MD Boudewijn F Kessing
email: b.f.kessing@amc.uva.nl

J M Conchillo

A J Bredenoord

A J P M Smout

A A M Masclee

Clinical relevance of transient lower esophageal sphincter relaxations (TLESR) in gastroesophageal reflux disease

Background

Transient lower esophageal sphincter relaxations (TLESR) are considered the physiological mechanism which enables venting of gas from the stomach and appear as sphincter relaxations which are not induced by swallowing. It has become increasingly clear that most reflux episodes occur during TLESRs and therefore play a key role in gastroesophageal reflux disease (GERD).

Aim

The aim of this study is to describe the current knowledge about TLESRs and its clinical implications.

Methods

Search of the literature published in English using the PubMed database and relevant abstracts presented at international conventions.

Results

Several factors influence the rate of TLESRs including anti-reflux surgery, meal, body position, nutrition, lifestyle and a wide array of neurotransmitters. Ongoing insights in the neurotransmitters responsible for the modulation of TLESRs, as well as the neural pathways involved in TLESR induction, have lead to novel therapeutic targets. These therapeutic targets can serve as an add on therapy in patients with an unsatisfactory response to PPI by inhibiting TLESRs and its associated reflux events. However, the TLESR-inhibiting drugs that are currently available still have significant side effects.

Conclusions

It is likely that in the future, selected GERD patients may benefit from TLESR inhibition when a compound is found without significant side effects.

Introduction

Under normal circumstances the lower esophageal sphincter (LES) and the crural part of the diaphragm both contribute to the creation of a constantly present local high-pressure zone which functions as an anti reflux barrier 1 . When a hiatal hernia is present the two components of the high-pressure zone can be separated: the esophagogastric junction (EGJ) and the LES are then located proximal to the diaphragm. Relaxation of the LES occurs during each swallow, allowing a bolus to pass into the stomach 2 . In addition, the LES relaxes during the so-called transient lower esophageal sphincter relaxation (TLESR). This phenomenon appears as a sphincter relaxation which is not induced by swallowing and this occurs about 3-6 times per hour 3-6 . TLESRs are considered the physiological mechanism which enables venting of gas from the stomach, also known as belching 7 . Distension of the stomach, such as caused by intragastric air and food, triggers a vago-vagal reflex which results in a TLESR. This reflex acts as a protective mechanism by preventing accumulation of excess amounts of gas in the stomach or gas entering the duodenum.

Since TLESRs were first observed in the early 80s it has become increasingly clear that most reflux episodes occur during TLESRs. Other mechanisms including straininginduced, coughing-induced and free reflux are responsible for the remaining part of reflux episodes which only become relatively and absolutely important in patients with severe disease associated with hiatal hernia. Since TLESRs play a key role in gastroesophageal reflux disease (GERD), understanding their background gives us more insight in the disease. This review will focus on the clinical implications of TLESRs by giving an overview of our current knowledge about the phenomenon and potential new treatments aimed at reducing TLESRs.

Definition of TLESRs

TLESRs have first been described by Dent et decline in pressure at the LES which is not induced by swallowing 9 . Holloway et al recently pointed to a substantial inter-and intra-observer variability in the detection of TLESRs using these criteria 10 . Additional objective criteria based on characteristics as crural diaphragm inhibition and a prominent after-contraction would help to reduce variability 10 .

LES relaxations resembling TLESRs may be induced by pharyngeal stimulation 2,11 .

However, in the fasting state, none of the LES relaxations induced by pharyngeal stimulation are associated with inhibition of the crural diaphragm and acid reflux 12 . In the postprandial periods, only 8% of the pharyngeal stimuli result in crural diaphragm inhibition. Furthermore, esophageal reflux was found only when LES relaxation was associated with diaphragm inhibition 12 . It has therefore been suggested that simultaneous diaphragm inhibition and LES relaxation is mandatory for the occurrence of gastroesophageal reflux.

