Discuss the control and importance of the sphincters of the GI tract.
Sphincters are used throughout the GI tract to control the movement of ingesta, including
both forward and reverse movements. Sphincters act to separate two areas of the GI tract,
by maintaining a positive resting pressure between the two areas of lower pressure that
predominate within the GI organs. They effectively serve as one way valves: as a general
rule, stimuli proximal to a sphincter cause sphincter relaxation, whereas stimuli distal to a
sphincter cause sphincter contraction. They are found at the top and bottom of the
oesophagus, at the exit of the stomach, between the bile duct and the duodenum, between
the ileum and the caecum and also at the exit of the GI tract – the internal and external anal
sphincters. They nearly all consist of smooth muscle, the exceptions being the upper
oesophageal sphincter and the external anal sphincter.
Upper Oesophageal Sphincter
The UOS separates the hypopharynx and the upper part of the oesophagus. It is composed
of striated muscle – the cricopharyngeus muscle. The muscle is innervated by the Vagus
nerve. It has the highest resting pressure of all the GI sphincters.
Very important during the processes of deglutition and respiration. Swallowing mechanism
controlled by the swallowing centre in the medulla, via cranial nerves V, IX, X and XII
(trigeminal, glossopharyngeal, vagus and hypoglossal). UOS closes during inspiration to
divert air towards the glottis and away from the oesophagus, whereas during deglutition the
UOS relaxes and the glottis closes off, vocal cords are pulled closer together to do this.
Respiratory centre of the medulla is directly inhibited by the swallowing centre for the short
time needed to swallow – deglutition apnoea.
Interaction is important because the airways and swallowing tract have to cross each other
at this point.
Lower Oesophageal Sphincter
Separates the oesophagus and the stomach, located just caudally to the diaphragm.
Consists of specialised smooth muscle, distinct from the smooth muscle on either side of it
both anatomically and pharmacologically.
Coordinates the movement of ingested food into the stomach. Deglutition or oesophageal
distension cause reduction in LOS pressure. Relaxation of LOS only occurs after UOS has
returned to its resting pressure.
LOS is normally tonically contracted, stimulated by SNS fibres and by vagal cholinergic
fibres, resting tone. Relaxation is induced by inhibitory vagal fibres, with VIP and NO release
having important inhibitory effects. LOS relaxes before food arrives, feed-forward vagal
reflex.
LOS contraction important in preventing gastroesophageal reflux -> oesophagtis, heartburn,
increased chance of oesophageal cancer. Gastrin, which increases gastric acid secretion,
also increases oesophageal tone.