Overview Motility refers to contraction and relaxation of walls and sphincters of gastrointestinal tract fragments and mixes food bolus in preparation for digestion and absorption propels food bolus along gastrointestinal tract Gastrointestinal contractions contractile tissue of gastrointestinal tract is almost exclusively unitary smooth muscle electrically coupled via gap junctions gap junctions permit rapid cell-to-cell spread of action potentials allowing coordinated smooth muscle contraction exceptions include pharynx, upper one third of esophagus, and external anal sphincter contractile tissue of these structures is striated muscle depolarization and contraction of inner layer of circularly-oriented muscularis externa decreases diameter of GI segment depolarization and contraction of outer layer of longitudinally-oriented muscularis externa decreases length of GI segment phasic contractions periodic contractions and relaxations occur in esophagus, antrum of stomach, and small intestine tonic contractions maintain a constant level of contraction ("tone") without regular periods of relaxation occur in lower esophageal (cardiac) sphincter, orad (upper-fundus and proximal body) region of stomach, iliocecal sphincter, and internal anal sphincter Slow Waves Slow waves unique feature of the electrical activity of gastrointestinal smooth muscle oscillating depolarization and repolarization of membrane potential of smooth muscle cells action potentials occur "on top of" slow wave if membrane potential is depolarized to threshold Origin of slow waves occur at interstitial cells of Cajal "pacemaker" for gastrointestinal smooth muscle slow waves occur spontaneously in interstitial cells of Cajal driving the frequency of slow waves Frequency of slow waves varies along the gastrointestinal tract each part of the gastrointestinal tract has a characteristic frequency stomach 3 slow waves per minute lowest frequency of slow waves in the gastrointestinal tract ileum 8-9 slow waves per minute duodenum 12 slow waves per minute fastest frequency of slow waves in the gastrointestinal tract sets frequency of action potentials which sets the frequency of smooth muscle contractions Mechanism of slow waves depolarization phase cyclic opening of Ca2+ channels inward Ca2+ current depolarizes cell membrane repolarization phase cyclic opening of K+ channels outward K+ current repolarizes cell membrane