Measuring TLESRs

Conventional manometry is currently still the golden standard to measure TLESRs and the objective criteria used to identify TLESRs are based on the manometric profile of the LES 9 . The so-called sleeve sensor was introduced in the late 70s and consisted of a several centimeters long water-perfused membrane which was placed in the LES 13 . This technique enabled continuous measurement of LES pressure and has led to a better understanding of GERD by identifying TLESRs as the main mechanism allowing gastroesophageal reflux. A second technique for prolonged LES pressure recording is an oil-filled chamber, the so-called sphinctometer 14 . This sphinctometer is able to measure LES pressure during an ambulatory measurement, and although LES basal pressure is not measured as accurately as with the water perfused sleeve sensor, the ability to detect TLESRs is comparable to the water-perfused sleeve 14 .

A relatively new technique to visualize esophageal pressure and to identify TLESRs is high-resolution manometry (HRM) 15 (figure 1). HRM uses a large number of closely placed sideholes or solid-state pressure transducers. The advantage of this technique is that it allows a more detailed view of pressure in the LES, esophageal body and UES. HRM has proven to be at least as effective as a sleeve sensor to identify TLESRs 16 .

When analyzing TLESRs with HRM one can consider the two signals with highest resting pressure LES pressure and these two channels have to fulfill the criteria for TLESR 16 . Alternatively, the so-called e-sleeve can be used which calculates the highest pressure in a several centimeters long segment, similar to a conventional sleeve measurement 17 . New techniques are still being developed and combined high-resolution manometry and impedance catheters are already available.

To investigate TLESRs it can be helpful to provoke TLESRs. Gastric air insufflation can be used to trigger TLESRs 18 . However, the gastric air insufflation technique has its limitations since the inflated air can be easily vented by a single belch. A second type of stimulus to provoke TLESRs is isobaric distension of the proximal stomach 18 . However, application of this stimulus is limited by a decrease in the rate of TLESRs during prolonged gastric distension 18,19 . This suggests adaptation to prolonged distension.

These findings demonstrate that this type of stimulus should not be used in sequential experimental conditions. Furthermore, it is not clear whether this stressor accurately simulates a physiological stressor for TLESRs. A third TLESR-provoking stressor is by a meal. In patients with symptomatic reflux, a meal induces a four-to fivefold increase in the rate of TLESRs and provides a stressor that simulates the physiological situation 20 .

Physiology

Main mechanisms of TLESRs

Studies employing the barostat technique indicate that TLESRs are triggered by gastric distension 18,21 . While still under debate, several studies suggest that tension receptors in the stomach appear to be more relevant than pressure receptors as the stimulus for transient LES relaxation 18,21,22 . The region of the stomach involved in triggering TLESRs has been assessed more precisely by selectively distending various gastric regions in the awake dog 23 . TLESRs could be triggered by distension in any part of the stomach. However, the subcardiac region of the stomach showed the lowest threshold for These events, in concerted action, thus result in complete relaxation of the EGJ by inhibiting LES and crural diaphragm, in suppression of esophageal peristalsis and esophageal shortening. The neural pathways involved will be discussed further in the section on neural pathways.

Longitudinal muscle contraction

Several investigators, using different measurement techniques, all observed contraction of the longitudinal muscle of the esophagus during a TLESR 27-29 . During TLESRs a unique pattern of longitudinal muscle contraction (LMC) occurs that starts in the distal esophagus, progresses in retrograde direction and is stronger in force than the LMC associated with a swallow 27 . Inhibition of the crural diaphragm after a LMC has been observed in cats 31 but not in humans. It has therefore been hypothesized that the LMC of the distal esophagus may be the primary motor event leading to LES relaxation 27 .

However, this hypothesis remains to be proven in humans.

