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REVIEW

Normal and abnormal electrical propagation in the small intestine W. J. E. P. Lammers Departments of Physiology, College of Medicine and Health Sciences, UAE University, Al Ain, United Arab Emirates

Received 21 May 2014, revision requested 3 June 2014, revision received 6 August 2014, accepted 19 August 2014 Correspondence: W. J. E. P. Lammers, MD, PhD, Veerkade 142, 1357PR Almere, the Netherlands. E-mail: wlammers@smoothmap. org

Abstract As in other muscular organs, small intestinal motility is determined to a large degree by the electrical activities that occur in the smooth muscle layers of the small intestine. In recent decades, the interstitial cells of Cajal, located in the myenteric plexus, have been shown to be responsible for the generation and propagation of the electrical impulse: the slow wave. It was also known that the slow waves as such do not cause contraction, but that the action potentials (‘spikes’) that are generated by the slow waves are responsible for the contractions. Recording from large number of extracellular electrodes simultaneously is one method to determine origin and pattern of propagation of these electrical signals. This review reports the characteristics of slow wave propagation through the intestinal tube, the occurrence of propagation blocks along its length, which explains the well-known decrease in frequency, and the specific propagation pattern of the spikes that follow the slow waves. But the value of high-resolution mapping is highest in discovering and analysing mechanisms of arrhythmias in the gut. Most recently, circus movements (also called ‘re-entries’) have been described in the small intestine in several species. Moreover, several types of re-entries have now been described, some similar to what may occur in the heart, such as functional re-entries, but others more unique to the small intestine, such as circumferential re-entry. These findings seem to suggest the possibilities of hitherto unknown pathologies that may be present in the small intestine. Keywords abnormal propagation, re-entry, slow wave propagation, spike propagation.

The history of our understanding of the electrical activities of the small intestine is, like with many other areas in medicine, full of innovations, breakthroughs and application of new technologies, well described by Szurszewski (1997, 1998), but also has had its fair share of discussions, misunderstandings and wrong interpretations. Fortunately, the story of the electrical activities of the small intestine has had its champions. We can safely start with Alvarez’s discovery that there exists a frequency gradient along the small intestine and that each piece, isolated from the rest, is spontaneously

active, but at a lower frequency than in the intact organ (1914). Later, Alvarez & Mahoney (1922) went on to record for the first time the electrical slow wave and related this to the observed contractions and contraction gradient. This was further expanded by Bass et al. (1961) who showed that the slow wave conducted as a ‘sleeve’ along the wall of the intestine. An interesting confusion first arose when slow waves were recorded from muscles while they did not contract, such as may occur in the stomach, but this was resolved when the spikes (‘action potentials’) were discovered in the wake of the slow waves by Richter

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(1924) and Bozler (1942). Bozler went on to show that the smooth muscles in the gastrointestinal system (GI) were electrically connected and essentially behaved as a syncytium. Finally, he showed mathematically that the extracellularly recorded slow wave is a derivative of the transmembrane potential (1946). Somewhat later, a discussion took place with the introduction of the oscillatory relaxation model to explain the generation and propagation of the slow wave, similar to what was proposed at that time in other syncytia such as cardiac muscles. An interesting and colourful debate occurred between supporters and opponents (Publicover & Sanders 1989, Sarna 1990a, Daniel et al. 1994) which was seemingly resolved by the entry of a new ‘kid on the block’; the interstitial cells of Cajal (=ICC). First described by Cajal in 1889, these cells, also residing alongside the myenteric plexus (=ICC-MY), were identified as the pacemaker cell of the small intestine thanks to the meticulous work of Lars Thuneberg (1982). The discovery of a mice model lacking these cells, and correspondingly lacking slow waves, laid the foundation for much future work (Ward et al. 1994, Huizinga et al. 1995). It is fair to say, however, that the exact mechanism of slow wave initiation and propagation has still, to date, not been fully resolved (Huizinga & Chen 2014, Sanders et al. 2014). Potentially, the development of ever more sophisticated modelling studies of the slow wave (Aliev et al. 2000, Corrias & Buist 2008) and recent modelling of intracellular events in these ICC’s (Lees-Green et al. 2014, Means & Cheng 2014) may ultimately shed some light in this very long tunnel.

