The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 2 (page 6)

2 What is a Sphincter?

The definition of sphincter (Greek sphinkter = that which binds tight, binder) remains elusive. Williams (l962) pointed out that early anatomists and surgeons saw and felt a narrow powerful band of muscle enclosing the distal end of the stomach and concluded that it was a sphincter controlling the width of the lumen; later observers thought the sphincter could be part of the circular musculature of the pyloric canal, constituting an inherent part of the gastroduodenal pump. He called the pyloric sphincter a muscularis propria ring, consisting mainly of circular but also containing some longitudinal fibres, which surrounds the pyloric aperture; it narrows the distal end of the stomach to the point at which the lumen can be occluded by a mucosal plug. The sphincter also takes part in pump-like contractions of the fan-shaped muscle to close the pyloric canal.

Edwards and Rowlands (l968) define sphincter as a muscle surrounding and serving to close an orifice; the cricopharyngeus is mentioned as an example of a tonically closed sphincter, detectable by manometry as a zone of sustained elevation of pressure, undergoing precisely timed and brief openings. However, the pyloric ring in man does not appear to be a tonically contracted sphincter which opens occasionally; it behaves as the distal end of the thickened muscle mass surrounding the "antrum," and forms part of this muscular unit.

According to Didio and Anderson (l968) a sphincter is a specialized accumulation of smooth muscle in the bowel wall which serves to slow down transit of contents and to prevent retrograde flow. It is characterized by a thickening of the circular muscle layer creating an observable constriction both on gross inspection and on radiographs. In some cases it causes a recordable zone of increased intraluminal pressure; at appropriate times it closes to prevent flow, while at other times it opens to facilitate emptying. According to these authors, the emphasis laid on the closing properties of sphincters is anatomically and physiologically inaccurate and clouds the understanding of the functional significance of sphincters; the mechanism for opening is equally important as the one for closing, and it would be preferable to regard all sphincters as gatekeepers.

Black's Medical Dictionary (l97l) defines sphincter as a circular muscle which surrounds the opening from an organ; by maintaining constantly a state of moderate contraction it prevents escape of contents.

Stedmanís Medical Dictionary (1972) describes a sphincter as an accumulation of circular muscle fibres or specially arranged oblique fibres, acting to reduce or occlude the lumen of a tube, the orifice of an organ or the cavity of a viscus; it is the closing component of a gatekeeper. Still another medical dictionary, Gould (1972), defines it as a muscle surrounding and closing an orifice. Ruch and Patton (l973) state that the pyloric sphincter is a true anatomic sphincter, formed by a distinct thickening of the circular fibres of the muscularis externa. It is open or its mucosa is in weak apposition during most of the gastric emptying cycle, both when the "antrum" is inactive and between "antral" peristaltic waves. However, it seems not to play an essential role in controlling gastric emptying. As far as the pyloric sphincter is concerned, there is a definite lack of agreement between anatomic predictions and physiologic thinking. The inferior oesophageal sphincter in man, on the other hand, cannot be recognized anatomically as a sphincter although it functions as such.

According to Gray's Anatomy (l973) the pyloric sphincter is a muscular ring composed of a thickened portion of the circular muscle layer, but also containing some longitudinal fibres which dip inwards to interlace with the circular fibres of the ring. Shepro et al. (l974) call the pyloric sphincter the circular muscle band that controls the opening of the stomach into the small intestine. Mehta et al. (l974) looked upon a sphincter as a zone of tonically elevated pressure; they failed to demonstrate such a zone at the pylorus.

In the gastro-intestinal tract, according to Alumets et al (l978), a particularly rich innervation of vasoactive intestinal peptide (VIP) nerves is seen in the region of sphincters; an evaluation of the density of these nerves may assist in anatomically defining a sphincter.

Wheater et al. (l979) state that the pyloric sphincter consists of an extreme thickening of the circular layer of the muscularis externa at the gastro-duodenal junction; in an accompanying illustration the sphincter is equated with the pyloric ring. Reeve (l98l) points out that the traditional definition of a sphincter is a ring-like muscle which controls the opening of a body orifice or constricts the lumen of a natural body passage, one of the essential functions being to delay the passage of intestinal contents. However, many other functions may be attributable to sphincters and anatomical studies of certain sphincters run into difficulty if the traditional definition is adhered to. The pyloric sphincter, for instance, is a ring of muscle consisting of an aggregation of the circular fibres of the muscularis externa at the terminal aspect of the stomach; it is not independent of the preceding part of the gastric musculature.

According to Thomas and Mann (l98l) it used to be thought that the function of a sphincter was to delay the onward passage of luminal contents, but it is now realised that this concept was too facile. Sphincteric zones act in a very complex regulatory fashion, and the organisation of a particular sphincter is not necessarily an exact replica of the others. A sphincter can be defined in different ways. Physiologically, it is an area which is tonically closed and which has the ability to relax and contract; some sphincters however, are not permanently closed and have an "open" mechanism. Anatomically it is a thickening of circular muscle fibres in a hollow viscus; the exception here is the intrinsic lower oesophageal sphincter in man. Pharmacologically a sphincteric region can be identified by the theory of "reciprocal innervation", whereby sympathetic stimulation produces sphincteric contraction and parasympathetic stimulation induces relaxation; these effects are opposite to those produced in neighbouring smooth muscle. This definition is considered to be sound for all smooth muscle sphincters (but is not applicable to the crico-pharyngeal and external anal sphincters, which contain striated muscle). Thomas and Mann (l98l) admit that none of these definitions is ideal.

Schulze-Delrieux and Shirazi (l983) state that the role of the pylorus as a sphincter remains controversial because the pylorus does not consistently close

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