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.