Chapter 16 (page 76)

The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 16 (page 76)

Chapter 16

Myoelectric Activity at the Gastroduodenal Junction

The presence of rhythmic electrical activity in the musculature of the gastric "antrum" of anaesthetized dogs was first reported by Alvarez and Mahoney (1922) and has since been confirmed repeatedly. Bozler (l945) recorded the electrical potentials of the dog, cat and guinea pig stomachs by means of non-polarizable differential electrodes. In the dog, the differential potential associated with each peristaltic contraction showed three main deflections, designated the R, S and T waves. The shape of the recorded potential was identical with that of cardiac muscle but differed from it in that it lasted from 5 to 8 seconds, i.e. it had a slow phase. Species differences were noted; in the guinea pig stomach a slow potential could be observed only in the region of the pylorus. In the cat the same action potentials as in the dog were obtained as long as the contractions in the stomach were weak. If movements were strong enough to be easily visible, brief spike potentials were superimposed on the slow phases; the number and frequency of the spikes increased with the strength of the contraction. At that time the significance of the spikes was not known.

Bass et al. (l96l) found that a rhythmic electric complex could be recorded regularly from the "antrum" in dogs. Called the basic electric rhythm (BER), its frequency was about 4.4 per minute. Bursts of relatively rapid changes in potential, fast or spike activity, occurred in association with approximately 40 percent of BER complexes. (In the duodenal bulb a similar basic electric rhythm and fast or spike activity was recorded.) Both types of activity became attenuated at the pylorus and usually disappeared in it. Although some of the fast activity of the antrum occasionally extended into the pylorus, it was felt that in the dog the pylorus acted as an "electric insulator" between the stomach and duodenum.

Daniel and Chapman (l963) pointed out that all investigators had recorded a more or less constant electrical wave in vitro from the body and "antrum" of both the resting and the contracting stomach. Its frequency varied from 3 to 6 per minute, depending on the species and the type of preparation. In the contracting stomach an electrical wave, which they called the primary wave, preceded each peristaltic contraction. In the dog's stomach these propagated electrical waves did not vary in shape with the presence or absence of motor activity. It was concluded that the electrical activity of the dog stomach preceded and appeared to control the spread of peristaltic contractions, but went on relatively unchanged in the absence of peristalsis.

Daniel and Chapman (l963) also investigated the electrical activity of the dog stomach by means of a monopolar technique. When the electrodes were oriented transversely to the longitudinal muscle fibres, the primary waves with a frequency of 3 to 6 per minute were the most prominent and consistent feature. In the upper third of the stomach the typical electrical wave was small and disappeared near the fundus. Activity progressed distally along an array of electrodes parallel to the greater curvature, at a velocity increasing from 0.3 cm per second in the body of the stomach to 3.0 to 4.0 cm per second as the wave neared the antrum, thus correlating electrical events with descriptions of a rapid spread of peristalsis in the antral region. Activity at the antral electrodes was often nearly simultaneous. In the antrum, but not in the body of the stomach, the primary wave was often followed by secondary smaller deflections which were sometimes fused; these were present only when visible, active peristalsis was evident. Visible antral peristaltic contractions followed the primary waves by 6 to 9 seconds and the secondary waves by 1 to 2 seconds. Daniel and Chapman (l963) described the events as follows: an electrical wave, probably originating in the cardia, travelled slowly down the stomach at a velocity of slightly more than 0.5 cm/sec until it reached the pyloric "antrum", where its velocity increased to 4 cm/sec. The wave recurred periodically at the same frequency of 3 to 6 per minute as the contractions of the stomach, and appeared to precede and initiate gastric motility. However, there was a variable relationship between electrical and mechanical events, as the primary electrical wave might continue unchanged after inhibition of motility. In the "antrum" of the dog secondary spikes accompanied motility and disappeared with it. Secondary spikes could only be recorded in the antral part of the dog's stomach, while in the cat and guinea pig stomach they occurred in the cardia.

Daniel (l965) studied the drug responses of the "antrum" and duodenal bulb in the dog, using intra-arterial infusions of drugs and combined recordings of electrical and mechanical activity. In the inactive antrum an electrical rhythm of 4 to 5 waves per minute occurred, resembling the QRS complex in the heart, and called the initial potential. It was propagated distally over most of the antrum at a velocity of 0.3 to 1.0 cm per second, but at a distance of 2.0 to 4.0cm from the pylorus the velocity increased, reaching 2.0 to 4.0 cm per second in the terminal two centimeters of the antrum; the amplitude also increased near the pylorus. In the active or contracting antrum, the above repetitive initial potential was followed by a second potential, consisting of repetitive negative going spikes in the terminal two centimeters; they did not appear to be propagated.

Daniel (l965) showed that infusion of adrenaline or noradrenaline caused inhibition of antral second potentials and contractions. Intra-arterial infusion of acetylcholine and nicotine led to the production or enhancement of second potentials and contractions; in addition, premature initial potentials might be produced and these were often propagated in an antiperistaltic direction. In the duodenum, slow waves occurred at a rhythm of l7 to l9 per minute, irrespective of mechanical activity; when contractions occurred they were preceded by a series of fast spike potentials. Acetylcholine and other cholinergic stimulants produced or increased fast spikes and contractions in the duodenum. Atropine prevented all the excitatory effects of acetylcholine and other cholinergic stimulants in both the antrum and duodenum.