Enteric nervous system

Gut-Brain Communication and the Microbiome

Extraordinary interest in the communication between the gut and brain over the past decade.
Focus on the role of the microbiome and substances within the gut wall in influencing health and wellbeing.
Understanding sensory nerve activation is crucial for understanding how the gut responds to different substances.
Discerning how the gut detects sensory stimuli is of supreme importance to understanding the control of the gut–brain axis.

Advances in Understanding Sensory Neurons

Important advances in knowledge of intrinsic and extrinsic sensory neurons.
These neurons communicate local reflexes within and outside the gut to the spinal cord and brain.

Historical Perspective on Gut Autonomy

Mid-1700s: Segments of intestine isolated from vertebrates can respond to stimuli independently of the brain and spinal cord.
Mid-1990s: Unequivocal evidence of the enteric nervous system (ENS) containing its own population of sensory neurons.
- These neurons can initiate reflex responses mediated solely via the ENS.

Recent Advances in Understanding Neural Reflex Responses

Past 5–10 years: Important advances in understanding how chemical and mechanical stimuli applied to the gut wall can elicit neural reflex responses within the ENS.
Major advances in understanding how extrinsic afferents (sensory nerve endings in the gut wall with cell bodies outside the gut) respond to certain stimuli.

Differences Between Intrinsic and Extrinsic Sensory Mechanisms

Even within the same bowel region and species, mechanisms that transduce mechanical or chemical stimuli into neural reflexes can differ substantially between intrinsic sensory neurons (in the ENS) and extrinsic sensory nerve endings (with cell bodies outside the gut wall).

Importance of Understanding Sensory Stimuli Detection

Understanding how the gut detects sensory stimuli is of supreme importance to understanding the control of the gut–brain axis.

Focus of the Review

Discussion of major advances regarding the mechanisms underlying the transduction of sensory stimuli in the gut wall into neural activity within intrinsic and extrinsic sensory nerves.

Enteric Nervous System (ENS)

The gastrointestinal tract is the only hollow organ with its own complete nervous system, the ENS.
The ENS can function fully independently of neural inputs from the central nervous system (CNS).

Evidence for ENS Autonomy

Bowel segments removed from vertebrates continue to generate complex propulsive neurogenic motor patterns ex vivo, even after severing all extrinsic nerves.

Historical Discoveries

1755: Von Haller
- Identified that intestines preserve peristaltic motion even after being deprived of communication with the brain.
1890: Lüderitz
- Provided the first clear description that local stimulation of the bowel could evoke polarized responses in isolated segments of the intestine.
Bayliss and Starling
- Confirmed polarized neural responses in exteriorized segments of dog intestine following local stimulation.
- Formulated the ‘law of the intestine’: “Local stimulation of the gut produces excitation above and inhibition below the excited spot”.
Trendelenburg (World War I)
- Coined the term ‘peristaltic reflex’ to describe the propulsion of content along isolated bowel segments.

Contemporary Understanding of Neurogenic Propulsion

Much progress identifying fundamental control mechanisms underlying neurogenic propulsion in the small and large intestine of small vertebrates and humans.
Confirms that the ENS alone can generate complex neurogenic motor patterns without CNS inputs.

ENS Structure

Consists of many thousands of discrete small ganglia with neural continuity, forming two distinct ganglionated neuronal plexuses: the myenteric and submucosal plexuses.

Innervation of Smooth Muscle

Unlike skeletal muscle (innervated only by excitatory neurons), gastrointestinal smooth muscle cells are densely innervated by both excitatory and inhibitory motor neurons.

Neuronal Heterogeneity

Each plexus contains a heterogeneous population of individual neurons with distinct neurochemical coding, projections, and functional roles.

Internodal Strands

Ganglia within the submucosal and myenteric plexuses are connected by internodal strands carrying axons over substantial distances (up to 13 cm).
- Facilitates rapid conduction of neuronal signals along the bowel.

Redundancy in the ENS

Considerable redundancy: loss of large numbers of enteric neurons does not necessarily lead to loss of motility or function.
Mice that lose at least half of their ENS can still generate rhythmic propagating neurogenic colonic motor complexes and live a normal lifespan.

