The relationship between slow waves and peristaltic reflexes has not been well analyzed. In this study, we have recorded the electrical activity of slow waves together with that generated by spontaneous peristaltic contractions at 240 extracellular sites simultaneously. Recordings were made from five isolated tubular and six sheet segments of feline duodenum superfused in vitro. In all preparations, slow waves propagated as broad wave fronts along the longitudinal axis of the preparation in either the aborad or the orad direction. Electrical potentials recorded during peristalsis (peristaltic waves) also propagated as broad wave fronts in either directions. Peristaltic waves often spontaneously stopped conducting (46%), in contrast to slow waves that never did. Peristaltic waves propagated at a lower velocity than the slow waves (0.98 ± 0.25 and 1.29 ± 0.28 cm/s, respectively; P < 0.001; n = 24) and in a direction independent of the preceding slow wave direction (64% in the same direction, 46% in the opposite direction). In conclusion, slow waves and peristaltic waves in the isolated feline duodenum seem to constitute two separate electrical events that may drive two different mechanisms of contraction in the small intestine.
- peristaltic reflex
- electrical mapping
- small intestine
the small intestine spontaneously produces a variety of motility patterns characterized as peristaltic or pendular, stationary or propagating, or twitch or segmental. Several electrical signals have been shown to be associated with these different types of contractions, including slow waves (4, 5), spikes (7, 18), or bursts (13, 16). The relationship between these different electrical signals and the patterns of contractions with which they are associated is not clear. It has been especially difficult (16) to describe the relationship between the slow wave and the peristaltic reflex (25).
For many years it has been known that the slow wave is able to propagate along the small intestine, and recently, the concept of propagation of peristaltic contractions (8) has re-emerged (6, 15, 25, 26). Gonella (13) identified the electrical potentials generated by peristaltic contraction as peristaltic waves. By recording peristaltic waves at several sites in the isolated rabbit duodenum, he demonstrated that these waves could propagate in either the oral or the aboral direction.
The aim of this study was to analyze and compare the patterns of electrical propagation of slow waves and peristaltic waves. By recording extracellular electrograms from 240 sites simultaneously we have been able to reconstruct the propagation of peristaltic waves and of slow waves in the same segment of the intestine. We showed that both waves propagate along the intestine as broad wave fronts that may travel in the same or in opposite directions from each other and may propagate in the oral or caudal direction. In addition, the peristaltic wave can at any time stop conducting spontaneously, whereas slow waves never do. From these results, it would seem that slow waves, with their associated spike patches (18), constitute a different form of contractile mechanism that operates separately from contractions induced by peristaltic waves.
These experiments used 11 mongrel cats of either sex (7 males, 4 females, 3.2 kg mean body wt) that were anesthetized with xylazine (10 mg/kg im) and ketamine (10 mg/kg im). After a midabdominal incision, a segment of the proximal small intestine was removed and transferred to a plastic dish filled with warm, oxygenated Tyrode where the duodenum was further trimmed to a length of ∼7 cm. In six preparations, the duodenum was opened along the mesenteric attachment to obtain a sheet of tissue, whereas in five tissues, the duodenal tube was left intact. The final preparation was positioned in a 200-ml tissue bath with the serosal side facing upward where it was superfused at a rate of 100 ml/min with a modified Tyrode's solution of (in mM): 130 NaCl, 4.5 KCl, 2.2 CaCl2, 0.6 MgCl2, 24.2 NaHCO3, 1.2 NaH2PO4, and 11 glucose saturated with carbogen (95% O2-5% CO2), whereas pH was kept constant at 7.35 ± 0.05 and temperature at 37 ± 0.5°C.
Electrical activity from the tissue was recorded at a large number of sites with a multiple-electrode assembly consisting of 240 Teflon-coated silver wires (0.3-mm diameter) glued together at an interelectrode distance of 2 mm in a 10 × 24 rectangular array. The tip of the electrodes extended 3 mm below the assembly to allow some solution to reach the serosal surface. The multielectrode assembly was carefully lowered on the preparation under constant visual guidance until the tip of the electrodes barely touched the serosal surface. Figure 1 C depicts the location of the electrode assembly covering a tubular duodenal segment. Electrical recordings were made unipolarly with a large silver plate in the tissue bath as the indifferent pole. All electrodes were connected through shielded wires to 240 alternating current preamplifiers (gain 4,000), and the recorded signals were subsequently filtered (bandwidth 2–400 Hz), digitized (8 bits, 1 kHz sampling rate per channel), multiplexed and stored. Details of the experimental setup and the recording system have been presented in previous communications (22). Recordings were made during spontaneous activities for periods of 15–30 min.
