervation experiments all point to the fact that vagal afferents to the GI tract are sensitive to, and tuned by, fluxes of CCK and other hormones of metabolism. Since some of these CCK-responsive vagal afferents are located in regions innervated almost exclusively by IGLE afferents (e.g., the “tension receptors” reported by Blackshaw and Grundy,27 see also Okano-Matsumoto et al.28), it appears a reasonable inference that vagal IGLE afferents do indeed comprise part of the pool of vagal afferents that dynamically integrate neural signals with local peripheral and circulating levels of CCK.

Another group of observations suggests that vagal IGLE afferents may have “smart” and adaptive arbors that supply substantial integrative potential out of all proportion to the number of neurites found within the nerve trunk. Other afferents (e.g., somatosensory afferents innervating the skin) sometimes support “axon reflexes” in which a membrane potential in a terminal branch invades other branches of the same afferent terminal, causing the release of transmitters or neuromodulators that produce local motor-like responses or tissue effects in the periphery and independent of any central relay. Such axon-reflex capability has been posited for vagal afferents as well,20, 29 and two features of IGLEs give the conclusion feasibility. Firstly, in a regional adaptation of the general IGLE morphology, some of the IGLEs located in the more distal corpus and antrum have not only many branches terminating in the characteristic plates of flattened, lamellar puncta, they also have other branches or collaterals that enter the more central region of the myenteric ganglia, display rounded, varicose contacts in proximity to postganglionic neurons (see Fig. 4A, 4B, and 4C). Secondly, as mentioned above, ultrastructural observations have reported presumptive synaptic vesicles in IGLE processes, suggesting that the afferent terminal arbors may release neuropeptides or other transmitter molecules in the target tissues.

Finally, and most speculatively in terms of IGLEs’ structured specializations shaping function, one might expect that IGLEs situated between the longitudinal muscle and the myenteric plexus might be more sensitive to longitudinal muscle stress whereas IGLEs located between the circular muscle and the plexus would be more affected by circular muscle stress.

Intramuscular arrays (IMAs)

IMAs were the second phenotype of vagal afferent terminals in the stomach to be recognized and well characterized morphologically. In 1992, our laboratory group29 labeled vagal afferent terminals with the anterograde tracer DiI, simultaneously characterizing the morphology of the endings and establishing, by limited nodose injections, that the terminals were vagal. Shortly thereafter, using HRP as an anterograde tracer, Wang and Powley30 further began to survey the endings and introduced the terminology of “intramuscular arrays.” As the name suggests and as we have described,31, 32 and in contrast with both vagal efferents and IGLE afferents both of which innervate the myenteric plexus situated between the two smooth muscle layers of the muscularis externa projecting to the stomach, intramuscular arrays or IMAs specifically innervate gastric smooth muscle proper. Individual IMAs distribute region-specific arbors either in the longitudinal smooth muscle layer superficial to the myenteric plexus or the circular smooth muscle layer deep to the myenteric plexus (and only rarely both muscle layers32).

Within the targeted smooth muscle layer, vagal IMAs arborize, branching so as to produce a series of short elements, bridging perpendicular to the smooth muscle fibers (Fig. 5B, 5C, and 5E), that bifurcate into long neurites which run parallel to and with bundles of smooth muscle fibers (Fig. 5A, 5B, 5C, and 5E) and the associated interstitial cells of Cajal (ICC) networks between muscle fibers (Fig. 5D). These long terminal branches express varicosities and flattened elements that appear to associate with and contact ICCs, forming an array of long parallel neurites. Immunohistochemical surveys of the IMAs indicate that the processes of the IMA-ICC complexes often intermingle with different neurochemically distinct afferent and efferent neurites.33 Ultrastructural observations of the IMAs corroborate the contacts with smooth muscles and ICCs, document the presence of vesicles, and illustrate that the IMA neurites and terminal branches often course in small fascicles of a few neurites.19

Figure 5.