Slow wave propagation in the small intestine The role of the ICC cells in the generation and propagation of the slow waves had not yet reached full publicity when we started developing high-resolution mapping of smooth muscles in the early 1990’s. Fortunately, there was already considerable experience in recording electrical activations from the GI smooth muscles (Bortoff 1967, Szurszewski et al. 1970, Specht & Bortoff 1972, Sarna & Daniel 1975, Publicover & Sanders 1984) that seemed to indicate that high-resolution mapping from the small intestine could be possible. A first high-resolution study on slow wave propagation was performed on isolated rabbit duodenum in vitro (Lammers et al. 1993). This first publication was really a feasibility study to demonstrate that it was not only possible but useful to record from a large number of electrodes simultaneously. Already in this first study, the variability in the location of the duodenal pacemaker, the possibility of multiple pacemakers operating simultaneously in a relatively small segment and the induced variation in propagation 350

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pattern became obvious (Lammers et al. 1993). This was elaborated in a further study on tissues obtained from cat small intestine where the activities of 1–5 different sites of initiations were evaluated (Lammers et al. 1996). But another confusion arose in the fact that, in our first propagation maps, conduction from the site of the pacemaker did not show a large degree of anisotropy. In those days, anisotropic patterns of propagation was a big topic in cardiac electrophysiology as it had been shown that the orientation of the myocardial cells had a strong effect on the propagation velocities in the longitudinal vs. the circumferential direction (Spach et al. 1988) even to such a degree that a strong anisotropy in itself could generate reentrant activities (Spach et al. 1988). Since, in the gut, the theory at that time was that slow wave propagation occurred along the longitudinal muscle wall (Bortoff et al. 1981), we had expected the propagation in that direction to be much faster than in the circumferential direction. This anisotropic pattern, however, was not visible in the propagation maps, and this author once had the privilege to discuss this issue with Bortoff at a poster presentation. Together, we hypothesized a role for the circular muscle in the propagation of the slow wave to counterbalance the anisotropic longitudinal propagation pattern. Fortunately, the description of a network of ICC-MY’s (Thuneberg 1982, Ward et al. 1994, Huizinga et al. 1995) seemed to explain the lack of an anisotropic propagation. Much later, in a more rigorous study (Lammers et al. 2002a), a small but significant anisotropic pattern was actually shown to exist, but with faster propagation in the circular, not in the longitudinal direction. A recent study, however, has meticulously computed the degree of isotropy of the ICCMY layer and reported that in the normal small intestine there is no degree of anisotropy at all (Gao et al. 2013). The fact that slow waves therefore have a slight anisotropic pattern of propagation in the circular direction remains unexplained. In the early years, attempts were made to emulate cardiac mapping by inducing pathology into the GI system. For example, in an in vivo model of feline small intestine, ischaemia-induced local areas of inexcitability, which varied in intensity and location, inducing numerous conduction blocks and ectopic pacemaking (Lammers et al. 1997). There was also the opportunity to solve a long-standing controversy, related to whether or not the antral slow wave could propagate across the gastroduodenal junction. This issue was studied in isolated feline stomach–duodenal preparations and showed unequivocally that there was an area of inexcitability distal to the pyloric ring (Lammers et al. 1998). Later, in a collaborative study, such a blockade was also shown to exist in mice and

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rats and that this area coincided with a significant reduction in the numbers of ICC-MY (Wang et al. 2005). Another long-standing issue relates to the decrease in slow wave frequency and the existence of ‘frequency plateaus’ along the length of the small intestine. To study this in detail, it was necessary to map the whole length of the small intestine simultaneously. We initially started with small animals (rats and rabbits) but quickly discovered that the small intestines, in vitro, showed numerous spontaneous contractions, that limited or blocked slow wave propagation. We unfortunately could not use nifedipine as this clearly diminished the amplitude of the extracellular slow waves (Bayguinov et al. 2011). The ideal preparation turned out to be the feline small intestine which fortunately did not have a too long small intestine; on average 111 cm (range 105–120 cm). Rows of 240 extracellular electrodes, spaced at 4–8 mm distances, were positioned along the length of the intestine in vitro, and recordings were performed from all electrodes simultaneously (Lammers & Stephen 2008). The results (Fig. 1) showed that (i) the majority of slow waves (73%) propagated throughout the whole length of the quiescent intestine; (ii) there was, in the intact intestine, only one pacemaker active, close to or at the pyloric junction; (iii) the velocity of propagation gradually decreased; (iv) spontaneous propagation blocks occurred throughout the intestine; and (v) there was a considerable variation in the location and