Evolutionary Development of the ENS

The evolutionary process of the ENS development is not restricted to vertebrates.
Invertebrates in the cnidarian phylum (e.g., Hydra) have an intrinsic nervous system without central ganglia (CNS) capable of generating propulsive peristaltic-like movements and expelling waste.

Functional Roles of the Myenteric and Submucosal Plexuses

The myenteric plexus coordinates muscle movements underlying propulsion.
The submucosal plexus is broadly involved in secretion and absorption.
The myenteric plexus is presumed to fulfill a similar role in invertebrates.

Importance of the ENS for Life

Vertebrates born with genetic mutations leading to complete loss of enteric ganglia in the terminal colorectum exemplify the ENS's importance.
Loss of enteric neurons over a substantial length of colorectum leads to lack of neurogenic motility patterns and improper expulsion of colonic contents, often resulting in megacolon and death shortly after birth.

Interest in Generating Enteric Neurons

Long-term interest in generating enteric neurons in aganglionic regions of the bowel for mammals with Hirschsprung disease or enteropathies associated with a deficiency in enteric neurons (e.g., Chagas disease).

Transplantation Studies

Recent studies showed that transplanted neural progenitor-containing neurospheres from mouse and human can integrate into the aganglionic mouse colon in vivo and ex vivo.
However, the extreme phenotype of the aganglionic mouse model often leads to offspring death within 1–2 months, making it difficult to assess treatment benefits.

Advances in Stem Cell Research

ENS progenitors from human pluripotent stem cells can be efficiently derived and isolated, differentiating into functional enteric neurons.
These precursors display targeted migration in the developing embryo of chicks and colonize the large intestine of adult rodents.

In Vivo Transplantation Success

In vivo studies have demonstrated successful transplantation and integration of enteric neural stem cells to restore nitrergic neurons in nitric oxide-deficient mouse colon.
Transplantation leads to a recovery of neuronal nitric oxide synthase-positive enteric neurons and restoration of motility, associated with the development of elaborate networks of transplanted cells.
This was a major advance as it demonstrated, for the first time, that enteric neural stem cell transplantation can improve colonic function.

Controversy Regarding Enteric Neuron Turnover

Original studies showed neurogenesis continues postnatally until about postnatal day 21, then declines.
Kulkarni et al. (2017) proposed rapid turnover in adult mice, with >85% of myenteric neurons in the adult mouse small intestine being less than 2 weeks old.
This high turnover rate would require substantial cell death, yet apoptosis is infrequent in the developing ENS of mice.
The study suggested that neuronal loss (due to apoptosis) is balanced by substantial turnover and neurogenesis, maintaining a constant number of myenteric neurons.
- This occurs via neuron formation from dividing precursor cells located within myenteric ganglia that express nestin and p75NTR.
- They also suggested a loss of approximately 4–5% of myenteric neurons daily, equivalent to about 30% every 7 days.

Unresolved Questions

If neurons turn over so rapidly, it's unclear how retrograde neuronal tracing studies can visualize labeled enteric neurons many days after tracer application.
It's also unclear how synaptic connections and neurotransmission persist with such rapid nerve cell body turnover.
Future studies are needed to better understand neuronal turnover rates and whether similar rates occur throughout the gastrointestinal tract.

Pacemaker Cells in the Gut

Like the heart, the gut contains non-neuronal pacemaker cells (interstitial cells of Cajal, ICC) that generate electrical rhythmicity (slow waves) in smooth muscle cells.
Slow waves cause phasic contractions without requiring enteric neuron activity.

Role of the ENS and ICC

The ENS is required for propulsion along the gut.
Pacemaker-type ICC (at the level of the myenteric plexus) and the electrical rhythmicity they generate are not required for normal gastrointestinal function, at least in the small intestine.

ENS vs. ICC Absence

Mice failing to develop an ENS but still developing ICC in the aganglionic smooth muscle region cannot generate sufficient propulsion or polarized contractions and die soon after birth.
This shows that ICC at the level of the myenteric plexus (ICCMY) were important for slow wave-mediated peristalsis, but these pacemaker cells and the rhythmic electrical depolarizations were unable to take over the critical role of the ENS.
Mutant mice lacking pacemaker-type ICC at the level of the myenteric plexus and consequently lacking electrical slow waves in the small intestine live a full lifespan with minor or no obvious gastrointestinal deficits and still generate propagating neurogenic contractions.

