Smooth muscle is responsible for the contractility of hollow organs, such as blood vessels, the gastrointestinal tract, the bladder, and the uterus. The human esophagus contains three layers of muscle in its walls; the outer longitudinal and inner circular layers of the main muscular coat and the muscular layer of the mucosa. Visceral smooth muscle cells make up all three layers along the entire organ except for the most rostral few centimeters. Smooth muscle contraction is critical to peristalsis in the human esophagus and the contraction involves activation of phospholipase. Other in vitro studies have shown that cultured human esophageal smooth muscle cells (HEsSMC) express voltage-dependent Na(+) channels.