Terminating motor events

Three different types of motor events in the esophagus lead to termination of a TLESR 32 (figure 1) . The first motor event is the swallow-induced primary peristalsis, characterized by UES relaxation with pharyngeal contraction and esophageal peristalsis progressing along the entire esophagus. The second and third type of motor events are full and partial secondary contraction. Full secondary contraction is defined as a completely propagated wave in the esophagus not associated with UES relaxation and partial secondary contraction involves only the distal esophageal segment. Early reports revealed that primary peristalsis was the most common motor event terminating a gastroesophageal reflux episode 3,8,33 . However, later studies concluded that the most common event that leads to termination of a TLESR is secondary contraction 2,29,32 . The type of terminating event can be predicted by the occurrence and nature of gastroesophageal reflux since in particular partial secondary contraction was more likely during distal gastroesophageal reflux and full secondary contraction was more likely during proximal reflux 32 . This pattern is not dependent on the acidity of the refluxate 32 .

TLESRs terminated either by primary peristalsis or partial secondary contraction were of equal duration but TLESRs terminated by full secondary contraction were shown to have a significantly longer duration of LES relaxation 32 . The duration of TLESRs is not dependent on whether or not it is associated with reflux 34 .

Factors contributing to TLESR frequency

Several physiological mechanisms influence the rate of TLESRs and symptoms. GERD patients often describe that the postprandial period is associated with increased symptoms. This clinical finding was objectively measured by Holloway et al as a four-to sevenfold increase in the rate of gastroesophageal reflux episodes through two mechanisms: a four-to fivefold increase in the rate of TLESRs and an increase in the proportion of TLESRs accompanied by reflux from 47% to 68% 20 . Both the increase in the rate of TLESRs and their association with reflux could explain why most GERD patients have increased symptoms in the postprandial period.

Whereas it is likely that the mechanism responsible for the postprandial increase in TLESRs involves an increase in proximal gastric volume, activating tension receptors in the proximal stomach, the percentage of TLESRs associated with acid reflux does not relate to proximal gastric volume 35 . With respect to different supine body positions it has been shown that the incidence of TLESRs as well as the incidence of TLESRs associated with acid reflux is higher in the right recumbent position when compared to the left recumbent position 36 .

Using esophageal pH-metry it can be observed that most reflux episodes and symptoms in GERD patients occur during the awake state. This is in accordance with the observation that the rate of TLESRs decreases during the night 8,32,37 . However, a subset of GERD patients still shows substantial acid exposure during the night. This can be partly contributed to the observation that although TLESRs are absent during stable sleep, they do occur during short arousal from sleep. This is in accordance with the finding that most nocturnal reflux events occur during arousals from sleep or when the subjects are fully awake 8,37 . In addition, other mechanisms of reflux are involved in patients with pathological nocturnal reflux including stress reflux and free reflux mechanisms 37 .

Neural pathways

Most of our knowledge regarding the neural pathways involved in the autonomic reflex that leads to a TLESR is derived from animal studies and it is assumed that TLESRs in humans follow similar pathways. Growing insights in the neural pathways and the neurotransmitters involved in the generation of TLESRs have led to clinical implications by the development of therapies aimed at inhibiting TLESRs.

Afferents

The reflex pathway of the TLESR commences with activation of gastric tension receptors primarily in the subcardiac region 23 (figure 2). The first study specifically describing neural pathways of TLESRs was by Martin

Neurotransmitters and receptors

Many neurotransmitters have been found to play a role in the neuromodulation of TLESRs. However, some neurotransmitters have already demonstrated not to be suited for therapeutic modulation. In the following only those neurotransmitters and receptors will be discussed which can potentially be used to treat GERD by inhibiting TLESRs.

GABA-B

Currently, the most extensively investigated neurotransmitter in the TLESR pathway is

Cannabinoid receptor 1

Two types of cannabinoid receptors are present in the human body, the cannabinoid receptor 1 (CBR1) and cannabinoid receptor 2 (CBR2). The CBR1 is mainly localized in the central nervous system 61 and the CBR2 is mainly associated with the immune system 62 . In an animal study, the CBR1 has been localized in the NTS and DMV, areas that are associated with central modulation of TLESRs 63,64 .