ICC and Neurotransmission

Evidence suggests another population of ICC lying adjacent to smooth muscle cells are important for neurotransmission, although this is controversial.
Excitatory and inhibitory neurotransmission persists in animals without this ICC population.

Sensory Innervation of the Gastrointestinal Tract

With the rapid increase in interest in sensory communication within the gut and along the gut–brain axis, our understanding of sensory innervation of the gastrointestinal tract has been the focus of much attention.
Since the gastrointestinal tract is the only internal organ that has evolved its own sensory neurons, there is intense interest in understanding how the mechanisms of activation of intrinsic sensory neurons differ from extrinsic sensory nerve endings in the gastrointestinal tract.
Understanding the relative contribution of intrinsic versus extrinsic sensory nerves to the generation of chemosensory and mechanosensory reflex responses is of supreme relevance to the development of future therapeutic interventions to modify gastrointestinal function.

Intrinsic Sensory Neurons

It was first demonstrated in the mid-1700s that the gastrointestinal tract is unique, in that it can respond to sensory stimuli in vitro.
Other visceral organs, such as the bladder, uterus, and lungs, rely on sensory stimuli being conveyed to the CNS via spinal or vagal afferents to elicit motor reflexes.

Historical Evidence for Intrinsic Sensory Innervation

Von Haller (mid-1700s) seems to have been the first to identify that the isolated gut could retain an ability to respond to sensory stimuli, despite losing connectivity with the spinal cord.
- He stated that, if he touched the intestines using a knife or corrosives, the intestine responded as if it were still connected to the spinal cord and brain.
- This finding was the first documented evidence that the gut could respond to stimuli without any apparent neural pathways between the gut and the CNS.

Electrophysiological Recordings

It was not until the first intracellular electrophysiological recordings were made from enteric neurons that evidence emerged for a class of sensory neuron actually within the ENS.
Compelling evidence that some enteric neurons in guinea pig small intestine exhibited sensory properties was not presented until the mid-1990s.

Dogiel Type II Neurons

These studies initially revealed that neurons with a Dogiel type II morphology were indeed responsive to chemical and mechanical stimuli.
Dogiel type II neurons have distinct characteristics, consisting of large cell somas and multipolar processes that ramify extensively within the myenteric plexus and have projections both into the mucosa and circumferentially, or aborally along the intestine.

Mechanosensory Properties of Dogiel Type II Neurons

The notion that neurons with a Dogiel type II morphology could function as mechanosensory neurons was also supported by Mao et al. in the mouse small intestine.
The authors showed that, during patch recordings from Dogiel type II nerve cell bodies that were immersed in a low $Ca^{2+}$ solution (to block all synaptic transmission), these neurons generated bursts of action potentials in response to mechanical compression of neighboring nerve fiber tracts.
Their findings provided additional evidence that mechanosensation might be a ubiquitous property of Dogiel type II neurons in the ENS of different species.

Dogiel Type I Neurons

While it had been thought that Dogiel type II neurons were the only intrinsic sensory neuron in the ENS, research has shown that other populations of enteric neurons with Dogiel type I morphologies can also have mechanosensory properties.
Dogiel type I neurons were originally classified as having small-to-medium-sized cell bodies with short, broad, flat dendrites and a single axon.
Later studies identified Dogiel type I neurons as being predominantly interneurons or motor neurons.

Prevalence of Mechanosensitive Neurons

In mouse ileum, neuronal imaging of the ENS has revealed that 22% of myenteric neurons are rapidly adapting mechanosensitive myenteric neurons while, in the colon, they represent 15% of myenteric neurons.
In guinea pig intestine, 45% of cultured myenteric neurons responded with mechanosensory properties.
Rapidly adapting mechanosensitive myenteric neurons have now been shown to be prevalent in the ENS of a variety of species and these neurons encode dynamic changes in force.
Action potential frequency:
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frequency \propto \deformation
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Sensitivity to Strain and Shear Stress

Notable, strain forces activated all classes of mechanosensitive enteric neurons, whereas shear stress was less effective.
Based on these findings, there is sound reasoning that neurons with either Dogiel type I or type II properties can not only respond to sensory stimuli as primary afferents but also receive synaptic inputs from other neurons.