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Intramuscular arrays (IMAs), in contrast to vagal preganglionic efferents and IGLE afferents, directly innervate the muscle layers and form arrays of branches that typically run with the ICC network found in the smooth muscle layers. Panels A, B, and C are photomicrographs of branches of the IMA digitized with Neurolucida in panel E. Panel D illustrates how IMA branches (dextran-biotin labeled brown fiber) course with ICCs (elongated purple cell—Vector® VIP labeled for cKit antibody) in the muscle layers. Panel F illustrates the specialized “web ending” variant of IMAs seen near the distal antral/pyloric insertion of sling muscle fibers. Panel G is a higher power image of the IMA apparatus and its contacts or varicosities illustrated in panel F. Scale bars = 20 μm (panels A−C), 10 μm (panel D), 250 μm (panels E and F), and 25 μm (panel G). Panels A–E reproduced by permission of Elsevier from Powley and Phillips.33

Systematic tracings and digital reconstructions indicate that IMA afferents most commonly consist of a “parent” neurite coursing to the innervation site and then ramifying into a single, often extensive, array of long, parallel terminal branches. Occasionally, a parent arbor will issue one or more secondary arbors.

Functionally, the operation(s) of IMAs remain an open empirical question. Based on the features of the afferents and several tentative inferences, we have provisionally suggested that vagal afferent IMAs have the architecture and contacts of stretch or length detectors (whereas IGLE afferents discussed above have the architecture and contacts of tension receptors). Our stretch-receptor suggestion21, 33 seems to fit the available evidence and to have heuristic value. It is, however, still provisional and certainly an approximation.22 Unfortunately, most electrophysiological analyses of vagal afferents that could potentially have distinguished between stretch or tension receptors have confounded the two types of force and thus do not permit any definitive conclusion. It is worth noting that in our initial description of IMAs,29 we too conflated tension and stretch reception and discussed IMAs in terms of “tension.” Consistent with a more particular and explicit distinction that IMAs may record stretch or length whereas IGLEs may record tension, Zagorodnyuk, Brookes, and coworkers23, 24 in their recent analyses of tracer-labeled IGLEs considered IGLEs to be tension receptors.

Vagal IMA afferents display particularly clearly the point that the complex vagal terminals in the stomach express predictable regional specializations in their morphology, which may generate different functional capacities. To date, the regional surveys of IMA specializations have been more comprehensive than the observations on gastric IGLEs or other afferents, and in the case of IMAs in particular, the regional specializations also seem consistent with the hypothesized role of stretch receptors.

At any rate, though, IMAs do display a multiplicity of adaptations to the region they innervate. IMAs in the forestomach are particularly densely distributed in both longitudinal (closer to greater curvature) and circular (closer to the lesser curvature) muscle layers, and the endings are extensive and elongated, features consistent with the reservoir function of the forestomach.30, 32 IMAs in the corpus and proximal antrum are prominent, particularly in circular muscle, consistent with mixing and pumping functions associated with the regions.32 In the antrum, near the pyloric attachment of sling muscles, IMAs also organize into a highly specialized web apparatus34 (Fig. 5F and 5G). The long sling muscles running superficially from the pyloric lesser curvature to the LES, are innervated by exceptionally elongated IMAs.34 And both the clasp muscles and sling muscles where they encircle the LES, are densely innervated by IMA afferents.35 The thick sphincter circular muscle ring of the pylorus is heavily innervated by relatively short IMAs, though with a densely arborized branching pattern.36 In the case of the pyloric IMAs, circumstantial evidence again suggests that IMA function may be modulated by CCK release: the pyloric IMAs appear colocalized with the distribution of CCK receptors in pyloric circular muscle,37 and the receptors appear to be densely distributed on the ICCs38 of the IMA-ICC complexes in that circular muscle.

And most generally, the stretch receptor hypothesis of IMAs is reinforced by the fact that the stomach, which because of a sphincter at either end operates as a reservoir of nutrients that distends, contracts and stretches with every meal, is extensively innervated with vagal IMA afferent terminals, whereas the intestines, which essentially constitute long “open-ended” tubes without sphincters (between the pylorus and ileocecal junction) have a minimum of IMAs.