After the experiment, representative samples of spontaneous activities were chosen every 5–10 min and the signals (8- to 12-s duration) were transferred to a personal computer for further analysis. Signals were digitally filtered (20-point moving average) to remove 50-Hz noise and were displayed on screen in sets of 24 channels at a time (Fig.1 B). Three types of signals were analyzed in this study:1) slow waves visible as slow mono- or diphasic deflections (0.05–0.5 mV) that last for 300–600 ms (5),2) individual di- or triphasic spikes of similar magnitude but much shorter duration (40–100 ms) (24), and3) peristaltic waves (13) that display multiple discharges lasting for 50–200 ms and often have a higher amplitude (0.1–1.0 mV) than slow waves or single spikes. Local activation time of a slow wave was identified by the moment of maximum negative slope and marked with a cursor (22), whereby reference time was determined by time of first detected slow wave (t = 0.0 s at electrode 1). In this study, the timing of peristaltic waves was also determined, and for this purpose, the occurrence of the first significant slope of the waveform was used (Fig. 1 B). After all recorded slow and peristaltic waves were analyzed, their activation times were displayed in the format of a grid of the original recording array of electrodes (Fig. 1 E). In the case of a low-quality signal or if no deflections were visible, those sites on the maps (i.e., Fig. 1,E and F) were left empty. In the maps, isochrones were drawn manually around areas activated in times steps of 0.5 s. Conduction velocity of a wave was calculated by the difference in activation time between two recording sites located 30–40 mm from each other after determining that the wave had propagated homogeneously between those two sites.
Student's t-test was used to determine statistical significance. As shown in Table 1, patterns of propagation of 24 waves, recorded in 11 different preparations (5 tubular and 6 sheet), were studied in this analysis.
Results displayed in Figs. 1 and 2illustrate several similarities and differences between the propagation of slow waves and peristaltic waves. Both propagate as broad homogeneous wave fronts across the entire width of the preparation and this pattern of propagation was seen in all activation maps (n = 24). However, velocity and direction of propagation of the two types of waves differed. Conduction velocity of the peristaltic wave is slightly slower than that of the slow wave (1.26 and 1.51 cm/s, respectively in Fig. 1). In Fig. 1, both waves traveled in the same caudal direction, but in Fig. 2, an example is shown in which the peristaltic wave propagated in a direction opposite that of the slow wave. In that same figure, the peristaltic wave front also crossed the second slow wave, apparently unimpeded by it. The propagation of that slow wave (SW2) however, was disturbed by the peristaltic front as visualized by the change in timing of the slow wave after the crossing. This disturbance was even stronger when the peristaltic wave crossed the third slow wave (SW3). Because of this effect, the propagation characteristics of the slow waves were always measured before the occurrence of a peristaltic wave.
Figure 2 also offers the opportunity to compare the propagation of slow waves, peristaltic waves, and individual spikes. Whereas both the slow waves and the peristaltic waves conduct as broad wave fronts across the width of the preparation, this is not the case for individual spikes initiated by the slow wave. As shown in Fig. 2 E, eight spike patches occurred after SW1, all of which showed only very limited spatial propagation as described in a previous study (18). As visualized by the ellipses in Fig. 2 B, all spike patches followed the propagating slow wave and appeared to be uninfluenced by the nearby peristaltic wave.
Another major difference between the slow wave and the peristaltic wave is the range of their propagation. Once a slow wave is initiated, it will continue to propagate until it reaches the border of the preparation or collides with another slow wave (21, 22). This is not the case with the peristaltic wave. In this study, several instances of spontaneous block of peristaltic wave propagation were found. As shown in Fig. 3 D, a peristaltic complex was initiated at a discrete site located close toelectrode 17, which propagated in both the caudal and oral directions. This propagation, however, stopped spontaneously after traveling only 8 mm in the caudal and 14 mm in the oral direction, thereby limiting the peristaltic wave to a relatively short segment of the preparation. Similar spontaneous conduction blocks were found in several other instances. Figure 3 E plots the lines of block of seven peristaltic waves. Blockade always occurred across the entire width of the preparation and in the circumference of the tube but never in the longitudinal direction. This is an important difference from the propagation of spikes where conduction block occurred in both the longitudinal and circular directions (Fig.2 E). Because of this difference, peristaltic waves never activated an area as small as a spike patch. Instead, peristaltic waves always activated a complete segment of the intestinal tube, which could, on occasion, be quite short.
Figure 3 also shows the pattern of spread of the peristaltic wave from its site of initiation. As can be seen, the long axis of the elliptical isochrones is parallel to the circumferential direction. A similar pattern was found at all sites of initiation of peristaltic wave (n = 7). Therefore, peristaltic waves seem to be initiated at discrete sites and propagate more rapidly in the circular than in the longitudinal direction.