Intracellular Studies

Intracellular electrophysiological studies have revealed that a population of Dogiel type I neurons (interneurons) in the myenteric plexus of guinea pig colon are mechanically sensitive and also receive fast synaptic inputs.

Differences in Mechanotransduction Mechanisms

Curiously, there are major differences in the mechanisms of mechanotransduction of Dogiel type II neurons in the guinea pig small intestine compared with Dogiel type I neurons in the large intestine of the same species.
In the guinea pig distal colon, Dogiel type I neurons are largely stretch sensitive and continue to fire action potentials when smooth muscle tension is reduced or abolished (that is, when the muscles are paralyzed).
By contrast, in the small intestine of the guinea pig, Dogiel type II neurons are sensitive to changes in muscle tension and, when the muscles are paralyzed, these neurons are markedly less responsive to changes in circumferential length compared with Dogiel type I neurons.

Factors Influencing Mechanotransduction

The reasons for the different mechanisms of mechanotransduction between the small bowel and colon are unclear but could be related to the different composition of the luminal contents and the fact that mostly liquid is propelled in the small intestine, whereas mostly solids are propelled in the distal colon.

Role of Enteroendocrine Cells and Serotonin

One of the major unresolved mysteries of Dogiel type II neurons is whether rapid release of substances (such as serotonin) from enteroendocrine cells can dynamically activate the terminals of neurons projecting to the mucosa.
There has been an assumption that endogenous serotonin is released from enterochromaffin (EC) cells following mucosal distortion and that this can directly activate the terminals of Dogiel type II neuron projecting to the mucosa.
Despite it being well accepted that exogenous 5-hydroxytryptamine (5-HT) can activate the terminals of Dogiel type II neurons directly, there is no obvious evidence that endogenous 5-HT released from EC cells can rapidly activate their terminals directly.
These neurons do form a unique sensory circuit, at least in guinea pig intestine, that involves preferentially activating ascending excitatory interneurons and excitatory motor neurons.

Role of Intestinofugal Neurons

Our understanding of the mechanisms underlying mechanotransduction of intestinofugal neurons in the ENS has progressed, although their precise functional role in the body remains somewhat mysterious.
Intestinofugal neurons are neurons with cell bodies in the gut wall, typically in the myenteric plexus, that have axon projections out of the gut wall to sympathetic neurons in prevertebral ganglia.
When activated by mechanical stimulation of the gut, these neurons generate fast synaptic potentials and sometimes slow synaptic potentials in sympathetic neurons.

Activation of Sympathetic Inhibition of Gut Motility

The current belief is that, when intestinofugal neurons are activated in the ENS, they probably function to cause increased sympathetic inhibition of gut motility via noradrenergic neurons in prevertebral ganglia.
To support this theory, there is clear experimental evidence that, when one segment of the colon is distended, extrinsic sympathetic reflex pathways can inhibit neighboring isolated segments of the colon in an organ bath.

Sensory Properties of Intestinofugal Neurons

In contrast to what is known about intrinsic sensory neurons in the ENS, which directly respond to chemical or mechanical stimuli without any synaptic transmission, intestinofugal neurons in the distal colon and proximal colon are largely second-order neurons that are activated indirectly by fast synaptic inputs from other enteric neurons that do respond directly to sensory stimuli.
However, in both regions of the bowel, a population of intestinofugal neurons seem to be directly responsive to mechanical stimuli, since blockade of synaptic transmission in the colon does not prevent distension-activated firing of these neurons.

Response to Stretch

It is known that intestinofugal neurons, at least in the distal colon of laboratory animals, are potently activated by changes in circumferential stretch, but do not respond to longitudinal stretch.
Curiously, this finding is in direct contrast to the mechanotransduction of spinal afferent endings in the colon (whose cell bodies lie outside the gut wall, in dorsal root ganglia), which are potently activated by both circumferential and longitudinal stretch.

Similarities between Intestinofugal and Intrinsic Sensory Neurons in the colon.