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length of the ‘frequency plateaus’ in the different preparations. Furthermore, after recording in the intact organ, the small intestine was ligated at 1–4 locations and the recordings were repeated. As shown in Figure 1, this led in the ligated segments to (i) decrease in distal frequencies; (ii) increase in slow wave velocity; (iii) disappearance of propagation blocks; (iv) emergence of multiple and unstable pacemakers; and (v) propagation from these sites in both the aboral and oral directions. We hypothesized from these findings that, in the intact small intestine, the slow wave propagates continuously in the refractory wake of the previous slow wave and that, with the gradual decrease in velocity, a significant number of slow waves were bound to collide against their predecessors and eventually disappear, thereby explaining the decrease in frequency (Alvarez 1914). As with many biomedical research endeavours, the aim of discoveries made in the laboratory is to implement these findings to the clinic and the bedside. A major step in that direction was made when we were invited to expand the high-resolution mapping technology onto the canine small intestine in vivo. In that situation, recordings were performed from a 240-rectangular electrode array at five different locations along the length of the small intestine. Results confirmed many of the results previously obtained in the in vitro models with the addition that any site could generate spontaneously slow waves, if only for a few beats, and hence, propagation in the small intestine

Figure 1 Slow wave ladder plots demonstrating propagation throughout the whole length of the small intestine. Left panel: in the intact preparation, the pacemaker is located at the most oral part of the duodenum and velocity decreases gradually throughout the organ. In this 1-min recording, 12 propagation blocks occurred at various locations, indicated by the red circles. In the right panel, the small intestine was ligated at four locations thereby inducing a propagation block at every ligation. Distal to these locations, new pacemakers emerged, firing at lower frequencies than before which led to higher propagation velocities and the disappearance of propagation blocks. (From Lammers & Stephen 2008 with permission). © 2014 Scandinavian Physiological Society. Published by John Wiley & Sons Ltd, doi: 10.1111/apha.12371

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was more variable in vivo than in vitro (Lammers et al. 2005). Another major step forward was made in collaboration with the GI motility group at the Bioengineering Institute in Auckland, New Zealand. The aim of that collaboration was to progress to the human GI system, concentrating especially on the stomach (reviewed in this issue by L. Cheng) and the small intestine. A new type of electrode array was developed which was flexible and could adapt to the shape of the stomach or the small intestine (Angeli et al. 2011). Initial results confirmed and expanded on previous results achieved in several animal models and in vitro studies (Angeli et al. 2013a).

Spike propagation in the small intestine From the early days of motility research in the small intestine, it was known that a second signal could occur in the small intestine, the ‘spikes’, also called ‘action potentials’ (Richter 1924, Bozler 1942) that are caused by the opening of voltage-dependent (Ltype) calcium channels (Droogmans & Callewaert 1986, Katzka & Morad 1989). Daniel, in 1959, already suggested that there were ‘two independent but interacting control mechanisms, that is, that responsible for the slow wave and that responsible for the action potentials’ (Daniel et al. 1959). It was clear that contraction, to a certain extent, was determined by the occurrence of these spikes. Daniel & Chapman (1963) suggested ‘an untested hypothesis that the slow wave is an advancing zone of enhanced excitability which, when further enhanced by local factors, leads to action potentials and contractions’. It was also known that a spike could propagate but only for a limited distance (Greven 1953, Daniel et al. 1960), and this obviously was another phenomenon that could be analysed with high-resolution mapping. Already, in the first high-resolution recordings of the isolated small intestine of the rabbit, spike potentials were seen right after the slow wave deflections (Lammers et al. 1993). A first attempt at visualizing the potential propagation of the spikes was carried out later on isolated small intestines from the cat (Lammers 2000), which showed the major difference between spike and slow wave propagation. Whereas the slow wave propagates until the limit of an organ or a tissue border has been reached, as shown in Figure 2c, this is not the case with spikes. Spikes propagate for limited distances, usually in the 5–25 mm range before terminating spontaneously (Fig. 2d). They occur in the wake of the slow wave, when the cells are depolarized during its plateau phase. This depolarization increases the probability for opening the L-type calcium channels that underlie 352

