A simple, inexpensive model to demonstrate how contraction of GI longitudinal smooth muscle promotes propulsion.

PERISTALSIS is a propulsive activity that involves both circular and longitudinal muscle layers of the esophagus, distal stomach, and small and large intestines. During peristalsis, the circular smooth muscle contracts behind (on the orad side) the bolus and relaxes in front (on the aborad side) of the bolus. At the same time, the longitudinal muscle relaxes orad to the bolus and contracts on the aborad side of the bolus. This activity is propagated over varying distances in the different segments of the gastrointestinal (GI) tract. Peristaltic movements are usually directed toward the anus and are due to the organization of the myenteric plexus, which resides between the circular and longitudinal muscle layers. It is relatively easy to understand how contraction of the circular muscle behind the bolus and relaxation of the circular muscle in front of the bolus propels the object along the GI system toward the anus. However, how contraction of longitudinal muscle on the aborad side of the bolus promotes propulsion is difficult for students to grasp. To help students understand how contraction of the longitudinal muscle on the aborad side of the bolus promotes propulsion, we used a simple, inexpensive model, a “finger trap” (also known as a Chinese finger trap, a Chinese finger puzzle, Chinese thumb cuff, Chinese handcuffs, and similar alternatives). The finger trap is a simple toy that traps the fingers in opposite ends of a small cylinder woven from bamboo and can be purchased online from a number of novelty toy suppliers or through amazon.com. Each student received a finger trap and marble (Fig. 1), and the instructor used a much larger ( 5 ft long) version of a finger trap (Fig. 2). Specifically, the instructor used a Jacket Rod Cover (Stick Jacket Rod Cover) that is constructed (woven) like a finger trap and maintains similar functional properties (i.e., the braided polyethylene filament behaves much like the finger trap). Its intended use is to fit over fishing poles to prevent rod tangles. The Jacket Rod Cover can be purchased online from various fishing supply companies or through amazon.com. To use this model, students were first told that when the longitudinal muscle contracts, the segment shortens (Fig. 3, A and B). Shortening of the segment enlarges (widens) the lumen,