2015B12 Describe the physiological factors that contribute to the competence and
tone of the lower oesophageal sphincter.

 

List:

·        Intro

·        Anatomy

·        Neural control

·        Physical factors

·        Hormonal factors

 

Intro:

Importance

·   Prevents regurgitation and aspiration

·   Especially important supine, under anaesthesia

Barrier pressure

·   Barrier pressure = sphincteric – intragastric

·   Normal pressures (cmH2O): barrier 26, sphincter 36, intragastric 10

·   Reflux occurs when barrier pressure <13cmH2O

 

Anatomy:

Inner sphincter

·   Lower 2-4cm oesophagus. Not anatomically distinct, not a true sphincter.

·   Tonic contraction of circular muscle

·   Provides 90% of basal pressure

·   Innervation: CNX, abundant

Outer sphincter

·   Right crus of diaphragm loops contralaterally around lower oesophagus. “Pinch cock” mechanism

·   Intermittent contraction

·   Applies co-ordinated pressure during inspiration, coughing

·   Inervation: phrenic

Oblique entry

·   Oblique passage of oesophagus into stomach

·   Gastric distension -> closure of orifice

·   “Flap-valve” mechanism

Intra-abdominal position

·   Sphincter is intra-abdominal

·   External pressure assists closure

 

Neural control:

Innervation

·   Meissner’s submucosal plexus

·   Auerbach’s myenteric plexus (between circular and longitudinal layers)

Intrinsic

·   Myenteric reflex: distension > peristalsis (e.g. swallowing)

·   Myogenic reflex: distension -> contraction

Extrinsic

·   PSNS: CNX via oesophageal plexus

·   SNS: cervical and thoracic SNS trunk via oesophageal plexus

·   Complex modulation of intrinsic circuits

 

Physical factors:

Gravity

·   Trendelenburg positioning -> ↑regurgitation

Pressure

·   ↑Abdo pressure -> ↓barrier pressure

o Obesity

o Pregnancy

o Laparoscopy

Sphincter

·   e.g. hiatus hernia -> ↓LOS competence

 

Hormonal factors:

↑ LOS tone

·   Gastrin

·   CCK

·   Motilin

·   Oestrogen

↓LOS tone

·   Progesterone (i.e. pregnancy)

·   VIP (causes reflex relaxation after swallowing)

·   GIP

·   Secretin

·   PGE2

·   Glucagon

·   Severe illness

 

 

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