Figure 1 - Primary peristalsis as recorded by an intraluminal manometry catheter.

Pharyngeal contraction coincides with relaxation of the upper esophageal sphincter (UES). This is then followed by sequential (peristaltic) phasic contraction along the esophageal body, which propels the swallowed bolus toward the stomach. The lower esophageal sphincter (LES) relaxes soon after the initiation of the swallow and remains relaxed until the peristaltic wave arrives. (Source: Goyal and Paterson,² with permission from The American Physiological Society.)