The site of action of CBR1 has been assessed and no CB1 receptor expression of vagal efferents projecting to the gastric fundus or LOS was observed 64 . This suggests that it is unlikely that the receptor is modulating vagal motor output directly 64 . Therefore, the most likely site of action of CBR1 agonists is via the CB1 receptor in the NTS that ultimately signals through vagal motor neurons.

Other neurotransmitters and agents

Several other neurotransmitters and agents have been identified that influence rate of TLESRs including nitric oxide (NO), opioids, anticholinergic agents and the neuropeptide CCK. However, these neurotransmitters have only modest effects on the rate of reflux, the effects apart from those on the LES are substantial or no oral agent for human use is currently available. Therefore, no important clinical role is anticipated for modulation of these neurotransmitters and receptors in the near future. Morphine has an inhibitory effect on the rate of TLESRs in GERD patients and to a lesser extent in healthy subjects 68 . The site of action is presumed to be both in the central nervous system and the myenteric plexus 68,69 . Opioid receptors can be targeted to decrease the rate of TLESRs by morphine. Although the first results with respect to TLESRs look promising, there are still too many unwanted systemic effects of morphine.

TLESRs triggering is also under neurohormonal control. Cholecystokinin (CCK) is a postprandially released GI peptide that has been shown to increase the frequency of TLESRs 65,70 . The rate of TLESRs induced by meals and gastric distension can be decreased with a selective CCK-A receptor antagonist 70-73 . However, the effects of CCK and CCK antagonists on the rate of acid reflux are only modest 74,75 .

Gastroesophageal reflux during TLESR

TLESRs are the main mechanism leading to acid reflux, they are responsible for 70% of acid reflux episodes 3,8,24,33,76-78 . Since GERD is characterized by reflux, TLESRs play an important role in the etiology of the disease. When TLESRs were first identified it was theorized that GERD patients should have a higher rate of TLESRs. However, the majority of the studies shows a similar rate of TLESRs in healthy subjects and GERD patients 4,5,79,80 . This means that in GERD patients there is a higher percentage of TLESRs that are associated with gastroesophageal reflux 34 . Therefore, a different underlying mechanism is necessary that allows reflux during a TLESR.

EGJ pressure gradient

Flow through a tube is linearly related to the pressure gradient, which is the major driving force of flow. Therefore, when reflux is to occur during TLESRs a positive pressure gradient between stomach and esophagus is required to allow retrograde flow of gastric content 35,81 .

In GERD patients, a slightly higher pressure gradient before and during a TLESR has been shown when compared to healthy subjects. This is caused mainly by an increase in intragastric pressure 35,81 . More importantly, the trans-sphincteric pressure gradient is Frankhuisen et al observed that healthy subjects have a decrease in intragastric pressure in the postprandial period, this decrease is not present in GERD patients 81 . These authors suggest that this may contribute to the increased postprandial reflux during TLESRs in GERD patients 81 . The mechanism underlying this phenomenon and its role in GERD has not been investigated.

EGJ compliance

EGJ compliance, as a factor contributing to GERD, was first investigated by Pandolfino et al using a modified barostat and radiography to measure the distensibility of the EGJ 82 . An increased compliance in GERD patients was observed 82,83 . This could explain why GERD patients show a loss of discrimination between air and liquid by the LES. Increased EGJ compliance was found in GERD patients with hiatal hernia 82,83 as well as in patients without hiatal hernia 83 .

Recently, a novel technique was developed that measures EGJ distensibility without the need for radigraphy measurement of the diameter of the distending balloon. This technique is known as impedance planimetry and the instrument is known as the functional luminal imaging probe (FLIP) 84 . By measuring symptomatic GERD patients without hiatal hernia and normal subjects it was observed that GERD patients exhibit a more distensible EGJ compared to controls 85 . This confirms that increased EGJ compliance in GERD patients could be a contributing factor to the increased association of TLESRs with gastroesophageal reflux in GERD patients.

