1. ELECTRICAL PHENOMENA IN THE HUMAN STOMACH
The main function of the stomach is to process and transport food. After feeding, the contractile activity of the stomach helps to mix, grind and eventually evacuate small portions of chyme into the small bowel (12), while the rest of the chyme is mixed and ground.
Anatomically, the stomach can be divided into three major regions: fundus (the most proximal), corpus and antrum (Fig.1.1). Histologically, the fundus and corpus are hardly separable. In the antral area, the density of the smooth muscle cells increases (11).
The stomach wall , like the wall of most other parts of the digestive canal, consists of three layers: the mucosal (the innermost), the muscularis and the serosal (the outermost). The mucosal layer itself can be divided into three layers: the mucosa (the epithelial lining of the gastric cavity), the muscularis mucosae (low density smooth muscle cells) and the submucosal layer (consisting of connective tissue interlaced with plexi of the enteric nervous system). The second gastric layer, the muscularis, can also be divided into three layers: the longitudinal (the most superficial), the circular and the oblique (Fig.1.2). The longitudinal layer of the muscularis can be separated into two different categories: a longitudinal layer that is common with the esophagus and ends in the corpus, and a longitudinal layer that originates in the corpus and spreads into the duodenum.
Figure 1.2. Structure of Gastric Muscularis: A -- the longitudinal layer (the area where the longitudinal fibers split is marked with a black circle); B - the circular layer; C - the oblique layer.
The area in the corpus around the greater curvature, where the split of the longitudinal layers takes place, is considered to be anatomically correlated with the origin of gastric electrical activity (11). The circular layer of the muscularis is continuous with the circular layer of the esophagus, but is absent in the fundus (12). The thickness of the circular layer increases in the antrum and especially in the pyloric sphincter (9). It does not continue into the duodenum. The oblique layer of the muscularis is clearly seen in the fundus and near the lesser curvature of the corpus, but the oblique fibers disappear distally (towards the antrum). The outermost main layer is the serosa (Fig. 1.3).
Figure 1.3. Cross section of gastric wall. Nerve plexi provide the interface between the mucosa and the muscularis, as well as between the longitudinal and circular layers of the muscularis (9, 12).
1.2. GASTRIC ELECTRICAL ACTIVITY
Intracellular electrical and contractile activity of smooth muscle fibers has been studied by many investigators (13, 14, 15). There is a general agreement regarding the relationship between these two phenomena. Many different terms have been used to describe gastric electrical activity: "slow potential", "slow wave", "initial potential", "control potential", "spiking", etc. In 1975 Sarna et al. (16) introduced a new terminology, that was adopted by the majority of the research groups working in the field.
Gastric electrical activity recorded in vitro is divided in two major types: Electrical Control Activity (ECA) and Electrical Response Activity (ERA). It has been shown (17, 18) that intracellular electrical depolarization due to spontaneous ionic exchange through the membrane precedes contraction of the gastric smooth muscle fiber. However, not all depolarizations are followed by contractile activity (18). This is an important difference between cardiac muscle fibers and gastric smooth muscle cells. The two different types of electrical activity, ECA and ERA, are clearly recognized in the cells of the distal two-thirds of the stomach (Fig. 1.4.). Many studies have attempted to determine histologically the origin of GEA in the stomach (19, 20, 21). It is definitely known, that smooth muscle cells of the fundus do not have the electrical and contractile behavior described above. Weber and Kohatsu (22) and Kelly and Code (23) pointed out as a possible origin of gastric electrical activity an area in the corpus along the greater curvature where the longitudinal muscle fibers that continue into the duodenum originate. This concept is rejected by many who feel that the lon-gitudinal layer in the stomach is integral with no area of split in the longitudinal fibers (24).
Figure 1.4. Electrical and contractile activity of gastric smooth muscle cell.
The mechanism of propagation of gastric electrical activity from one cell to another is still unknown. There is no doubt that regardless of the exact mechanism of propagation, there is a full electrical coupling of different parts of the normal stomach, in other words there are well defined temporal and spatial rules that regulate the propagation of gastric electrical activity. The nature of this regulation is not entirely known (25).
ECA is considered to be the initial rapid depolarization of the cell and is a necessary, but not a sufficient condition for contraction to occur. It is periodic by nature, with a period of about 20 seconds (or frequency of 3 cycles per minute, cpm).
