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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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