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the spikes. Once the slow wave has repolarized to its resting potential, these spikes do not occur any longer. We initially thought that spikes would propagate along the whole depolarized area of the slow wave, but that is not the case; spike propagation is more limited. The reasons for that are not clear. By juxtaposing all the spike patches that occur after a slow wave (Fig. 2d), it can be seen that the edges of sequential patches did not align with each other but that the patches showed large overlaps. This seems to imply that there is not an underlying anatomical or structural reason, such as the edge of a bundle or a local accumulation of fibrous tissue, which could account for the termination of spike propagation. Noteworthy is the fact that spike patches seem to be larger if the slow wave propagation was inhomogeneous such as when two slow waves propagate towards each other and collide. Immediately, following the moment of collision, there will be a larger depolarized area in the tissue and this does seem to offer more area for the spike to propagate, although, again, spikes do not propagate through the whole depolarized area (Lammers & Slack 2001). It could be that it is the shape of the slow wave plateau, often showing a dome shape in intracellular recordings, that could play a role in enabling local spike propagation if, as suggested, the ‘open probability of L-type Ca2+ channels is steeply dependent on voltage’ (Sanders et al. 2014) but this needs to be further elucidated. Several years later, upon invitation by Jan Schuurkes and Luc Ver Donck, we performed high-resolution electrical mapping in anaesthetized dogs in vivo. As in rabbits and cats, spikes often occurred following the slow waves. When the propagation patterns of these spikes were analysed, similar results were obtained to that obtained in the cat, that is spikes propagated for limited distances before terminating spontaneously. However, another discovery was made. During the analysis, two types of spike patches emerged; a first type that showed predominant propagation in the longitudinal direction and a second type with propagation in the circumferential direction. The second type of spike almost always originated from the antimesenteric border and seemed to straddle the intestinal tube. Propagation velocity was much faster in the circumferential spikes than in the longitudinal spikes (Lammers et al. 2003). However, it must be clear that the concept that longitudinal spikes propagate in the longitudinal muscle layer and the circumferential spikes in the circular layer is only based on these extracellular recordings and their analysis. To prove definitely that this is the case requires direct recordings from within the two muscle layers themselves, combined with an overall serosal mapping of the area.

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Figure 2 Propagation patterns of slow waves, spikes in spike patches and of spike bursts in the isolated feline small intestine. (a) Recording from one site, at low speed, depicts 14 successive slow waves and one peristaltic complex. (b) Set of 24 electrograms, selected from the box indicated in (a), displayed at a faster speed, show three successive slow waves travelling in the aboral direction. A peristaltic wave originated from the caudal side of the preparation and propagated in the oral direction. At approx. the level of electrode #13, the peristaltic wave passed the second slow wave and appeared to continue unimpeded its propagation while the propagation of SW2 was slightly disturbed as indicated by the change in the slope of the SW2 arrow. Approx. 3.5 s later, the peristaltic wave crossed the third slow wave causing a stronger disturbance in the propagation pattern of that slow wave. Spike clusters, which occurred after the first slow wave (SW1), are indicated by ellipses. (c) Propagation map of SW1 illustrating the homogeneous sequence of aborad propagation. (d) Composite map of the eight spike patches that occurred after SW1. Note the overlap between individual spike patches and the sequence of appearance in the aborad direction following the aborad propagating slow wave. (e) Propagation map of the antidromic propagating peristaltic wave illustrating the homogeneous sequence of its propagation (slightly modified from Lammers et al. 2002b, with permission).

Spikes may also occur in bursts, independent of the slow wave, and demonstrate a different propagation pattern from that of the slow wave (Lammers et al. 2002b). In one study in which we could record from the isolated feline small intestine during spontaneous

peristaltic activity, bursts of spiking activity were seen to propagate, as broad wave fronts in either the orad or the aborad direction, just like the slow waves (Fig. 2). Their velocities were slightly lower than that of the slow waves but they often stopped propagating

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spontaneously, something that slow waves never do. Interestingly, as shown in Figure 2, both wave fronts, on occasion, were seen to ‘cross’ each other. In such a case, it was always the propagation of the slow wave that was disturbed, never that of the peristaltic wave. We suspect this could be due to the forceful contractions generated by the peristaltic wave but the reasons for that are not clear.

