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

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

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

Intrinsic

·   Meissner’s submucosal plexus

·   Auerbach’s myenteric plexus: between circular and longitudinal layers

·   Myenteric reflex: distension > peristaltic wave (sensed and controlled by the above).
E.g. reflex dilatation then contraction after swallowing.

·   Myogenic reflex: distension -> contraction

·   Swallowing: ↑VIP -> reflex relaxation

Extrinsic

·   PSNS: CNX via oesophageal plexus.

·   SNS: cervical and thoracic SNS trunk via oesophageal plexus.

·   Complex modulation of intrinsic circuits

↑ LOS tone

·   Gastrin

·   CCK

·   Motilin

·   Oestrogen

↓LOS tone

·   Secretin

·   GIP

·   VIP

·   Progesterone

·   PGE2

·   Glucagon

·   Severe illness ? exact cause

Intra-abdominal hypertension

·   ↑Abdo pressure -> ↓barrier pressure

o Obesity, pregnancy

o Reverse Trendelenburg position

o Laparoscopy

Sphincter dysfunction

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