At present, what is known about the mechanosensation of intestinofugal neurons in the colon is similar to our current knowledge of intrinsic sensory neurons in the colon, namely that they are both largely insensitive to changes in muscle tension.
That is, paralysis of smooth muscle contraction and a reduction in muscle tone or tension in the colon of laboratory animals have no detectable effect on stretch-activated firing of either class of enteric neuron.
In addition, intestinofugal neurons actually decrease their firing during muscle contraction.

Primary Response of Intestinofugal Neurons

Miller and Szurszewski suggested that intestinofugal neurons respond primarily to changes in length rather than acting as tension receptors, and function to monitor changes in intracolonic volume.
This hypothesis was based on their findings that clearly demonstrated an increase in intestinofugal neuron firing with an increase in circumferential length and a decrease in firing with contraction, that is, upon a decrease in intraluminal volume.

Differences Compared to Spinal Afferent Nerve Endings

This aspect is vastly different from spinal afferent nerve endings that innervate the same region of the colon, which show increased firing of action potentials with increases in muscle tension (under either isotonic or isometric conditions).

Ionic Mechanisms Underlying Sensory Transduction

The ionic mechanisms underlying the transduction of mechanical and chemical stimuli of intrinsic sensory neurons in the ENS are not well understood.
It is known that mechanotransduction of spinal afferent endings is very rapid, involving direct mechano-gated channels, and does not require exocytotic release of transmitters since mechanotransduction persists in zero calcium solution and is not blocked by conventional stretch-activated ion channel or transient receptor potential channel blockers.
This aspect suggests that stretch-activated ion channels involve direct physical mechanotransduction.

Role of Piezo Ion Channel Family

Whether members of the Piezo ion channel family are involved, as they are for direct mechanotransduction of somatic afferents in skin, awaits further study.

Potential of Intestinofugal Neurons

Intestinofugal neurons can also be directly mechanically sensitive and respond to Von Frey hair probing, confirming early suggestions that a population of these neurons can indeed be directly mechanosensitive.

Generation of Neurogenic Motor Patterns

There have been some important advances in our understanding of the intrinsic neuronal mechanisms and pathways that underlie distension-evoked peristalsis.

Early Theories on the Role of Serotonin

Originally, in the 1950s, work from Büllbring’s laboratory suggested that the release of 5-HT from the mucosa was important for distension-evoked peristalsis to occur, findings that were largely based on the observation that endogenous 5-HT was released when peristalsis occurred and that exogenous 5-HT could also elicit peristalsis.
Indeed, since then, others have also proposed that endogenous 5-HT release from the mucosa had an essential role in the generation of colonic peristalsis or colonic migrating motor complexes (CMMCs).

Contradictory Evidence

Since 5-HT antagonists could also block these motor patterns in the colon of laboratory animals, there seemed to be a compelling case that endogenous 5-HT was important for distension-evoked peristalsis to occur.
This notion has been contradicted in the past decade when a selective small-molecule inhibitor of the enzyme tryptophan hydroxylase 1 (TPH1) was given to conscious mice to selectively block synthesis of mucosal 5-HT.

Effects of TPH1 Inhibition

This drug was found to have no effect on gastric emptying, colonic transit, or overall gastrointestinal transit times, raising doubts about earlier suggestions that mucosal 5-HT was an essential player in gastrointestinal motility
In support of these findings, studies showed that, in conscious mice, genetic ablation of the gene Tph1 also led to no changes in gastrointestinal transit in vivo and CMMCs still occurred.

Real-time Amperometric Recordings of Serotonin Release

Real-time amperometric recordings of 5-HT release have provided powerful insights into the role of mucosal 5-HT release.
Studies showed that release of endogenous 5-HT that occurred at the same time as peristalsis or CMMCs was actually a consequence of the contraction underlying peristalsis and not the underlying cause of peristalsis or CMMCs

Effects of Serotonin Reuptake Inhibitors

Although pharmacological or genetic blockage of mucosal 5-HT synthesis does not reduce gastrointestinal transit in vivo, studies have shown that inhibitors of serotonin reuptake cause a reduction in the threshold to elicit propulsive contractions in the small intestine of guinea pigs.
It was concluded that background release of endogenous 5-HT from the mucosa alone is inadequate to modify the threshold for propulsive motor activity.

