Chapter 2 (page 8)

The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 2 (page 8)

3. Peristalsis

Bayliss and Starling (l899, l901) formulated the "Law of the Intestine" to provide an explanation for peristalsis (peri + Greek stalsis = contraction). They found that the response of the small intestine to a local stimulus consisted of contraction of the muscularis externa immediately above, and relaxation immediately below the point of stimulation. It was attributed to a reflex which involved the myenteric plexus and was independent of the external innervation of the intestine. Cannon (l911) called it a "myenteric" reflex; the biphasic wave of relaxation and contraction was found to pass over the muscular layer in an aboral direction for short distances from the point of stimulation. Later workers, notably Brody et al. (l940), Alvarez (l940) and Bozler (l949), failed to detect a forward relaxation phase, possibly through inappropriate application of the stimulus, but the myenteric reflex as originally described has been confirmed repeatedly.

Contraction of a smooth muscle cell is associated with change in potential of the cell membrane; the potential depends on the distribution of electrolytes between the cell and the extracellular space. Bozler (l941) found that two main types of spontaneously arising changes in the membrane potential may be detected, viz. slow potential variations or basal electrical rhythm (BER), and spike potentials. Spike potentials occur during depolarization of the BER and are associated with mechanical activity.

Alvarez (l948) pointed out that the term "peristalsis" was often used carelessly to describe different types of motor activity. Bulbring (l958) divided gastrointestinal motility into peristalsis, resulting in forward movement of contents, and pendular movements, i.e. regularly occurring contractions of short duration, also known as segmenting waves. The latter constitute the most important motor activity in the small bowel. Bulbring et al (l958) observed that some investigators failed to make a strict distinction between peristalsis and other intestinal contractions. It was suggested that the term "peristalsis" should be limited to movements activated by the peristaltic reflex. This was defined as an intrinsic intestinal reflex, mediated through a local neural pathway and initiated by an increase in intraluminal pressure (usually a bolus), in which co-ordinated movements of the longitudinal and circular musculature of the wall occurred, propelling luminal contents in a cephalo-caudal direction.

In experimental studies Hukuhara et al. (l958, l96l) and Nakayama (l962) demonstrated two separate intrinsic intestinal reflexes, viz. an intrinsic muscular and an intrinsic mucosal reflex. In the former, chemical or mechanical stimulation of the serosa or muscular coat caused relaxation of the muscle both above and below the point of stimulation. In the latter various mucosal stimuli were associated with contractions of the muscularis externa above and relaxation below the point of stimulation. The effects were abolished by the application of cocaine to the mucosa but not by external denervation, showing it to be a true reflex; the associated electrical depolarization curve gave an indication of a preceding wave of inhibition under some circumstances. The intrinsic intestinal mucosal reflex conformed to the "Law of the Intestine".

Contractions of the longitudinal and circular musculature during peristalsis are out of phase by 90 degrees, according to Davenport (l961). On distension of the lumen the longitudinal muscle contracts, followed by progressive contraction of the circular layer. The circular layer begins contraction when contraction of the longitudinal layer is half complete; contraction of the circular layer is complete when relaxation of the longitudinal is half complete.

Texter (l963, l964) reiterated that "peristalsis" was often used inexactly as a synonym for propulsion. Although propulsive waves occur in the oesophagus, stomach and colon, propulsion often results from pressure gradients caused by segmental contractions occurring some distance orally from the point of flow. According to Texter (l963) peristaltic activity is uncommon in man.

Peristalsis is defined rather superficially in scientific (Van Nostrand l968) and medical dictionaries (Black l971; Dorland l988; Stedman l972; Blakiston Gould l972). The consensus appears to be that it is a vermiform or a progressive, wave-like movement in tubular organs, consisting of alternating waves of relaxation and contraction in the muscular coat, by means of which the contents are propelled. Horrobin (l968) describes it as a contraction preceded by relaxation, spreading down a long length of intestine to propagate the contents, and depending on an intrinsic intestinal reflex. Both Horrobin (l968) and Christensen (l97l) found that peristalsis seldom occurred in the normal small intestine of man.

Weisbrodt (l974) pointed out that at one extreme the term was used in a general sense to describe any type of recurrent contractile activity; at the other extreme it was used to indicate a specific reflex movement in isolated segments of small bowel. It was usually used as a synonym for propulsion.

It appears to be generally accepted that peristalsis occurs in, and is confined to the muscularis externa; the mucosal layer is usually considered to be of little consequence in gastrointestinal motility. However, as it is the innermost layer of the wall, the mucosa is in intimate contact with luminal contents, and the question arises whether it is not in some way involved in motility. Normally the mucosa of the stomach and other regions of the gastrointestinal tract is elevated into folds which are clearly demonstrable radiologically. Movements of the folds are well-known to radiologists and have been recorded by numerous observers in normal subjects, while atypical or absent movements are well documented expressions of pathological conditions. However, few attempts have been made to determine if a regular or consistent motility pattern of the folds exists.

In a systematic radiological study normal movements of mucosal folds in the gastrointestinal tract were recorded (Keet l974); radiology was combined with intraluminal pressure measurements in the investigation of fold movements in the distal stomach and duodenum (Keet et al. l978). The effects of pharmacological agents on fold movements were studied, and in vivo animal observations in the small intestine of the Cape baboon (Papio ursinus ursinus) were done (Keet l974).

It was found that the folds usually have an irregular or reticular pattern "at rest", i.e. presumably in the motor quiescent phase of the interdigestive myoelectric complex. Whenever the lumen is distended by air, gas or ingesta, all