Chapter 16 (page 77)

The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 16 (page 77)

Bortoff and Weg (l965) studied the relationship between antral and duodenal slow waves at the gastroduodenal junction. Using feline anatomical preparations, they confirmed that spontaneous electrical activity of the pyloric "antrum" consisted of periodic depolarizations; these antral slow waves could be associated with spike potentials which were thought to initiate contractions. There was an extension of antral slow waves across the pylorus into the proximal duodenum; consequently muscular contractions initiated in the antrum could extend into the duodenum, thereby co-ordinating the activities of the antrum, pylorus and duodenal bulb. Although the results were not as definite in the dog as in the cat, they differed from those of Bass et al. (l961), who had concluded that the pylorus acted as an electric insulator, separating the electrical activity of the stomach from that of the duodenum. Bortoff and Weg (l965) found that extension of antral slow waves into the proximal duodenum could be eliminated by a transverse incision through the musculature at the gastroduodenal junction, the mucosa and submucosa being left intact; this indicated that continuity of the gastroduodenal musculature was a necessary condition for transmission of antral slow waves into the proximal duodenum. It was surmized that electrical slow waves were generated by longitudinal muscle cells; they could be recorded in the stomach in the absence of any apparent mechanical activity.

Carlson et al. (l966) simultaneously recorded intraluminal pressures, intramural electrical activity and contractions as seen cineradiographically, in fasted dogs. In the gastroduodenal junctional zone the electrical activity consisted of cyclic changes in potential, recognized as BER of the antrum, and occurring at a rate of 5.1 per minute. Between rhythmic antral BER complexes, elevations with superimposed rapid spike activity, also described as "fast activity", occurred. The presence or absence of motor activity did not affect the frequency of antral BER cycles, but did affect the contours. Motor action was associated with spike configurations; in every instance of cineradiographically identified contraction, the electric record showed associated spike activity. According to Carlson et al. (l966), a recognizable interval usually elapsed between the appearances of BER complexes at separate electrodes in the upper part of the stomach. The mean velocity of the conduction of a BER complex increased as it approached the pyloric "canal". In the body of the stomach there was a slow propagation of 0.5 cm per second, increasing to about 2.0 cm per second in the antrum. In the terminal three centimeters of the antrum simultaneous or nearly simultaneous BER complexes were recorded from different electrodes; this was consistent with the development of a simultaneous or nearly simultaneous contraction of the entire terminal antrum as seen at cineradiography.

Motor activity in the pyloric "canal", as in the antrum, was associated with fast activity in the electrical record, and occurred with the same frequency and rhythm as the fast activity in the adjacent antrum. Contraction of the pyloric canal occurred simultaneously with, or shortly after, the onset of a terminal antral contraction (TAC). (Comment: The pyloric canal was equated with the pyloric aperture). The electrical activity in the proximal duodenum was characterized by cyclic changes in potential, with a mean rate of l7.2 per minute, designated BER of the duodenum. Duodenal contractions occurred synchronously with the BER, but their precise timing with reference to contractions in the adjacent pyloric canal was irregular; BER of the stomach and duodenum did not appear to be in phase.

Although the technique of obtaining electrical records from cutaneous electrodes, called electrogastrography, had been known for a number of years, it was further developed by Nelson and Kohatsu (l968). These authors defined the slow wave in the stomach as a controlled, rhythmic, regularly propagated, moving annulus of electrical depolarization travelling from the cardia to the pylorus, and accelerating during its passage; it could be viewed as a conducted action potential. When mechanical or contractile waves were present, the electrical and mechanical waves were synchronous. The relationship of peristaltic to electric waves could be considered as locked in time but graded in amplitude from no coupling (i.e. a mechanically quiescent stomach) to complete coupling (i.e. a peristaltic wave of maximum amplitude synchronous with each electrical wave). There was a 1:1 time relationship of the peristaltic and electrical waves. In human subjects studied by means of surgical implantation of stainless steel electrodes directly into the muscle, it was found that the rate in the fasting stomach was 3 ± 0.4 cycles per minute.

Daniel and Irwin (l968) pointed out that muscular contractile activity in the stomach was rhythmic and propagated in a well co-ordinated way. Rhythmic contractions in unanaesthetized man recurred at a mean frequency of 3 per minute and in the dog at 5 per minute. Regular, propagated electrical activity was associated with this regular contractile activity. The rhythm of the elctrical activity was the same whether the stomach was contracting or inactive; during contractile activity, a second electrical component appeared. In the inactive stomach the rhythmically occurring electrical complex, previously called the basic electric rhythm (BER) or pacesetter potential, was termed the "initial potential" or "initial polarization" by Daniel and Irwin (l968). It seemed to commence some 15 to 20 cm above the pylorus in human subjects and was normally propagated toward the pylorus; both the size and the rate of propagation of the initial potential increased as it progressed. In the anaesthetized dog it had a propagation velocity of 0.1 to 0.2 cm per second near its origin, increasing to 1.5 to 4.0 cm per second in the antrum. The same general scheme had been noted previously by Carlson et al. (l966). According to Daniel and Irwin (l968) the more rapid spread of electrical activity over the antrum presumably provided the mechanism responsible for its behaviour as a motor unit.

In the active or contracting stomach, a second electrical deflection occurred, corresponding in time to the mechanically recorded contractile activity. This had previously been called "spiking potentials", "fast activity" or "action potential"; Daniel and Irwin (l968) suggested the term "second potential". It was phased by the initial potential and was typically recorded as a prolonged negative deflection, or as a series of negative spikes. Spikes were usually seen only in the terminal 2.0 or 3.0 cm of the dog antrum.

Abolishing contractile activity with moderate doses of catecholamines or atropine was associated with disappearance of the second potential according to Daniel and Irwin (l968). Activation of contraction in a previously inactive stomach resulted in the reappearance of second potentials. Thus it appeared that second potentials were associated with, or initiated, the contractile process. The second potential, unlike the initial potential, was not propagated; it could be produced locally by the local intra- arterial infusion of acetylcholine, without appearing at electrodes a few millimeters distant in either direction. It was surmized that the second potential as well as its associated contractile activity might be produced or affected by local release of chemical mediators or neurohormones, i.e. it appeared to be under local control.