No anatomic sphincter exists at the junction of stomach and esophagus. However, the circular layer of smooth muscle, which is a bit more developed than the neighboring regions, functions as a physiologic sphincter.
As the food descends from the through the esophagus into the stomach, relaxation of the circular layer of muscle at the lower end of esophagus occurs ahead of the peristaltic wave. This makes sure that the food enters the stomach and is not blocked in the lower part of esophagus. Once the food has passed to the stomach, the tonic contraction of this sphincter prevents the contents within the stomach from regurgitating into the esophagus.
The closure of the gastroesophageal sphincter is under vagal control. And because the vagus nerve contains only the parasympathetic fibers, it is correct to state that this sphincter is controlled by the parasympathetic nervous system. The closure can be augmented by the hormone gastrin and inhibited by hormone secretin. Cholecystokinin and glucagon have also an inhibitory effect on the closure of the gastroesophageal sphincter.