Gastric mucosal arbors

While vagal IGLE afferents and IMA afferents comprise the general categories of vagal afferents that innervate the muscularis externa, the mucosa and submucosa of the stomach wall are innervated by a different class of complex vagal terminal. Of the three broad phenotypes of gastric afferent terminals, these are the last to be characterized morphologically with tracer injections. We recently described their architecture, established (by nodose injections of the tracer) that they are vagal, and designated them “gastric antral mucosal arbors” because of their architecture and location in the gastric glandular mucosa.31 The architecture of the mucosal endings in the corpus or most of the stomach have yet to be fully examined.

Nonetheless, though only recently and partially described morphologically and mapped, the broad outline of their architecture has come into focus. The pattern again suggests that the gastric mucosal arbors have topographies consistent with extensive, broadly tuned structures capable of integrating considerable information on stomach function. As illustrated in Figure 6, individual vagal afferents course through the submucosa to begin arborizing at the base of the gastric glandular mucosa. The parent neurite divides repeatedly, forming a number of higher order branches that create dense networks of afferent terminals in the deeper layers of the mucosa and a subset of terminals that course along the inner epithelial wall of the gastric glands to continue to terminal endings immediately below the lumenal mucosal lining (Fig. 6A). These gastric mucosal arbors consist of varicose branches, many of which run in close proximity to EEC (Fig. 6B), potentially affected by the paracrine release of those EEC,26, 39 as well as by the secretory products elaborated by the local gastric glands and mucosa. Presumably, those higher order terminal branches coursing up to the lumenal epithelium of the gastric mucosa would also respond directly or indirectly to the contents and composition of the gastric chyme.40

Figure 6.

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Vagal mucosal arbors are afferents that arborize deep in the mucosal layer and send a subset of their branches paralleling the gastric glands and reaching the basal side of the epithelial wall in direct contact with the contents of the stomach. Panel A illustrates a dextranbiotin filled mucosal arbor (brown branches) in the mucosal layer. Panel B illustrates how the mucosal fibers branch and ramify in the deeper mucosal layers in the zone heavily populated with EEC (in the specimen in panel B, the brown dextran-biotin labeled afferent articulates with EEC immunohistochemically stained for gastrin (steel gray secondary)). Scale bars = 100 μm.

It should also be noted that the arbors of the antral mucosal afferents are “bushy” or extensive in z-, as well as in x- and y-, up to about 500 µm in x- and z-, and about 200 Δm in y- (within the thickness of the mucosa). One significance of this “bushy” pattern of arborization, if the afferent had mechanoreceptive properties, would be that the arbor might not have major orientation sensitivity, but rather might respond more to net pressure or deformation, regardless of orientation. Such functional patterns have been described electrophysiologically for vagal afferents in the mucosa,41, 42 and bushy 3D arbors sensitive to deformation or pressure in any orientation might well provide the functionality for the stomach to monitor the pressure associated with contractions, peristaltic waves, grinding of chyme, or maceration of food material.

Considerable evidence suggests that these arbors have been partially characterized electrophysiologically (and, if so, they are apparently polymodal arbors, and perhaps with multiple receptive fields43), but they, as discussed, have only recently—and partially—been morphologically characterized.

Esophagus and intestines: sympathetics and spinal visceral afferents

A primary focus of the present review is the vagal innervation specifically of the stomach. This focus was adopted in the interest of space, but also because the gastric innervation, compared to that in the rest of the GI tract, has been the most thoroughly inventoried. The conclusion that the autonomic innervation of the GI tract is complex and extensive, based on a variety of distinctive phenotypes specialized to different regions, however, appears to be general, possibly ubiquitous. Certainly, the pattern of extensive, complex, and specialized smart terminals is not limited to the stomach, to the vagus, or to parasympathetic projections.