Role of Exogenous Serotonin and 5-HT Receptors

However, if exogenous 5-HT was applied to the lumen, or there was increased release of endogenous 5-HT, this lowered the threshold of propulsive contractions in the small bowel of guinea pigs, which was mediated by the 5-HT3 and 5-HT4 receptors.
Indeed, others had shown some time ago that mucosally applied 5-HT reduces the threshold for peristalsis in the small intestine of guinea pigs, likely mediated by the 5-HT3 receptor located on the mucosal but not serosal surface of the bowel.
This aspect was determined when 5-HT3 antagonists affected the peristaltic threshold when luminally applied but not when serosally applied.

Peristalsis and the Myenteric Plexus

These findings supported earlier studies showing that distension-evoked peristalsis and the propulsion of natural fecal content did not cease when the mucosa was removed and all dynamic release of endogenous 5-HT from the mucosa was prevented, leading to the conclusion that the mechanotransduction process underlying distension-evoked colonic peristalsis by natural fecal pellets required only the myenteric plexus and smooth muscle layers.
Similar results were obtained in the small intestine, where peristalsis was still shown to occur in intact isolated preparations where the mucosa and submucosa had been dissected away.

Mechanosensory Elements Critical for Distension-evoked Colonic Peristalsis

Taken together, these findings led to the inescapable conclusion that distension-evoked peristalsis involved an intrinsic neural reflex circuit in the myenteric plexus, whose stretch receptors must have been located outside the mucosa.
The only remaining possibility was that the distension-evoked circuit lay in the myenteric plexus

The Role of Connectivity Between the Circular Muscle and Myenteric Ganglia.

Further studies in the colon showed that the mechanosensory transduction process required for activation of distension-evoked ascending excitatory and descending inhibitory nerve pathways (at least in the colon) was critically dependent upon connectivity between the circular muscle and myenteric ganglia.
Removal of the circular muscle prevented distension-evoked ascending excitatory and descending inhibitory pathways, but removal of the longitudinal muscle from the myenteric plexus did not reduce mechanotransduction.
In addition, removal of the circular muscle from the myenteric plexus abolished the vast majority of ongoing fast synaptic potentials in the ENS of the distal colon but removal of the longitudinal muscle did not.
One possibility is that intrinsic sensory neurons in the colon have fine nerve projections that lie in the circular muscle layer that are length-sensitive and these are destroyed when the circular layer is removed.
Indeed, both Dogiel type II neurons and mechanosensitive Dogiel type I neurons have been shown to have fine processes in the circular muscle.

Compelling Evidence about Enteroendocrine Cells

Compelling evidence exists that populations of enteroendocrine cells in the gut are mechanosensitive and can release high quantities of 5-HT.
However, evidence from various laboratories has shown that this release of 5-HT is not required to initiate distension-evoked peristalsis nor does it seem to be required for normal orderly transit of content in vivo.

Concentration of 5-HT and its Effects on Target Effector Cells

There is no doubt that large quantities of 5-HT can be released into the bloodstream in response to distension (much of which is taken up by platelets). However, the final concentration of 5-HT that reaches the target effector cells is unclear. A major focus of future research will be to unravel the functional role of enteroendocrine cells in living animals.
Indeed, there is evidence for major changes in expression of the serotonin reuptake transporter and increased expression of mucosal 5-HT in mammals with inflammatory bowel disease and it is possible that endogenous 5-HT is largely responsible for the changes in ENS excitability only in disease states.

Electricity of Isolated EC Cells

Direct recordings from isolated EC cells revealed that they are indeed electrically excitable and express major ion channels for the generation of action potentials. This expression includes L-type $Ca^{2+}$ channels, which are essential for contraction or distension-evoked release of 5-HT.

Mechanosensitive Ion Channels in Gut

The identification of the mechanosensitive ion channels that underlie distension-evoked neurogenic motor patterns in the gut, such as peristalsis, is of major interest, in particular the Piezo family of mechano-gated ion channels and their role in stretch-evoked reflexes in visceral organs such as the gut.