Hiatal hernia

Research on the rate of TLESRs in patients with hiatal hernia has yielded conflicting results 86,87 . However, by investigating reflux mechanisms in GERD patients with and without hiatal hernia it has been shown that TLESRs play a less important role in the occurrence of reflux in GERD patients with a hiatal hernia than in GERD patients without a hiatal hernia 86,87 . Only 40% of acid reflux episodes were associated with

Nutrition and lifestyle

Obesity is a known risk factor for GERD 88 . By comparing obese subjects without GERD with normal weight patients without GERD it was observed that obesity is associated with an increased rate of TLESRs as well as with an increased association of TLESRs with gastroesophageal reflux 89 . Furthermore, a higher pressure gradient was measured during TLESRs in obese subjects compared to normal weight subjects 89 and an increased proximal extent of the refluxate was found to be associated with high body weight 90 .

Since a more proximal extent has been demonstrated to result in increased reflux perception, obese patients are therefore more likely to develop reflux symptoms 91 .

The correlation between fat intake and GERD or esophageal acid exposure has been thoroughly studied and the results from these studies show controversial results. Efforts to measure the effect of fat intake on to the rate of TLESRs have also been made but no effect on the rate of TLESRs was found when comparing isocaloric and isovolumetric meals with different fat percentages 92,93 . Although population studies often show a positive correlation between fat intake and GERD symptoms, only BMI appears to be an important risk factor 94 . Despite the fact that objective markers do not show a correlation between fat and gastroesophageal reflux, the relative fat content of food affects symptom frequency. The correlation between carbohydrates has not been investigated extensively.

However, a study show an increased rate of TLESRs after ingestion of carbohydrates due to colonic fermentation 95 .

TLESRs and anti-reflux surgery

Fundoplication is the most widely applied surgical intervention for GERD patients. It is effective in reducing gastroesophageal reflux, both acid and non-acid 96 . The mechanisms through which fundoplication reduces reflux are complex and incompletely understood.

Currently, it is believed that three mechanisms may play a role. The first mechanism is anatomical restoration of the EGJ by repairing a hiatal hernia if present. Secondly, antireflux surgery decreases the distensibility of the EGJ 85,97 . The third mechanism by which anti-reflux surgery (complete and partial fundoplication) causes a reduction in reflux episodes is a decreased rate of TLESRs 98-101 . Furthermore, the percentage of TLESRs associated with reflux was found to be decreased after fundoplication 98,99 .

The mechanism by which a fundoplication results in a decreased rate of TLESR is unclear. Fundoplication reduces the volume of the proximal stomach, theoretically leading to a decrease in gastric distensibility and a decreased gastric receptor activation.

Using isobaric distension, however, it was shown that proximal gastric compliance is not significantly different between post-fundoplication patients, GERD patients, and healthy controls 102 . Altering the anatomy of the proximal stomach during fundoplication could in theory lead to local damage to the vagus nerve thereby decreasing vagal output resulting in TLESRs and thus to a decrease in reflux after fundoplication. However in a study that tested this hypothesis it was found that gastric emptying actually increased and affected vagus nerve integrity was present in only 10% of patients 103 .

The decreased rate of TLESRs in post-fundoplication patients is in accordance with the clinical finding that patients often present with symptoms of abdominal bloating following fundoplication. It is believed that the increased threshold for TLESRs following fundoplication inhibits the protective mechanism of TLESRs thereby leading to accumulation of excess amounts of gas in the stomach and gas entering the duodenum 99 .

Future clinical implications

Proton pomp inhibitors (PPI) have proven to be a clinically successful therapy for GERD.