Slow wave arrhythmias in the small intestine As with the study of the slow wave, disturbances in the rhythm and the propagation of the slow wave in the small intestine had already been noted many years ago, although this topic has not been studied as vigorously as with similar disturbances in the stomach. Many types of interventions have been used to induce these disturbances (Schaap et al. 1990, Vantrappen et al. 1991). Seidel et al. (1999), in a rabbit model in which four serosal electrodes had been implanted, showed that intestinal ischaemia often induced what they labelled ‘the first demonstration of intestinal tachyarrhythmias’. Important is their observation that while one electrode detected the arrhythmia, another electrode, located not so far away, showed normal slow wave rhythm, indicating that discrete segments of the small bowel were being activated at different rates (Seidel et al. 1999). The local effects of ischaemia were also observed on slow wave propagation in the anaesthetized cat small intestine in vivo which showed that ischaemia caused areas of conduction blocks and the appearance of ectopic pacemakers, although tachyarrhythmias were not seen (Lammers et al. 1997). The difference with Seidel’s observation could be due to a point raised by Huizinga (1998) that one advantage of ICC-MY being organized in a network could be that damage to a few cells could be corrected by the rest of the network. This concept was further investigated in a cellular automaton model of the small intestine (Lammers et al. 2011). In that model, it was shown that random small deletion of individual cells had little effect on overall propagation pattern and velocity of propagation. Depending on the degree of connectivity between individual cells, up to 50% of all cells in a population could be made inactive before major propagation problems ensued. Obviously, if the damage affects lager areas or the whole bowel, even the best-coupled network will then no longer cope. Such disturbances affecting the whole bowel could occur, for example, during the administration of morphine in the canine small intestine whereby morphine had ‘a dramatic effect on the spatial and temporal organization’ of the slow wave and on motility (Sarna & Otterson 1990). Christensen, using intra-luminal 354

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electrodes inserted in humans, studied the effects of fever and a variety of diseases on the basic electrical rhythm and noted, for example during induced fever, that after about 5 min, the ‘orderly sequence of slow waves disappeared and the electrical record showed rapid irregular waves of varied form’ (Christensen et al. 1966). Inflammation was often used in animal models to induce ectopic foci, probably due to a decrease in intercellular coupling (Der et al. 2000). Other interventions that may cause intestinal arrhythmias are intestinal obstructions (Chang et al. 2001), diabetes (He et al. 2001) or major ICC-loss (Sanders et al. 2002). Until recently, the presence or absence of an arrhythmia was determined by looking at the shape or the frequency of the slow wave, using a limited number of electrodes, or by recording the motility of the intestine (Scheffer & Smout 2011). That, as such, is certainly enough to determine that there is a problem. However, if one wants to determine the mechanism of the arrhythmia, more work needs to be carried out to determine the causative agent. In the case of arrhythmias, an important tool is high-resolution electrical mapping (Shenasa et al. 2009). As discussed in several studies in this issue and at the beginning of this review, this technology makes it not only possible to determine the normal origins and propagations of electrical impulses, but also that of abnormal propagations. One possible and potentially important mechanism of intestinal arrhythmias is the occurrence of reentries in the small intestine. Interestingly, the first demonstration of re-entry in the small intestine was observed in silico, in a computer simulation study (Gizzi et al. 2010) where the authors developed a 3-D model and, through thermal imbalance, induced conduction blocks and circus movement of the slow waves. They were actually studying the mechanism of paralytic ileus and hypothesized that the thermal imbalance induced by the surgery could induce these motility disturbances. A first demonstration of re-entry in a living small intestine, in vitro, was obtained by our group in the context of studying the long-term effects of diabetes on slow wave propagation in the rat small intestine (Lammers et al. 2012). It was surprising in the sense that such small re-entries had not yet been seen in such a narrow tube as the small intestine of the rat. After all, a re-entry has a certain size and if the circuit is too large and the available tissue too small, then re-entry simply cannot occur. The relationship between size of the tissue and size of the circus movement was already well established in cardiac electrophysiology many years ago. Mines (1913) and Lewis (1925), who were the first to describe circus movements in the heart, discussed the importance of the wavelength. This was later formu-

