The lower esophageal sphincter is tonically constricted, but relaxes just prior to the primary or secondary waves of peristalsis passing through. Rarely, the lower esophageal sphincter does not relax satisfactorily, resulting in a condition called achalasia.

Achalasia acardia is when the esophagus muscles fail to relax, also a myenteric plexus problem
- If the sphincter of the top end of the stomach isn't functioning, and there is no peristalsis, the patient is suffering from achalasia acardia.  This is an issue related to the parasympathetic nervous system.