Despite the success of PPI treatment there is still a substantial subgroup of patients that show no symptomatic response or an incomplete response to PPI therapy 104 . This so- called refractory GERD is often due to an erroneous diagnosis of GERD in patients with functional disorders such as functional dyspepsia. However, since PPI therapy does not reduce the total number of reflux events but only reduces the acidity of the refluxate, refractory symptoms can also be caused by persisting non-acid reflux 105 . PPI-resistant patients are usually referred to a gastroenterologist. After having confirmed that the patient indeed had GERD the gastroenterologist has little to offer besides increasing the PPI dosage. When PPI threatment fails, surgical intervention is the last therapeutic option available. Drugs that reduce the rate of TLESRs could provide the much needed additional therapy in PPI-resistant GERD patients, in particular when the symptoms are due to non-acid reflux. Since TLESR inhibitors will be able to reduce the number of reflux events and PPI therapy will reduce the acidity of the remaining reflux events much is expected from combination therapy. Several receptors have been targeted to reduce the rate of TLESRs but only three receptors are currently under consideration for therapeutic modulations.

GABA-B receptor modulators

GABA-B agonists are considered among the most promising agents that are able to reduce the rate of TLESRs and associated reflux. Several studies with GABA-B agonists have been performed in humans. These studies all point to a reduction in the rate of TLESRs and of gastroesophageal reflux 53,56,106-109 . A frequently studied GABA-B agonist is baclofen which not only reduces the rate of TLESRs, but also has been demonstrated to increase LES pressure in humans 53,107 . These effects of baclofen contribute to the reduction of gastroesophageal reflux in patients with hiatal hernia 110 .

Unfortunately, baclofen has many side effects on the central nervous system, including drowsiness, confusion and impaired memory 111,112 . Therefore, more specific, peripherally acting GABA-B agonists are currently being developed that aim to overcome the central side effects 56,113 .

A recent study assessed the addition of the more peripherally specific GABA-B agonist lesogaberan to conventional PPI therapy in patients with refractory symptoms and a decrease in reflux events and an increase in LES pressure was observed 114 . However, 17/35 some central side effects effects like headache and paraesthesia may occur when using lesogaberan 113 . Widespread application of GABA-B receptor modulators as add on therapy in refractory GERD patients is likely to take place when more specific agents with fewer side effects become available.

MgluR5 receptor modulators

A dose-dependent decrease in the rate of TLESRs is observed in response to a mGluR5 antagonist in animal studies 115,116 . These findings suggested a potential therapeutic role for mGluR5 antagonists in the treatment of GERD. Recently, the effects of the mGluR5 antagonist on esophageal acid exposure in humans was assessed and a decrease in esophageal acid exposure after receiving an mGluR5 antagonist was observed 117,118 .

No results have yet been published that measure TLESR rate in humans after receiving mGluR5 antagonists. More importantly, the side effects of mGluR5 antagonists need be studied in more detail since only limited results are available before a good assessment of its future clinical implications can be made.

CBR1 receptor modulators

The effect of CBR1 agonists on the rate of TLESRs has been studied in animal models 64,119,120 and humans 120 . It was found that CBR1 agonists reduce the rate of TLESRs. These results confirm that cannabinoid receptors are involved in the neuromodulation of TLESRs. Although research on CBR1 is still limited the available results suggest a possible future role for CBR1 agonists in the pharmacomodulation of GERD. There are no available data on side effects or on the effect of reducing the rate of reflux in GERD patients.

Conclusion

The physiological response of the human body to gastric distension is a vago-vagal reflex which induces relaxation of the LES, a phenomenon known as TLESR. Gastroesophageal reflux occurs mainly during TLESRs which makes this phenomenon an important factor in the pathogenesis of GERD. Several receptors in the TLESR reflex arch have been identified which can potentially be used as a target in therapy. Most likely, TLESR- inhibiting drugs would be useful as an add-on therapy for patients on PPI. The combination therapy would result in a decrease in the number of reflux events through the use of TLESR inhibitors and a decreased acidity of the remaining reflux events through the use of PPI. Although the number of TLESRs as well as the number of reflux events can be decreased by the TLESR-inhibiting drugs that are currently available, there are still unwanted side effects. It is likely, however, that in the future, selected GERD patients may benefit from TLESR inhibition when a compound is found without significant side effects. 111x139mm (300 x 300 DPI)

