The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 13 (page 54)


This region roughly encompasses the proximal one sixth of the stomach. The muscular coat is at its thinnest in this region and consists of outer longitudinal, middle circular and inner oblique fibres. The entire cardiac mucosal zone as well as the upper part of the oxyntic zone are located in this region. During radiographic examinations this part of the stomach is seen to be capable of slow expansion and contraction (depending on the degree of filling), but no other intrinsic motility, such as peristaltic activity, is discernible. This correlates well with manometric findings, in which it has been shown that motor activity in the fornix consists almost entirely of slow, low amplitude phasic changes in pressure or tone (Lind et al. l961; Code and Carlson l968; Granger et al. l985). It has also been found that myoelectric activity is absent in the fornix (Kelly and Code l97l; Koch et al. l987).

Corpus and Sinus

This region extends from the base of the fornix as far as an imaginary line 3.0 to 4.0 cm orally to the pyloric aperture. The muscular coat in its proximal part again consists of outer longitudinal, middle circular and inner oblique fibres, while the oblique layer terminates in its distal part. The major part of the oxyntic mucosal zone as well as the proximal part of the pyloric mucosal zone are located in this region. (The extent of the pyloric mucosal zone is discussed in Chap. 5).

During radiological examinations narrow, annular constricting waves, moving in a caudal direction (i.e. peristaltic waves) are seen in this part. They commence as shallow circumferential indentations of the barium column in the corpus, some distance above the incisura angularis, and may or may not become deeper as they proceed. This agrees with manometric and other physiological findings, where it has been shown that peristaltic contractions originate in the orad part of the corpus and migrate towards the pylorus (Smith et al. l957; Rhodes et al. l966; Carlson et al. l966; Granger et al. l985; Koch et al. l987). On the basis of intraluminal pressure changes, these peristaltic waves are divided into Type I waves (producing pressure increases of less than 5.0 cm of water) and Type II waves (producing pressure increases of more than 5.0 cm of water) (Code et al. l952; Smith et al. l957; Carlson et al. l966). The two types are essentially similar, being simple, monophasic waves, differing only in amplitude (Code and Carlson l968). It was surmised that the main function of Type I waves was mixing (with a secondary function of propulsion) and the main function of Type II waves propulsion (and secondarily mixing).

While both Type I and Type II waves are of a peristaltic nature, a third type namely Type III waves, may occur. These are complex waves, characterized by a rise in base-line pressure on which either Type I or Type II waves are superimposed. They are seldom present and of little consequence in the stomach (Code et al. l952). Radiologically Type III waves are usually not recognizable in the stomach.

The frequency of barium-induced peristaltic (i.e. Type I and Type II) waves in this part of the stomach, as seen radiologically, is approximately 3 per minute in man and 5 per minute in dogs (Smith et al l957; Rhodes et al. l966; Carlson et al. l966). Underlying myoelectric activity occurring here consists of slow waves (also known as basic electrical rhythm, pacesetter potential or electrical control activity) and spike activity (spike bursts, action potential or electrical response activity) (Chap. 16). Slow waves originate on the greater curvature in the upper part of the corpus, where spontaneous depolarizations occur at a frequency of 3 cycles per minute in man (Kwong et al. l970; Couturier et al. l972; Funch-Jensen l987), and 5 cycles per minute in canines (Weber and Kohatsu l970; Kelly et al. l97l). Contraction occurs when spike activity is superimposed on slow waves.

Distal 3-4 cm

The muscular coat is at its thickest in this part of the stomach and consists of outer longitudinal and inner circular fibres. The thickening of the circular musculature commences almost imperceptibly 3.0 to 4.0 cm from the pylorus, increases gradually in an aboral direction and ends abruptly in the muscular ring (i.e. the muscular component of the pyloric ring) which surrounds the aperture. There is no structural discontinuity between the musculature of this region and that of the more proximal part of the stomach on its oral side (the sinus). Aborally the circular musculature of the ring is sharply demarcated from that of the duodenum by a fibrous septum (Chap. 3).

The musculature of the distal 3.0 to 4.0 cm of the stomach forms the pyloric sphincteric cylinder, as described by Cunningham (l906), the circular musculature of which consists of a system of rings or loops according to Forssell (l913), Cole (l928) and Torgersen (l942) (Chap. 3). It was shown that the loops deviate from the lesser curvature, where they meet in a muscle torus or knot, to encircle the greater curvature in a fan-like shape. The right muscular loop forms the peripheral part of the pyloric ring (Chap. 11). The greater curvature part of the left loop (which is less well-developed than the right) is situated 3.0 to 4.0 cm orally to the right loop and corresponds to the sulcus intermedius; the circular loops are connected by intervening circular as well as by the overlying longitudinal fibres.

The interior of this part of the stomach is lined by pyloric mucosa. However, the pyloric mucosal zone is not limited to this region, but extends orally into the more proximal part of the stomach (i.e. the sinus) for a variable distance (Chap. 5).

Myoelectric activity in the distal 3.0 cm of the stomach differs from that in the corpus and sinus in that a marked velocity increase in basic electrical rhythm occurs here (Chap. 16). This is associated with a "rapid spread of peristalsis" or a "nearly simultaneous contraction" of this part of the stomach, responsible for its behaviour as a motor unit (Chap. 16).

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