Only the appearance of the plateau of the ERA, the second type of activity, and eventually the second component of the ERA, the spikes that are superimposed over the plateau of the ERA, indicate that contraction will follow. It is clear, that ERA itself is separated into two components: plateau and spikes. These components are not independent. Spike bursts cannot exist without the plateau, while the opposite can happen, the plateau of ERA might follow ECA without any spikes. The force of contraction that will follow in that case, however, will not be higher than 0.25 N. The presence of spikes indicates that a contraction with greater strength is expected. ECA is always present in the stomach, while ERA with its component(s) manifests itself occasionally (12, 16, 17). Most authors (20, 21, 24) relate the occurrence of ERA to the injection of Ca2+ ions into the smooth muscle cell. They point out that the plateau of ERA is possibly related to the opening of the so called "slow Calcium channels" of the cell membrane, while the spikes are produced after the injection of Ca2+ ions through the "fast Calcium channels". There are some suggestions also that similarly to the ECA, the first component of ERA, the plateau, is always present in the stomach, but it has to reach a certain threshold level for contraction to occur (14, 15).
During the gastric fasting state, Code and Marlett (26) pointed out that GEA can display the features of the so called Migrating Myoelectrical Complex (MMC), a phenomenon initially described by Szurszewski in the small bowel (27). The MMC can be separated into four phases: quiescent phase 1, when only ECA is present; transitional phase 2, when cycles of ECA without any ERA are mixed with cycles in which both ECA and ERA are present; phase 3, during which ECA is always followed by ERA with its two components (in other words, spike activity is always superimposed on the plateau of ERA); and transitional phase 4, very similar to phase 2. The total duration of all four phases is about 1.5 - 2 hours, the active phase 3 usually lasts 10 - 18 minutes. After feeding, the pattern of MMC changes probably because the stomach becomes active until it empties (28). The fasting pattern of MMC returns 6 - 8 hours after feeding. Although the strength of contractile activity varies postprandialy, quiescent periods are not observed in normal subjects (28). In all in vitro measurements of GEA, special methods of recording with microelectrodes are used. These experiments are quite complicated and are performed under special laboratory conditions (11, 12, 14, 15).
The relationship between extracellular and intracellular recordings of gastric electrical activity is a complex and controversial issue (29, 30, 38). The approach followed in our studies is to consider one isolated cell as a monopole, i.e. a single (point) source or sink of current within a conducting medium. The cell culture from which an extracellular recording is considered a current dipole (combination of a current source and a current sink). The potential Vm produced by the monopole can be given with (38):
where Io is the current of the source, is the conductivity of the medium through which the current is flowing, i = Io/r, r is the distance between the monopole and the measuring point, and is 3.1415. The potential V produced by a current dipole (combination of a current source and a current sink, for example current flows out of the membrane of a cell at one point, and back in at another one nearby) in a conductive medium is expressed with:
where d is the displacement of the current source caused by the dynamics of the depolarization wave and denotes the differentiation. This equation indicates that V can be considered differentiated with respect to Vm. The problem is that this differentiation is not versus time, but versus d, which is the displacement of the current source. Ultimately, d is a function of time, but this does not mean that the extracellular signal can be considered a true time derivative of the intracellular one. In fact, it can be concluded that the waveform of the extracellular signals differ from the one of the intracellular, which is related in part to the fact that the depolarization wave in the living tissues is not static.
Equation [1.1] indicates that the extracellular potential would repeat the waveshape of the intracellular one only if the cell is considered isolated from the outside world. This assumption is entirely hypothetical and has only a theoretical rather than a practical value. In the real situation, i.e. when a group of cells are considered a current dipole situated in a conductive medium, the extracellular potential would have, in general, different waveshape than the intracellular one (see equation [1.2]). Some authors claim that extracellular waveshape resembles the waveshape of the first derivative of the intracellular signals (29, 30). Smout (12) and other European authors (11, 14) recorded monopolarly the extracellular voltage with implanted serosal electrodes and speculated that it is also inverted with respect to the intracellular potential. It should be mentioned, however, that it is quite difficult to determine the extracellular waveform experimentally mainly because of the so called "injury potential" (29). This potential is a result of damaging the cells by the extracellular electrode. The "injury" can make the transmembrane resting potential in the affected area close to zero (this potential is normally negative), and permeability to ions may become very high.
