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15 sec

FIGURE 7 Intraluminal pressures recorded from five sensors in the stomach. Note that there is little activity in the orad region and that peristaltic contractions increase in force and velocity as they near the pylorus. (Modified from Johnson LR, ed., Gastrointestinal physiology, 6th ed. St. Louis: CV Mosby, 2001.)

are propelled back into the stomach because the wave of contraction has closed the distal antrum. This ''retro-pulsion'' back into the stomach thoroughly mixes the contents and reduces the size of solid particles.

Contractions of the caudad area of the stomach are initiated by intrinsic electrical activity generated by pacemaker cells known as the interstitial cells of Cajal, which have unstable resting membrane potentials. The pacemaker conductance is regulated by periodic releases of Ca2 + , resulting in a rhythmic depolarization and repolarization. These cyclic fluctuations are called slow waves, and if they reach a threshold level of depolarization, they trigger a contraction. These electrical changes are often referred to as pacesetter potentials or the basic electrical rhythm of the stomach. These latter designations refer to the fact that the slow waves determine the timing and pattern of contractions. Slow waves are always present even if not of sufficient magnitude to produce contractions. In humans, their frequency is constant and ranges between three and five cycles per minute. Thus, as previously noted, the frequency of contractions is also three to five per minute.

Whether an individual slow wave results in a contraction and the amplitudes of contractions are determined by hormonal and neural events dependent on the nature of the gastric contents and the digestive state. Slow waves originate from an area in the middle of the stomach, near the border of the orad and caudad areas. This is often referred to as the gastric pacemaker. The slow wave then spreads toward the gastroduodenal junction, increasing in both velocity and amplitude as it nears the pylorus. The increases in velocity and amplitude account for the similar changes in the gastric peristaltic wave as it moves toward the duodenum.

The interstitial cells of Cajal are intercalated between the intramural neurons and the smooth muscle cells and possess receptors for neurotransmitters and some hormones including CCK. Additional evidence indicates that the interstitial cells of Cajal are coupled with smooth muscle cells via gap junctions. Similar arrangements of these cells also exist in the small and large bowels and are responsible for generating the electrical activity resulting in the movements of these tissues as well.

Three gastric slow waves and their resulting contractile events are depicted in Fig. 8. The slow wave consists of an initial rise (depolarization) called the upstroke potential and a relatively flat plateau potential (Fig. 8A). For contraction to occur, the plateau potential must exceed threshold, as in Fig. 8B. At times, the plateau potential may have a number of spike potentials superimposed on it (Fig. 8C). These lead to increased strength of contraction and may initiate and prolong contractions. Spike potentials occur most frequently in the muscle of the distal portion of the antrum. Once threshold is reached, the greater the amplitude of the slow wave, the greater the force of contraction.

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