Chapter 16 (page 79)

The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 16 (page 79)

According to Szurszewski (l98l) a phasic gastric contraction is usually a mechanical manifestation of an electrical event occurring in smooth muscle cells. A spontaneous electrical signal originates in the musculature of the midcorpus, which can be identified as a pacemaker region. The signal complex is propagated circumferentially around the stomach and longitudinally to the gastroduodenal junction. In canines intracellularly recorded action potentials from antral circular musculature, in the region extending from the intermediate sphincter to the gastroduodenal junction, distinguishes it from more orad regions of the stomach; similar features were likely to exist in human musculature. (Comment: The intermediate sphincter is synonymous with the left pyloric loop as described in Chap. 3). The "antrum" can be divided into orad and terminal parts, according to Szurszewski (l98l). In the corpus and proximal antrum the electrical wave forms are the same, and coincide closely with contraction of the circular muscle fibres; in the terminal 2.0 to 3.0 cm of the antrum, characteristic electrical activity is seen during contraction. The action potential in this region has an initial, rapid depolarization and a plateau potential with oscillations in potential superimposed on the plateau potential.

Stoddard et al. (l98l) reiterated that gastric myoelectric activity is characterized by the presence of regular slow waves in the distal two-thirds of the stomach, the orad third being an electrically silent area. The mean slow wave frequency is species dependent, being approximately 3 cyles per minute in man and 5 cycles per minute in dogs, with little day to day variation. The rhythm is normally remarkably stable, with only occasional irregularities of a few cycles' duration.

You and Chey (l984) pointed out that it had not been clarified whether or not a slow wave (pacesetter potential or PSP) per se was associated with a mechanical contraction. A prevailing view was that gastric contractions resulted from the occurrence of spike activity (action potential), and not from the slow wave alone; the function of the slow wave was considered to consist of setting the pace and direction of gastric contractions. However, some authors had reported an intimate relationship between contractile activity and PSP in the canine stomach, whether spike activity was present or not; a similar relationship had been found in human antral muscle segments (You et al. l980). These observations suggested that PSP in the stomach might also promote phasic contractions. In a study of the relationship between electrical and mechanical activities of the "distal antrum" in humans and canines, it was found that phasic contractions were recorded by sensitive ministrain gauges implanted on the serosal surface, although electrical activity showed only PSP's without action potentials or second potentials. (The distal antrum was defined as the region 2.0 cm proximal to the pylorus). Intraluminal manometry could not always recognize these phasic contractions; the number of contractions recorded by manometry was less than 50 percent of the PSP's accompanied by action potentials or second potentials. No gastric contractions were recorded when pacesetter potentials occurred without action potentials.

You and Chey (l984) found that gastric dysrhythmia, including tachygastria, could be induced by epinephrine. During dysrhythmia phasic contractions disappeared and no contractions occurred in association with action potentials. Tachygastria was considered to be present when PSP occurred regularly in a frequency of more than 7 cycles per minute; bradygastria indicated a frequency of less than 3 cycles per minute. Because of the insensitivity mentioned above, intraluminal manometry might not be able to detect gastric dysrhythmia.

Geldof et al. (l986) reiterated that gastric myoelectric activity could be recorded by peroral (suction) electrodes, by serosal electrodes placed at laparotomy, and by electrodes attached to the abdominal skin (electrogastrography). In a series of patients with unexplained nausea and vomiting, these authors recorded abnormal myoelectric activity by means of electrogastrography in almost 50 percent; the abnormal recordings were characterized by instability of the gastric pacemaker frequency, tachygastrias and absence of the normal increase in amplitude in the postprandial recording.

Discussion

Periodic, rhythmic electrical activity, propagated in an aboral direction, occurs in the musculature of the stomach in man and other higher vertebrates. These waves, consisting of cyclical changes in potential, are variously known as primary waves, basic electrical rhythm (BER), slow waves, initial potential, pacesetter potential (PP) and electrical control activity (ECA). The waves occur at a rate of approximately 3 per minute in man and 6 per minute in dogs. They are omnipresent, precede and control the spread of peristalsis (Daniel and Chapman l963), but continue unchanged in the absence of peristalsis and do not indicate contractile activity.

Bursts of relatively rapid changes in potential, variously known as secondary waves, fast or spike potentials, second potential, fast activity, action potentials and electrical response activity (ERA), occur in association with some BER complexes. They are not propagated, appear to initiate contractions, and are associated with motor action.

A number of authors demonstrated a velocity increase in BER from approximately 0.3 cm per second in the corpus of the stomach to 3.0 or 4.0 cm per second in the terminal two to three centimeters of the "antrum", both in dogs (Daniel and Chapman l961, l963; Carlson et al. l966) and in man (Daniel and Irwin l968; Duthie et al. l97l). This corresponded to a "rapid spread of peristalsis" in, or "a nearly simultaneous contraction" of the terminal antrum. According to Daniel and Irwin (l968) the more rapid spread of electrical activity over the "terminal antrum" presumably provided the mechanism responsible for its behaviour as a motor unit.

The rapid progression of BER in the "terminal antrum" probably also points to some distinguishing feature or specialization of the musculature in this region. It is surmized that it is linked to the specialized musculature of the pyloric sphincteric cylinder described by Cunningham (1906), Forssell (1913) and Torgersen (l942), with its typical motor activity as discussed in Chapters 3, 13 and 15.