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lated mathematically by Wiener and Rosenblueth as the distance travelled by the depolarization wave during the duration of the refractory period (Wiener & Rosenblueth et al.1946). This concept was later re-introduced by the group of Allessie et al. (Smeets et al. 1986, Rensma et al. 1988). The minimal size of a circus movement is determined by the speed of propagation and the time it takes for the tissue to restore from a previous activation, that is, the refractory period. The faster the speed or the longer the refractory period, the larger the circle. On the other hand, if the propagation is very slow and/or the refractory period very short, then the diameter of the circus movements will be smaller. There is not much information available concerning the refractory period in smooth muscles but indirect evidence, such as obtained during pacing, seem to suggest that the refractory period is close to the normal slow wave interval (Lammers et al. 2012). That is much longer than what is seen in the heart muscle. In the heart muscle, the refractory period ranges from 150 to 400 ms, approx. 30% of the total cardiac cycle length, depending on cardiac rate and location (Denes et al. 1974). A few studies have measured the refractory period in gastrointestinal muscle and showed that the refractory period was much longer in the GI tract and often lasted more than 70% of the intrinsic cycle length (Specht & Bortoff 1972, Sarna & Daniel 1974). However, the velocity is definitely much slower (1–10 cm s 1; Lammers et al. 2005) than in the canine atria (100 cm s 1; Rensma et al. 1988). Therefore, the combination of a long refractory period and a slow velocity makes a wavelength of 1–2 cm possible with a diameter of approx. 0.5–1.5 cm, a size that can be accommodated in the intestinal wall of the rat. Similar calculations predict that such re-entries may also occur in the human GI, especially in the ileum where conduction is slower than in the duodenum (Lammers et al. 2012). The relationship between wavelength and re-entry described above relates in the first place to so-called functional re-entries (Allessie et al. 1977). These are circus movements whereby their properties (size, velocity and refractoriness) are determined by the underlying electrophysiology of the tissue. However, the concept of the wavelength, to a certain degree, could also have implications for circus movements that are not functional, that is partially or totally determined by other factors such as anatomical boundaries or, as shown most recently, by the size of the intestinal lumen. Angeli et al., in the pig in vivo (Angeli et al. 2013b) showed the possibility of a slow wave rotating around the circumference of the small intestine, hence labelled ‘circumferential re-entry’, a phenomenon that was also predicted by Gizzi et al. (2010). Whereas this type of arrhythmia may occur in

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the relatively normal small intestine, it is not hard to predict that such events may occur more often in dilated loops where the diameter of the intestinal loop has increased and probably the velocity decreased. In fact, we may soon come to a situation, already visualized in the stomach (Lammers et al. 2008), of highly irregular propagating events in the wall of the small intestine. S.B. Subramanya, B. Stephen, S.S. Nair, K.H. Sch€afer & W.J. Lammers (unpublished data) used ethanol to slow down propagation velocities of the slow waves in the rat small intestine and described a variety of propagation disturbances varying from very small circuits to several intertwined re-entrant loops (Fig. 3; S.B. Subramanya, B. Stephen, S.S. Nair, K.H. Sch€afer & W.J. Lammers, unpublished data). The question arises whether re-entrant arrhythmias could play as prominent a role in provoking motility disturbances as it could do in the stomach and certainly does in the heart. Probably not. In the small intestine, next to normal or abnormal propagating slow waves, there are other important systems for generating and controlling motility (Huizinga & Lammers 2009). The role of spike bursts, for example, plays an important role in the context of the migrating myoelectric complexes (Szurszewski 1969). In that situation, in the empty organ, a sequential spread of a high degree of spiking activity has been shown to migrate slowly down the stomach and the small intestine, presumably to get rid of any indigestible left-overs in the intestinal lumen (‘housekeeper function’) and we could imagine that the propagation of individual spikes in these migrating complexes may look similar to what was shown during peristaltic activity in Figure 2. In other words, spiking behaviour is also determined by other systems such as the enteric nervous, hormonal and immunological systems. The pathology of the small intestinal motility is quite varied (Husebye 1999) but, unfortunately, still little is definitely known. In conclusion, high-resolution electrical recording of the small intestine is a useful approach to detect and analyse normal and abnormal initiation and propagation of several electrical impulses in the small intestine. In recent years, this approach has increased our understanding of basic physiology and pathophysiology of this organ, using both in vitro and in vivo models, in animals and in humans. At the same time, this work has also revealed our relative lack of knowledge and understanding of the mechanism of many such electrical events and their correlation with the mechanical actions of the small intestine.