Rather than reflecting a specific stomach feature, complex and specialized vagal terminals are found throughout the GI tract. Vagal preganglionic motor fibers are found from the esophagus, through the stomach, as well as through the intestines to at least the proximal colon.44 Similarly, vagal IGLE afferents are found from the esophagus to the proximal colon.14, 16, 17 Vagal IMAs are apparently found in the esophagus, the stomach and its sphincters, and within the intestines (low density; Phillips and Powley, unpublished data). Furthermore, as we have recently described in a survey of the proximal small intestines, the vagus elaborates two additional and unique phenotypes of afferent terminals in the postgastric GI tract. One of these afferents produces terminal branches that coil around intestinal crypts or glands (see Fig. 7B), and the second intestinal phenotype forms arbors in the villi of the intestine (Fig. 7A).

Figure 7.

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Vagal afferents innervate villi and crypts (or intestinal glands) throughout the small intestines. Panel A illustrates a villus ending (brown neurites labeled with dextran-biotin) in the distal jejunum. Vagal villus afferents throughout the small intestine issue multiple branches that course along the basal side of the epithelial wall and run apically to the villus tips. Several adjacent villi can be innervated by one afferent arbor. Along the small intestine, the vagal villus afferents exhibit local specializations with, generally, more numerous branches in individual villi in the proximal intestines and less numerous branches in the distal intestines. Panel B illustrates the arbor of a villus crypt—or gland—afferent (brown neurites labeled with dextran-biotin) encircling multiple neighboring glands immediately below intestinal villi. This afferent, located in the distal duodenum, characteristically links several neighboring glands into a presumptive receptive field. Scale bars = 100 μm.

Moreover, extensive and specialized endings are not restricted to the vagus. For example, postganglionic sympathetic axons (originating in the celiac and superior mesenteric ganglia) innervating both the stomach and intestines form complex and polytopic projections and have complex distributions45 (Fig. 8). Similarly, the special vagal efferent projections (which are issued by the neurons of the compact or “retrofacial” subnucleus of the nucleus ambiguus) terminate in esophageal striated muscle and end in complex, extensive and polytopic patterns consistent with the smart terminal inference.35 In addition, visceral afferents issued by the “sacral” component of the “craniosacral” parasympathetic division of the autonomic nervous system supply complex afferents to the distal GI tract. Brookes and colleagues23, 24, 46, 47 have applied tracers to the distal stump of fiber bundles (including dorsal root afferents) to the distal colon and rectum and observed a rectal IGLE-like profile (rIGLEs) in the rectum. Spencer and coworkers47, 48 have injected dextran conjugates into the lumbosacral DRGs and reported a multiplicity of types and variants of afferent arbors in the distal GI tract.

Figure 8.

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Sympathetic efferent fibers innervating the gut exhibit complex terminal arbors. The labeled fiber in this photomicrograph (brown neurite labeled with dextran biotin) courses through and contacts myenteric ganglion cells (entering at lower right) and then turns and continues to extensively arborize in the smooth muscle wall, where it produces branches coursing both longitudinally (top to bottom of figure) and circularly (left to right of figure). Scale bar = 31 μm. Reproduced by permission of John Wiley & Sons from Walter et al.45

Receptors, hormones, and paracrine integration

Though, in the initial sections of this review we have concentrated on a few examples of receptor molecules where there is good evidence for a coincidence of the maps for individual vagal efferent or afferent arbor phenotypes and the maps for distributions of identified receptor or binding site types, a considerable amount of additional evidence indicates more generally— with less clear (or unexamined) coincidence of a single phenotype and particular receptor type—that vagal projections to the stomach are rich with hormone, paracrine factor, and another molecular binding site expression‎. For example, vagal afferents (precise target and architecture unspecified) examined in terms of nodose ganglia neuronal somata expression‎ (mRNA, immunohistochemistry, etc.) produce receptors for CCK, serotonin, lipid amides, Y2, CART, MCH-1, T1R2/T1R3 heterodimers, GPR120, etc.49, 50, 25 The case is compelling that vagal afferents including those that innervate the stomach are effectively influenced by the local tissues, the chemistry and the environment of the circulation, and the contents of the GI tract. Given the high percentages of afferent somata in the nodose ganglion that express the various separate types of paracrine/hormone/etc. binding sites, it must follow that some or all individual nodosal afferents express binding sites for multiplicities of different hormonal and humoral signals.