  al in the early 80s of the past century as transient episodes of what was then thought 'inappropriate' complete LES relaxation lasting 5-30 seconds 8 . Objective criteria for the manometric detection of TLESRs have been defined by Holloway et al in the mid 90s. A TLESR is thereby defined as an abrupt

triggering

  TLESRs and is therefore considered the gastric region primarily responsible for triggering TLESRs 23 .Gastric distension induces an autonomic reflex involving four different events :1. Relaxation of the LES by activation of inhibitory neurons in the myenteric plexus by the vagus 24 2. Suppression of motor output to the crural diaphragm leading to crural diaphragm inhibition with subsequent relaxation 24-26 3. Suppression of esophageal peristalsis 2,24 4. Contraction of the distal esophageal longitudinal muscle leading to esophageal shortening[START_REF] Babaei | A unique longitudinal muscle contraction pattern associated with transient lower esophageal sphincter relaxation[END_REF][START_REF] Shi | Distinct patterns of oesophageal shortening during primary peristalsis, secondary peristalsis and transient lower oesophageal sphincter relaxation[END_REF][START_REF] Pandolfino | Transient lower esophageal sphincter relaxations and reflux: mechanistic analysis using concurrent fluoroscopy and high-resolution manometry[END_REF][START_REF] Dogan | Axial stretch: A novel mechanism of the lower esophageal sphincter relaxation[END_REF]

  et al who observed abolition of TLESRs and gas venting after vagal blockade in the dog 38 . These authors pointed to the importance of intact vagal pathways in inducing TLESRs and concluded that sensory signals from the stomach are projected to the brain through afferent sensory fibers of the vagus 38,39 . The relaxation of the crural diaphragm that occurs simultaneously with LES relaxation is also mediated through vagal afferents 25,26 .

  Terminating synapses of vagal afferents have been found in the nucleus tractus solitarius (NTS) 40,41 and stimulation of the NTS subsequently evokes LES relaxation 42 . However, signals from the NTS do not provide signals to the LES directly, instead, signals from the NTS are relayed to the dorsal motor nucleus of the vagus (DMV) 43 . Motor function of the entire gastrointestinal tract including esophagus and LES is modulated by the DMV 44 . Signals from the DMV to the LES originate from two functional areas inside the DMV known as the rostral and caudal area 45,46 . By assessing the function of both regions it was observed that stimulation of the caudal DMV resulted in decreased LES pressure whereas stimulation of the rostral DMV resulted in increased LES pressure 45,47 . Although central modulation of LES relaxation in response to gastric distension has been investigated extensively, central modulation of the crural diaphragm has received somewhat less attention. Recently, Niedringhaus et al observed that stimulation of the DMV not only affects LES activity but also crural diaphragm activity 47 . Using retrograde tracers it was demonstrated that this stimulation is a result from direct projection from DMV to crural diaphragm 48,49 .After applying retrograde tracers in both crural diaphragm and LES, double labeled cells were observed in DMV and NTS. These findings indicate that LES and crural diaphragm share similar central pathways that modulate TLESRs situated in DMV and NTS.EfferentsMotor function signals to the LES and crural diaphragm are conducted through the efferent motor tract of the vagus 50 . These signals are relayed to the myenteric plexus from where they are distributed throughout the esophageal body and LES 48-50 . The crural diaphragm is not only innervated through efferent vagal endings but is also innervated by the phrenic nerve 49 . The brain stem sites responsible for this dual innervation are yet to be defined 49 .