Distribution of Piezo Channel Isoforms

A study published in 2018 showed that, in guinea pig intestine, the mechanosensitive ion channel Piezo 1 is expressed in enteric nitrergic and some cholinergic enteric neurons, whereas Piezo 2 was rarely expressed in any enteric neurons.
We wait with excitement to learn whether Piezo channels are involved in the stretch activation of peristalsis.

Piezo 2 Channels in EC Cells

Interestingly, EC cells have been shown to express the mechanosensitive ion channel Piezo 2, which is located in close apposition to vesicles containing serotonin. These studies also showed that mechanical sensitivity of a subset of isolated EC cells relies on Piezo 2 ion channels.
This finding could be important because this channel has emerged as a major player in mechanotransduction in Merkel cells in skin to modulate the conversion of touch into itch sensation.
Additionally, studies suggest that some population of enteroendocrine cells also make synaptic-like connections with vagal afferent endings, very much like the Merkel cell–neurite complex in the skin.
It was concluded from these studies that synaptically released glutamate is used by an epithelial sensor cell in the gut to rapidly transduce luminal stimuli to the CNS.

Muscle Tone/Tension requirement

There are major differences in mechanotransduction processes that convert mechanical distension of the small intestine and colon into neurogenic motor patterns.
In the small intestine, the mechanisms underlying distension-evoked peristalsis require muscle tone or tension in the smooth muscles. This is supported by direct recordings from intrinsic sensory neurons in the small intestine that display substantially reduced stretch-sensitivity following muscle paralysis. By contrast, in the colon, paralysis of tone or tension in the smooth muscle has little or no effect on distension-evoked peristaltic reflex pathways or on the propagation of peristalsis.

Neural Activation by Mechanical Stretch

This finding means that the sensory neurons underlying the activation of colonic peristalsis are primarily length sensitive rather than muscle tension or tone sensitive. The precise ionic transduction process by which mechanical stretch activates enteric neurons in the colon to elicit peristalsis or CMMCs is not fully understood.

Chemotransduction and Modulation of ENS Activity

Although endogenous 5-HT release from the mucosa is not required for normal gastrointestinal motility or transit in healthy bowel, there is evidence that endogenous 5-HT release from the mucosa can modulate ENS activity and gastrointestinal motor patterns via particular chemosensory stimulants applied to the lumen.

Nutrient-induced Segmental Contractions

For example, in the isolated small intestine, studies have suggested that endogenous 5-HT modulates the activity in intrinsic neural circuits that underlies nutrient-induced segmental contractions.
Segmental contractions are different from classic peristaltic contractions in that segmental contractions have been long proposed to involve myogenic electrical rhythmicity in the smooth muscle and consist of phasic contractions generated by repetitive non-neuronal depolarizations of the muscle cells.
In contrast, peristalsis is critically dependent upon the ENS and cannot occur when ENS activity is absent.

Myogenic Rhythmicity

Interestingly, studies have shown that segmental contractions that are induced by nutrients, at least in guinea pig small intestine, can occur when slow waves and electrical rhythmicity are not recorded. This finding raises serious doubts about whether myogenic rhythmicity in the smooth muscle is in fact a prerequisite for segmentation to occur.

Mediators of Segmentation

Evidence supporting the notion that serotonin and cholecystokinin (CCK) released from the mucosa may be involved in segmentation was provided when nutrient-induced segmentation (induced by decanoic acid) was found to be depressed when 5-HT receptor antagonists (granisetron and SB-207266) and CCK antagonist devazepide were applied to the lumen.
It was proposed that decanoic acid activated a novel intrinsic pathway in which segmentation involves 5-HT3 and 5-HT4 receptors and CCK1 and CCK2 receptors

Enterotoxins Effect on Motility

Potent enterotoxins, such as cholera toxin, can also potently modify gastrointestinal motility by stimulating secretomotor neurons, leading to the release of 5-HT from enteroendocrine cells.
Most commonly, cholera toxin causes diarrhea by increasing the secretion of water into the intestinal lumen

Biphasic Response of Cholera Toxin

Studies have shown that cholera toxin administered into the lumen of the jejunum ex vivo can have opposing effects on distinct motor patterns. For example, in organ bath experiments using isolated segments of guinea pig small intestine, cholera has been reported to enhance propulsive contractile activity, whereas in the same preparation it can also suppress nutrient-induced segmentation.
It was also proposed that cholera toxin is able to stimulate more than one intrinsic neural pathway in the ENS and that the segmenting pattern and propulsive motor patterns are differentially regulated.

