There are two sphincter muscles located in the esophagus: The lower esophageal sphincter (LES) and the upper esophageal sphincter (UES). When the lower esophageal sphincter is not functioning properly, there is a back flow of stomach acid into the esophagus. If this happens two or more times a week, it can be a sign of gastroesophageal reflux disease, or GERD.
As with the lower esophageal sphincter, if the upper esophageal sphincter doesn't function properly, acid that has back flowed into the esophagus is allowed into the throat and voice box. When this happens, it's called Laryngopharyngeal Reflux, or LPR.