Peristaltic waves were found to be initiated at sites that appeared to be unrelated to the sites from which slow waves were initiated. An example of this is shown in Fig. 4. The earliest slow wave emerged from a distant oral site and entered the mapped area at t = 0 ms as a broad wave front. Somewhat later, at t = 490 ms, a second slow wave was initiated near electrode 19. The peristaltic wave also happened to be initiated within the mapped area at t = 2,560 near electrode 15, ∼9 mm away from the nearest site of slow wave initiation. In Fig. 4 E, the location of these sites of initiation is plotted with those from three other similar examples recorded in different preparations. There is no apparent spatial relationship between the pairs of slow wave and peristaltic wave initiation sites.
In contrast to this lack of spatial relationship between the site of initiation of the two types of waves, we found a small, but possibly important, relationship between the timing of the initiation of the peristaltic wave and the passage of the slow wave. In seven cases in which the moment of initiation of the peristaltic wave could be determined, the peristaltic activity was always initiated within a period of 2 s after the preceding slow wave (mean 35%; range 23–53% of the preceding slow wave interval). In other words, the moment of initiation of a peristaltic wave at a particular site always occurred in the wake of the previous slow wave and never occurred 1 or 2 s before the next slow wave. It should be stressed that this temporal relationship is only true for the moment of initiation of the peristaltic waves. Thereafter, because of differences in direction of propagation and because of the difference in conduction velocities, there was no longer any temporal relationship in the propagation of both types of waves.
Table 1 summarizes all the findings in this study. Data recorded from both intact tubular preparations and cut, flattened, sheet preparations are listed separately. This was done to determine whether cutting and flattening the intestinal tube effected propagation of the two kind of waves. Spontaneously occurring peristaltic waves were found in five tubular preparations (tissues A–E) and in six sheets cut open (tissues F–K). Cutting open the preparation did not affect the conduction velocity of either the slow wave [at 1.30 and 1.29 cm/s; P = not significant (ns)] or the peristaltic wave (1.00 and 0.97 cm/s;P = ns). The conduction velocity of the peristaltic wave, however, was significantly lower than that of the slow wave in both types of preparations (0.98 ± 0.25 and 1.29 ± 0.28 cm/s, respectively; P < 0.001; n = 24 waves).
Both slow waves and peristaltic waves spontaneously propagated in either the oral direction (4 and 12 waves, respectively), in the caudal direction (11 and 5 waves, respectively), or in both directions (9 and 7 waves, respectively). Furthermore, in seven cases, the peristaltic and slow waves propagated in the same direction (Fig. 1), whereas in four other cases, they propagated in opposite directions (Fig. 2). In the majority of cases (13), both propagation patterns were complex in that waves emerged in both directions from local sites of origin (Fig. 4).
Finally, Table 1 also shows the extent of the area of the intestine activated by the slow wave or peristaltic wave. This area was determined by the number of electrodes that recorded a slow wave or a peristaltic wave. Whereas slow waves always activated the whole mapped area (100%), this was not always the case for the peristaltic waves. In 13 cases, the whole area was activated by a peristaltic wave, but in 11 cases a segment of the mapped area only was activated (as in Fig.3). On average, 76% of the tubular preparation was activated by a peristaltic wave (range 38–100%) and 72% of the sheet preparations (range 25–100%). There was no significant differences between sheet and tubular preparations.
In this study, first comparisons have been made between the spatial and temporal patterns of propagation of slow waves and peristaltic waves. Our overall results suggest that these two waves are distinct and separate phenomena, implying different mechanisms of propagation. This conclusion is based on 1) a small but significant difference in conduction velocity, 2) a lack of spatial relationship between the propagation patterns of the two waves that could propagate in the same or in opposite directions,3) a lack of temporal relationship between the propagation of the two waves, and 4) lack of any spatial relationship between the sites of initiation of slow and peristaltic waves.
Our results confirm and expand on Gonella's work (13,14). With a limited set of 1–6 extracellular electrodes, Gonella was able to record spontaneous peristaltic waves in the rabbit duodenum preparation and demonstrate that 1) the peristaltic wave propagated from one electrode to the next, 2) this propagation could occur in either the oral or the aboral direction, and3) the peristaltic wave would conduct for a variable distance. Due to the limited number of electrodes, Gonella was not able to compare the propagation of the peristaltic and the slow wave.