Our focus in this review is on how the crosslinking, coordination and integration of terminals morphologically “hardwired” in the vagal−gut architecture may generate processing in the periphery. In addition to this hardwiring of processing syntheses, however, it is also important to recognize that such inherent integrative capacity is further amplified by sensitivity to endocrine/paracrine factors and dynamic humoral adjustments of the innervation. Considerable evidence now indicates that vagal efferents and afferents (typically not specified in terms of their phenotypes and targets and in many cases only characterized as “vagal”—insofar as the associated response(s) can be blocked by vagotomy—rather than even specifically motor or sensory) are influenced by, i.e., respond with various up- or downregulations to, the conditions of the gut. These labile gut factors include diet, microbiome, disease, and insult. For example, manipulations of the microbiome of the gut certainly affect the brain−gut connectome51 and can reflect dynamic changes of the afferents that monitor the status of the internal milieu. Furthermore, the cascade of inflammatory responses and associated cytokines that can impact the GI tract can produce major changes in the vagal pathways comprising the brain−gut connectome.52,53

Therapeutic manipulations of the brain−gut connectome

Given the myriad GI disorders that are poorly treated pharmacologically (e.g., gastroparesis, reflex disorder, eating disorders, obesity, IBS, celiac disease; etc.), the need for prescriptions for selective manipulations of the vagus and/or optimally locating of GES electrodes is apparent. The fact that vagal projections to the upper GI tract are smart, or morphologically “wired” for coordination or integration, has significant promise for therapeutic interventions. On the other hand, assessments of the vagus nerve, some of its efferents and afferent phenotypes, and some of its terminal concentrations are too incomplete to yet formulate comprehensive prescriptions. Too little is yet known about the precise locations of the different neurons (i.e., their neurites) in the vagus nerve or their terminal distribution patterns in the GI tract to specify a prescription for selective activation of particular vagal effectors, nonetheless a review of the smart vagal projections to the stomach highlights several principles: (1) Successful vagal nerve stimulation (VNS) will likely have to be engineered to selectively affect specific phenotypes of vagal efferent and/or afferent neurites. (2) For a given disorder, a decision will have to be made as to whether to directly or indirectly (by engaging vagovagal reflex circuitry) affect GI function. (3) Successful gastric electrical stimulation (GES) will likely have to be engineered to selectively affect specific sites with concentrations of particular phenotypes efferents or afferents. (4) Again, in the case of GES for a particular disorder (as in the case of VNS), a choice of directly or indirectly engaging motor outflows will have to be made. (5) Successful VNS or GES will likely have to be tuned or engineered to selectively affect one phenotype of efferent or afferent intermingled with other projections of other phenotypes of similar caliber or similar target sites.

Summary

In general, observations of the architecture of the different autonomic projections to the gastrointestinal tract revealed by neural tracers underscore two conclusions: (a) the autonomic innervation of the GI tract, or the degree of communication between the brain and the gut is underestimated and cannot be adequately understood from simple counts of cell bodies or axons responsible for the coordination and (b) the autonomic projections to the gut are characterized by extensive and smart terminal arbors that may perform substantial integration in the periphery, even before the signals are communicated to the CNS.

Acknowledgments

The authors thank C.N. Billingsley, M.-C. Holst, J.B. Kelly, F.N. Martin, G.C. Walter, and F.-B. Wang for their expert help in the use and refinement of the several different tracer protocols as well as their technical assistance employed in tissue preparation. The morphological analyses of the authors were supported by NIH grants OT2OD023847 and DK-027627 to TLP and RJP.

Footnotes

1

Some sources treat the plate as an “IGLE”; whereas other sources refer to the entire afferent issuing the complete set of the one plate and all others issued by a common neurite as an “IGLE”. For consistency and to avoid ambiguities, here, we distinguish the plate associated with a particular ganglion as an “IGLE” and we refer to the entire afferent as an “IGLE afferent.”

Competing interests

The authors declare no competing interests, and the National Institutes of Health provided all monetary support.

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