Chapter 13 (page 58)

The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 13 (page 58)

Finally, a simultaneous widening of the two legs or loops of the inverted V occurs, causing the disappearance of the pseudodiverticulum. This results in a triangular region of tight contraction (Fig. 13.10). The pyloric canal is now fully formed and runs through the contracted region as a thin channel containing one or more barium-lined longitudinal mucosal furrows.

Fig. 13.10. Sketch of maximal contraction. D.B., duodenal bulb; P.C., pyloric canal; M.C., muscular contraction

The events on the lesser and greater curvatures, together with the narrowing of the lumen, occur simultaneously in one smooth, integrated, uninterrupted movement.

This then constitutes a maximal contraction of the distal 2.0 to 3.0 cm of the stomach. It occurred in all normal cases. After two to three seconds the contraction relaxed, the lumen reassumed its "resting" diameter, and the process was repeated, showing it to be of cyclical nature. A "pyloric cycle" denotes the time from commencement of one contraction to the commencement of the next.

Frequency

The frequency of pyloric cycles per minute was determined as follows: In 50 of the subjects a stage was awaited in which maximal contractions occurred regularly. At the commencement of one of these contractions an assistant with a stopwatch would be told to "start!". At the end of 30 seconds the assistant would exclaim "stop!". The number of cyclical contractions per 30 second period per subject would be counted, from which the average number of contractions per minute per subject could be established. This proved to be approximately three and a half cycles of contraction per minute. (Because of various factors, e.g. the delay in responding to "start" and "stop", the correct figure is estimated to be somewhat less and probably between 3 and 3Æ cyles per minute).

Amplitude

A maximal contraction wave was associated with a sharp increase in intraluminal pressure, ranging up to 34 mm Hg (vide supra: See also Chapter 15).

Anatomical Correlates

An attempt was made to correlate details of the contractions of the distal 3.0 to 4.0 cm of the stomach, as revealed by radiology, with the muscular anatomy as described by Cunningham (l906), Forsell (l913), Cole (l928) and Torgersen (l942). The findings, which were analyzed in 320 cases (Keet l957), can be described as follows:

From the validation studies it is concluded that dynamic narrowing of the barium- containing lumen, as seen during a maximal contraction, is caused by muscular contraction of the walls. In order to visualize the shape and extent of muscular contraction, the method of focussing on the "black" areas of contraction surrounding the "white" barium-filled lumen is used. Using this perspective, it appears that the arrival of a peristaltic wave at a point 3.0 to 4.0 proximal to the pyloric aperture, initiates contraction of the various divisions of the pyloric sphincteric cylinder; normally this progresses uninterruptedly to culminate in a tight, maximal contraction of the entire cylinder.

One of the first events to occur, namely widening of the pyloric ring on the lesser curvature side (Fig. 13.8), tallies with commencing contraction of the pyloric muscle torus or knot, which is located in this situation. (This is also evident on the radiograph shown in Fig. 13.6).

Commencing formation of a gastric loculus on the greater curvature side (Fig. 13.8) tallies with early contraction and approximation of the right and left pyloric loops (the latter is adjacent to the stationary peristaltic wave on the greater curvature).

On the lesser curvature the indentation caused by continuing contraction of the muscle torus fuses with that of the stationary peristaltic wave, to cause a single region of contraction (Fig. 13.9). At this stage the right and left pyloric loops radiate in a fan-like shape from the muscle torus to surround the greater curvature, where two contraction rings are seen. The rings compress the gastric loculus, resulting in the formation of a physiological pseudodiverticulum (Fig. 13.9) (see also Fig. 13.11).

Fig. 13.11. Radiograph of normal, physiological pseudodiverticulum. Note single area of contraction on lesser curvature and two loops on greater curvature

Continuing contraction of the muscle torus and the two loops further compresses the pseudodiverticulum, causing its disappearance and resulting in a single, cylindrical region of tight contraction (Fig. 13.10). The compressed lumen at this stage is not more than 2-3 mm in diameter; it extends through the centre of the maximally contracted cylinder as a thin tube, often containing one or more longitudinal mucosal furrows (see Fig. 13.15B).

It is probable that narrowing of the lumen is brought about by contraction of the circular, and approximation of the loops by contraction of the longitudinal muscle fibres of the sphincteric cylinder.