Mucosal Exposure and ENS Activity

Support for the notion that ENS activity can be modulated by agents such as cholera exposed to the lumen comes from direct intracellular recordings that have shown increased excitability of myenteric Dogiel type II neurons, which was proposed to occur via a mechanism that involved NK3 receptors but not 5-HT3 receptors.

CMMC and oestrus Cycle (Cholera Toxin)

A particularly interesting finding was reported when cholera toxin was infused into the colon and the effects on motility were compared not only between male and female mice but also between the stages of the oestrus cycle in female mice.
It was found that cholera toxin immediately and reversibly inhibited CMMCs in female mice that were in the oestrus stage of the cycle but not in female mice in pro-oestrus or in male mice. The inhibitory effects of cholera toxin on CMMC frequency seemed to be mediated by the 5-HT3 receptor and EC cells since these effects of cholera toxin were absent from female mice in which the enzyme TPH1 was genetically ablated.

EC Cells Concentration in oestrus Stage

These investigators showed that the number of EC cells that contain 5-HT was approximately 30% higher in female mice during the oestrus stage than in female mice in the pro-oestrus stage or in male mice.

Chemosensory Activation of the Mucosa on ENS

Taken together, while mutation of the gene Tph1 that synthesizes mucosal 5-HT does not lead to changes in gastrointestinal transit in vivo in mice, there is still evidence that chemosensory activation of the mucosa can lead to changes in ENS circuits via 5-HT receptors and possibly EC cells.
In support of the notion that intrinsic sensory neurons can also respond to chemosensory stimuli, direct recordings from Dogiel type II neurons have shown that these neurons do respond to electrical stimuli of the mucosa or to chemical stimulation of the mucosa with 5-HT or acetate.

Neurotransmitters in the ENS

Despite the large number of different neurochemicals synthesized in the different classes of enteric neurons, most neurochemicals shown to be expressed and synthesized in the ENS have not been demonstrated to behave as neurotransmitters.

Acetylcholine as Major Excitatory Neurotransmitter

It is well established that the major excitatory neurotransmitter underlying enteric neuroneuronal (a synaptic junction between two neurons) transmission is acetylcholine acting on nicotinic receptors on enteric neurons. In most species, blockade of nicotinic transmission abolishes all fast synaptic neurotransmission.

Co-Transmitters

In a smaller population of neurons, there is evidence that after nicotinic receptor blockade, adenosine triphosphate (ATP) acts on P2X receptors and serotonin acts on 5-HT3 receptors to elicit fast excitatory postsynaptic potentials.

Glutamate as a Neurotransmitter

Interestingly, although glutamate is a major fast excitatory neurotransmitter in the CNS, evidence for glutamate as a neurotransmitter in the ENS is poor.
Indeed, glutamatergic receptors are expressed in some enteric neurons, but electrophysiological studies have found the evidence for glutamate as a neurotransmitter in the ENS to be weak or absent. Interestingly, a study using calcium imaging from myenteric neurons in mouse colon suggested that endogenous glutamate may mediate part of the slow excitatory postsynaptic potential (EPSP), particularly in neurons that express calbindin.

Slow EPSPs

In addition to fast EPSPs in the ENS, there is sound evidence for the existence of slow EPSPs in myenteric neurons. ATP has been shown to mediate slow EPSPs via P2Y1 receptors and there is also compelling evidence that tachykinins can mediate slow EPSPs in myenteric neurons via the NK1 receptor.

Submucosal Plexus Neurotransmission

In the submucosal plexus, there is evidence that acetylcholine, ATP, and 5-HT mediate fast synaptic transmission mediated by nicotinic, P2X, and 5-HT3 receptors. In addition, ATP can mediate an intermediate and slow EPSP in some submucosal neurons and 5-HT can also mediate some slow EPSPs in myenteric neurons via action on the 5-HT7 receptor.