The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 2 (page 7)

off the gastroduodenal junction or affect the rate of gastric emptying. According to Wingate (l987) it is uncertain whether the pylorus is indeed a sphincter; it depends on what is meant by the term "sphincter". The limitation of language has not yet allowed a comprehensive definition of the concept.

An illustrated medical dictionary (Dorland l988) again defines sphincter as a ringlike band of muscle fibres that constricts a passage or closes a natural orifice; the pyloric sphincter is said to be a thickening of the circular muscle of the stomach around its opening into the duodenum.


It is clear that consensus has not been reached on the definition of "sphincter" in general terms. It would appear, however, that the definition as stated in most medical dictionaries is totally inadequate.

A time-honoured view is that a sphincter is a band of circular musculature which is tonically contracted most of the time, creating a zone of sustained elevation of pressure preventing flow of intestinal contents in either direction; it relaxes intermittently, usually in reciprocity with an oncoming peristaltic wave, to allow flow. In this view a sphincter functions more or less independently of the surrounding musculature and probably has a separate innervation.

As far as the pylorus is concerned, the question arises whether the pyloric ring should be looked upon as a sphincter in the sense implied above. Is it a ring consisting totally or almost totally of muscular fibres, or is it merely a constriction which impedes flow by its narrowness and poor distensibility, as suggested by Stadaas and Aune (l970) and by Schulze-Delrieu (l983)?

Another view of sphincters is that proposed by Thomas and Mann (l98l), who mention the possibility of sphincteric zones acting in complex regulatory fashion. However, on anatomical grounds it is difficult to accept the theory of reciprocal innervation, in which a particular nerve stimulus is purported to produce opposite effects in the "sphincteric" and the adjacent smooth musculature.

A third possibility is that a rather intricate sphincteric mechanism may exist at junctional zones, e.g. the pyloroduodenal junction. In this instance localized rings of circular musculature are inherent components of a muscular region or cylinder consisting of both circular and longitudinal fibres, the entire structure functioning as a unit.

It is proposed to discuss these and related questions in more detail in subsequent chapters.


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