  gamma-aminobutyric acid (GABA). GABA-B receptors are located at both central 51,52 and peripheral 53-56 sites in the TLESR reflex arch. Central GABA-B receptors are located in NTS and DMV 51 . The central effects of application of GABA-B agonists on the NTS include a reduction of the number of NTS neurons receiving vagal input 52 . Peripheral GABA-B receptors are located in both afferent 55 and efferent 53,54 pathways. Furthermore, GABA-B agonists decrease the vagal afferent response to gastric distension 55 .Studies assessing the effects of GABA-B agonists on crural diaphragm relaxation are limited, but it has been demonstrated that GABA-B agonists block the reflex inhibitory pathway to both LES and crural diaphragm in cats 57 . This could theoretically be caused by the fact that LES and crural diaphragm relaxation share pathways 48 .Metabotropic glutamateMetabotropic glutamate receptors (mGluR) are present throughout the central and peripheral nervous system and are classified into group I, II, and III mGluR. Group I consists of mGluR1 and mGluR5 which are excitatory neurotransmitters, whereas group II and III are inhibitory neurotransmitters 58 . mGluR expression has been demonstrated on gastric vagal afferents by retrograde tracers in the ferret 59 . The known inhibitory effect of group II and III mGluR and the excitatory effect of group I mGluR were also demonstrated as a decrease and increase in vagal afferent mechanosensitivity 59 .

  abundant in gastric vagal afferent neurons but sparse in fibers within NTS vagal subnuclei 60 . This suggests that mGluR5 plays a prominent role at gastroesophageal vagal afferent endings but a minor role in central vagal pathways.

  Nitric oxide (NO) is the major inhibitory postganglionic neurotransmitter in the pathways to the LES. It has been observed that NO increases the rate of TLESRs and a blockade of NO receptors results in a decrease in the rate of TLESRs in dogs 65 . NG-monomethyl-Larginine (L-NMMA), a specific NO synthesis blocker, reduces the rate of TLESRs induced by gastric distension by more than 75% 66 . The rate of TLESRs in the postprandial period was reduced by 25% 67 . The therapeutic potential of L-NMMA has still to be determined and no orally effective agent is available yet.

  accompanied by acid reflux compared to TLESRs without acid reflux 35 .

TLESRs 86 .

 86 Patients with hiatus hernia more frequently have reflux associated with low LES pressure, swallow-associated normal LES relaxations, and straining during periods with low LES pressure 86 . Furthermore, investigations on EGJ compliance in GERD patients with and without a hiatal hernia show that GERD patients with hiatal hernia have increased EGJ compliance compared to GERD patients without a hiatal hernia 83 . It has been suggested that this could contribute to the rate of gastroesophageal reflux in patients with hiatal hernia 83 .

FiguresFigure 1 .

 1 Figures

Figure 2 .

 2 Figure 2. Neural pathways involving TLESRs. The TLESR reflex pathway commences with gastric distension of primarily the subcardiac region of the stomach. Sensory signals are projected to the nucleus tractus solitarius (NTS) through afferent sensory fibers of the vagus. Motor function signals project from the dorsal motor nucleus of the vagus (DMV) to the LES and diaphragm through vagal efferents. The diaphragm also receives motor function signals from the phrenic nucleus which are conducted through phrenic efferents.

  A, Blackshaw LA, Brändén L, Carlsson A, Jensen J, Nygren E, et al. Cannabinoid receptor agonism inhibits transient lower esophageal sphincter relaxations and reflux in dogs. Gastroenterology 2002;123:1129-1134. 120. Beaumont H, Jensen J, Carlsson A, Ruth M, Lehmann A, Boeckxstaens G. Effect of delta9-tetrahydrocannabinol, a cannabinoid receptor agonist, on the triggering of transient lower oesophageal sphincter relaxations in dogs and humans. Br J Pharmacol 2009;156:153-162.

Figure 1 .

 1 Figure 1. HRM plots of three TLESRs with three different terminating motor events. A: Primary peristalsis. B: Full secondary contraction. C: Partial secondary contraction.

Figure 1 .

 1 Figure 1. HRM plots of three TLESRs with three different terminating motor events. A: Primary peristalsis. B: Full secondary contraction. C: Partial secondary contraction.

Figure 1 .

 1 Figure 1. HRM plots of three TLESRs with three different terminating motor events. A: Primary peristalsis. B: Full secondary contraction. C: Partial secondary contraction.

Alimentary Pharmacology & Therapeutic