Future directions of research There are plenty of opportunities for further research in many directions (Cannon 1902). Additional work

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Figure 3 Example of slow propagation in a loop mimicking a double re-entrant activity. (a, b) Show propagation from two successive focal discharges (location indicated by the star along the upper border). Propagation from this area proceeded down the lower left part of the map before rotating around the edge of a block and propagating back towards the upper part of the map. As shown in the electrograms (e) and indicated by the red line drawn at the moment of the second discharge, propagation was so slow that the second impulse was initiated while the previous wave was still propagating in the mapped area, indicating that there are two wavelets propagating simultaneously in this small area (10 9 10 mm). In fact, as indicated in (c) in which one frame of the accompanying movie is plotted, an alternative explanation could be re-entry around the central block with the wave propagating outside the upper border (indicated by the dashed arrow). In the third cycle, however (d), the focus has shifted away from the upper border, indicating that this pattern of activity was caused by focal discharges. (from S.B. Subramanya, B. Stephen, S.S. Nair, K.H. Sch€afer & W.J. Lammers, unpublished data with permission).

in in vitro preparations could, for example, concentrate on the interaction between electrical activities and motility (Lammers et al. 2001). In the rabbit model, we have been able to study the interaction between the slow wave and pendular contractions (Lammers 2005) but this could be further extended to other contractions such as the peristaltic reflex (Trendelenburg 1917, 2006, translated in English). A most recent publication (Huizinga et al. 2014) proposes the interaction of two pacemaker networks to explain the 356

pattern of segmental contractions first described by Cannon in 1902 (Cannon 1902). Would it be possible to map from both the serosal and the mucosal side of the intestinal wall simultaneously to analyse these interactions? Additionally, little is known of the effects of many pharmaceuticals on propagation patterns. But the major thrust for the future has to be to our patients. For this to happen, we need much more information regarding electrical propagations in the

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small intestine in health and disease. The problem here is that the signals are much weaker than those, for example, from the heart and thus cutaneous signals from the small intestine are very weak (Smout et al. 1980, Chen & McCallum 1991). Other modalities could be envisaged such as recording the magnetic counterpart of the electrical signal, as is being performed in a study on human pregnant myometrium (Eswaran et al. 2004). More simply, development of new electrode types could be envisaged for recording through the mucosal barrier, or from the serosal side via laparoscopy (O’Grady et al. 2009). At the end of the day, we need more information in order to understand and ultimately to be able to treat our patients. Electrical recordings, together with other modalities, may form one way forward towards that goal.

Conflict of interest There are no conflicts of interest with this paper. This review gives me the opportunity to thank all those who have helped me for many years in the studies described in this review, and it is a pleasure to recognize the contributions of Mrs Betty Stephen, Mrs Kholoud Arafat, Mr S Dhanasekaran, Mr A Wahab, Mr S Singh and Mr A El-Kays. On this occasion, I would also like to pay tribute to the work of several predecessors that have greatly helped in my work. The publications and the figures from the work of Drs. Atanassova, Bortoff, Christensen, Daniel, El-Sharkawy, Gonella, Publicover, Sarna, Schulze, Specht and Szurszewski have been a source of inspiration to me and have helped make high-resolution mapping of smooth muscles possible. This work has been supported from 1992 till 2014 by many grants from the College of Medicine & Health Sciences, UAE University, Al Ain, UAE. An animation of the pseudo-re-entry shown in Figure 3 is available on YouTube: http://youtu.be/89wNId5xaCY.

References Aliev, R.R., Richards, W. & Wikswo, J.P. 2000. A simple nonlinear model of electrical activity in the intestine. J Theor Biol 204, 21–28. Allessie, M.A., Bonke, F.I. & Schopman, F.J. 1977. Circus movement in rabbit atrial muscle as a mechanism of tachycardia. III. The “leading circle” concept: a new model of circus movement in cardiac tissue without the involvement of an anatomical obstacle. Circ Res 41, 9–18. Alvarez, W.C. 1914. Functional variations in contractions of different parts of the intestine. Am J Physiol 35, 177– 193. Alvarez, W.C. & Mahoney, L.J. 1922. Action currents in stomach and intestine. Am J Physiol 58, 476–493. Angeli, T.R., O’Grady, G., Erickson, J.C., Du, P., Paskaranandavadivel, N., Bissett, I.P., Cheng, L.K. & Pullan, A.J. 2011. Mapping small intestine bioelectrical activity using high-resolution printed-circuit-board electrodes. Conf Proc IEEE Eng Med Biol Soc 2011, 4951–4954.

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