The intracellular electrode configuration is monopolar, while the extracellular signals are recorded using both monopolar or bipolar recording techniques. Additional change in the waveform can be expected if the recording technique (monopolar or bipolar) is altered. When recording bipolarly with the extracellular serosal electrodes (usually implanted stainless steel or silver needles) about 2.5 mm apart along the longitudinal axis, the recorded signal represents the difference between the two extracellular signals. If the waveform of the extracellular signals was found to be close to the waveform of the first derivative of a typical intracellular signal, the waveform of these bipolar signals can be estimated with the second derivative of the intracellular signal. The pattern of this signal is shown on Fig. 1.5. The abbreviation SDB (short distance bipolar) will be used for this type of recording.
Figure 1.5. Intracellular (A, B) and extracellular (C, D) electrical activity (ECA and ERA). The distance between the cells A and B is approximately 2.5 mm in distal direction. The biphasic signal E is the difference between C and D.
An increase of the distance between the two extracellular electrodes would change the waveform of the recorded signal dramatically (Fig. 1.6). The term LDB (long distance bipolar) will be used for these recordings.
The relationship between the waveform of the signal and the electrode configuration will be discussed in more detail when modeling of the electrical field produced by the stomach is described.
Figure 1.6. Long distance bipolar (LDB) signal is obtained when the distance between the extracellular bipolar electrodes is greater than 2 cm. Note that the signal has two waves in the 20-second interval.
In SDB recordings, the plateau of ERA is hardly recognizable, but spikes are well grouped following the successive biphasic ECAs. On the contrary, in LDB recordings one can easily miss spikes, but the plateau level is recognized better. Of course the ideal technique would be to record monopolarly a signal that is close to the first derivative of the intracellular gastric electrical signal. Unfortunately, many noise components, that would be rejected as common mode signals with the bipolar recording technique, would now pass the input stage of the amplification system and the signal-to-noise ratio would become worse. That is why bipolar recordings are more popular when studying extracellular GEA.
In some early studies of gastric electrical activity, the term "electrogastrography" (EGG) was used for any extracellular recordings performed in vivo (3, 7, 8). This term is, however, now used almost exclusively for cutaneous recordings of gastric electrical activity obtained with abdominal electrodes (9, 10, 11, 17).
Depending on the electrode configuration bipolar EGG could be SDB or LDB. Standard electrocardiographic (EKG) electrodes arranged in different configurations on the abdomen are routinely used for bipolar EGG recordings in many laboratories. The radius of a standard disposable EKG electrode is about 0.5 cm. When the centers of the two cutaneous electrodes from one bipolar pair are closer than 1.5 cm no signal can be recorded even with the most recent amplification techniques, probably due to a combination of limited resolution and many external noise factors. This implies that these recordings are actually LDB (Fig.1.7, 31).
Many different configurations of cutaneous electrodes have been suggested (9, 10, 11, 12, 17). Mirrizzi et al. (31, 32, 33) modeled the electrical field produced by the human stomach and pointed out that the optimal position of the EGG electrodes is along the projection of the stomach axis on the abdomen. This concept was also proven experimentally and has been utilized by most research centers in the world (10, 17). If electrogastrograms are actually cutaneous LDB recordings of GEA, the first and most important restriction on the EGG recordings is imposed: cutaneous signals can provide no more information than internal LDB signals. Ideally, EGG signals should be 100% comparable to LDB recordings. However, there are three major differences between EGG and internal LDB signals:
Figure 1.7. Typical electrogastrographic (EGG) signal recorded bipolarly using a pair of standard electrocardiographic (EKG) electrodes 4 cm apart. Only ECA can definitely be recognized.
(a) the active surface areas of the recording electrodes (stainless steel or silver needles internally and standard EKG electrodes cutaneously) are quite different;
(b) the distance between the source of the electrical field (the stomach) and the electrodes is much greater in EGG recordings;
(c) external noise on the abdominal surface is much greater compared to that in internal recordings. How these differences affect the ability of EGG to represent internal GEA is the subject of a separate discussion.