College Answer
For a good answer candidates were expected to give a description of the lower oesophageal sphincter (the intrinsic and extrinsic sphincters and flap-valve), that it maintains a resting pressure of 15-25 mmHg above gastric pressure which prevents gastro-oesophageal reflux, which relaxes after swallowing, that the resting tone is maintained by myogenic and neurogenic mechanisms and the effects of hormones upon the sphincter (ie gastrin, motilin and α adrenergic stimulation increases and secretin, glucagon, VIP and GIP decrease tone).
Recommended sources: Ganong Review of Medical Physiology Chp 26.
Discussion
- Physical factors:
- Upright posture: gravity prevents the upward movement of stomach contents
- Right lateral position decreases reflux, as it puts the pylorus in a dependent position, promoting gastric emptying (Loots et al, 2012)
- Anatomical factors:
- The angle of His is an acute angle between the oesophagus and the gastric fundus, which contributes to the prevention of reflux mainly by creating a mucosal flap valve.
- Mucosal flap valve: this is a "180-degree musculomucosal fold apposite to the lesser curvature of stomach" created by the intraluminal extension of the angle of His. It blocks the oesophageal opening.
- Posterolateral location of the gastric fundus keeps gastric contents away from the oesophageal opening
- Sphincters:
- Lower gastroesophageal sphincter is the smooth muscle that encircles the lower oesophagus.
- Structurally identical to the circular smooth muscle in the rest of the oesophagus
- Seems thicker because it is constantly tonically contracted.
- Diaphragmatic crura (especially the right crus) is a band of skeletal muscle fibres which form the sides of the oesophageal hiatus, the opening through which the oesophagus descends.
- Resting sphincter tone
- Exerts a pressure that is greater than gastric pressure
- Usually 15-25 mmHg in total (but it only needs to be about 2-3 mmHg higher).
- This tone is influenced by neurogenic, myogenic and hormonal factors:
- Neurogenic control is by the vagus nerve:
- Vagally mediated inhibition relaxes the lower oesophageal sphincter by a nitric oxide mediated mechanism
- Myogenic influence on tone is mainly due to functional differences in the smooth muscle cells in the lower oesophagus, as compared to the rest of the oesophagus:
- Lower oesophageal smooth muscle has more α-actin and basic essential light chains
- That smooth muscle is constantly in a state of depolarization because of a higher resting membrane potential, though to be due to its greater chloride conductance
- Hormones that increase lower oesophageal sphincter tone:
- Gastrin
- Motilin
- Catecholamines (α-adrenergic effect)
- Substance P
- Bombesin
- Galanin
- Pancreatic polypeptide
- Hormones that decrease lower oesophageal sphincter tone:
- Secretin
- Glucagon
- VIP and GIP
- Cholecystokinin
- Somatostatin