There have been few studies that attempted to compared slow waves with peristaltic waves or with peristaltic contractions. Huizinga et al. (16), in a model of distension-induced peristalsis in the isolated mouse small intestine, showed that slow waves with associated spike activity could induce pulsatile outflow in synchrony with the slow wave. In addition, distension in that preparation induced burst-type activity associated with significant intraluminal pressure increases. Due to the limited number of recording electrodes and the resulting lack of temporal resolution, they were unable to determine the propagation characteristics of these bursts and to differentiate between slow wave-induced spiking activity and that of bursts. They were, however, able to show that tetrodotoxin abolished the bursts without affecting the slow wave, whereas in W/Wv mice, which lacked interstitial cells of Cajal (10), slow waves were abolished, whereas burst-activity could still be evoked (16).
Our results show that the peristaltic wave propagates, a finding that also gains support from several studies in which peristaltic contraction or movement was monitored. Cannon (8), in isolated feline small intestines, had already shown that induced waves of contraction would propagate in both directions. More recently, Hennig et al. (15) showed that propagation of peristaltic contractions usually occurred in the caudal direction and occasionally in the oral direction. In addition, peristaltic contractions may occur in empty tubular preparations and even in flat sheets (6), similar to our preparations. Spencer et al. (25), in the isolated guinea pig small intestine, also showed that peristaltic contractions propagated, and that this could occur in empty tubular preparations and in preparations opened along the longitudinal direction. In another study, Spencer et al. (26) also showed that peristaltic contractions could propagate in both the anal and oral direction. Unfortunately, in the guinea pig model, it is not possible or very difficult (9, 11, 23) to record slow waves, at least in vitro (12), making it impossible to compare the propagation and interaction of slow and peristaltic waves.
Several studies have classified patterns of contractions in the small intestine into a neurogenic and a myogenic component (1-3,6, 10, 15-17, 28). This classification is mainly based on the fact that tetrodotoxin or atropine do not abolish all motility in the small intestine although they block peristaltic contractions and propagation (16, 25). Results from our study support a dual mechanism of contraction in the small intestine. One type of motility is caused when spike patches occur in association with the propagating slow wave (18). As spike patches activate limited areas, these contractions are relatively weak and in rhythm with the slow wave, usually visible as pendular movements (19). The other type of contraction, peristaltic contraction, is much stronger, as suggested by the multiple spike discharges, and by the fact that these peristaltic waves propagate across the whole width of the intestinal segment, thereby activating a much larger area. Peristaltic waves are not in rhythm with the slow wave and, in fact, occur much less frequently, as shown in the top traces in Figs. 1-4. It is, therefore, also important to realize that, although all spikes induce contraction, they occur in different groupings and patterns (spike patches or peristaltic waves) and hence generate different types of contractions (pendular contractions or peristaltic contractions).
In vitro, in the duodenum, our present results show that the two mechanisms seem to operate largely separately, because little temporal or spatial interrelationship or interaction could be found between these two events. That is not to say that these two mechanisms are totally independent from each other. In fact, some interrelationship between them is only to be expected, if one accepts that the enteric nervous system, next to its dominant role in its control of the peristaltic reflex, most probably also contributes to the triggering of individual spikes during the slow wave complex (27). In addition, it would seem highly unlikely that the enteric nervous system would also not have a role to play in determining site and frequency of the initiation of slow waves (3). In other words, the distinction into a neurogenic and a myogenic component might be too simplistic and inadequate to describe two mechanisms driven (peristaltic contraction) or modulated (slow waves, spike patches) by the enteric nervous system.
Limitations of this study should be clear and conclusions reached must also be bound by these limitations. In the first place, the study was performed in one species, the cat, and in one part of the small intestine, the duodenum. Second, the peristaltic waves studied were of the spontaneous type, and no information is available concerning their similarity or difference from those induced by fluid distension, local stretch, or mucosal stroking.
These limitations need to be addressed and more work is required to elucidate the interplay between the two types of motility patterns in the small intestine. High resolution electrical mapping offers a novel approach to study this interplay by using the extracellular electrical signals recorded from the muscle layers that generate these motility patterns. It should also help to elucidate the nature of the electromechanical coupling that must take place during contraction. Correlating these electrical signals with contraction signals (19) would be a further step in obtaining the data required to expand our understanding of motor control in the small intestine.
This work was supported by the Research Committee of the Faculty of Medicine and Health Sciences, United Arab Emirates University Grants CP/98/5 and NP/2000/19.
Address for reprint requests and other correspondence: W. J. E. P. Lammers, Dept. of Physiology, Faculty of Medicine and Health Sciences, PO Box 17666, United Arab Emirates University, Al Ain, United Arab Emirates (E-mail:).
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- Copyright © 2002 the American Physiological Society