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ILLUMINATIONS

Interactive Classroom Demonstration of Skeletal Muscle Contraction

Department of Chemical, Biological and Environmental Sciences, Mount Royal College, Calgary, AB Canada T3E 6K6 jmeeking{at}mtroyal.ab.ca

Students in a traditional lecture-style course often struggle to visualize the sequence of events in the molecular mechanism of skeletal muscle contraction as represented in anatomy and physiology textbooks (e.g., Ref. 1). The author of this article has successfully used the following interactive method with first-year nursing students. If the students are able to state the functions of calcium, troponin, tropomyosin, and cross-bridges, the demonstration requires only 15–20 minutes of class time. The size of the actin and myosin filaments can be varied according to the class size.

For a sample class of 15 students, the required materials are: six apples (actin molecules), six bananas (myosin molecules), six signs with ATP in bold print on one side and ADP + P_i in bold print on the other side, and six signs with Ca²⁺ in bold print on both sides.

To set the stage for the demonstration, first set up a thin filament line of six students. Each student in that line will flex the right elbow and place an apple with the stem end facing anteriorly in the palm of the right hand (Fig. 1 A). The apple represents an actin molecule, and the stem indentation represents the active site for binding with the myosin cross-bridge. Students will cover the actin-active sites with their left hands and line up all the left forearms, forming one continuous tropomyosin molecule. The troponin molecules are represented by the heads of the students. Next, set up a thick filament line of six students facing the thin filament. Each student in that line will hold a banana in the right hand. The bananas (myosin cross-bridges) of all students in the line should be held at a similar angle, pointing toward the opposing actin molecule. Each myosin cross-bridge is in the high-energy conformation. Each of these students will place an ATP/ADP + P_i sign on the right forearm, near the banana, with the ADP + P_i side visible. Three students, holding a Ca²⁺ sign in each hand, will be sequestered in the sarcoplasmic reticulum (SR), which can be represented by a barrier of chairs or extra students. The interactive part of the demonstration has four steps. 1) Announce that the action potential has arrived via the T-tubule. The SR calcium channels open, and the Ca²⁺ escapes. 2) The three Ca²⁺ students move into position behind the thin filament, placing a Ca²⁺ sign on top of the head (troponin) of each thin-filament student. The binding of Ca²⁺ causes the tropomyosin (left hand and forearm) to move away from the binding site of actin. Ca²⁺ remains attached to troponin. 3) The myosin molecules (bananas) are "at the ready," and bind to uncovered active sites. Fig. 1 B illustrates the cross-bridge formation. All bananas then pivot in the same direction, representing the power stroke, and the actin filament is moved along the myosin filament. (The myosin cross-bridges stay attached to actin until they receive the new ATP molecules.) During the power stroke, the ADP + P_i sign detaches. This sign should be lifted and flipped over so as to become an ATP sign, which then reattaches to the myosin (at the forearm). 4) As the ATP sign attaches to the myosin, the cross-bridge (banana) detaches from the actin (apple). The ATP is converted to ADP + P_i, so the myosin students flip their signs again. The myosin cross-bridge, now in the high-energy state, reattaches to actin, and the above steps repeat. At some point, ask the Ca²⁺ students to detach and return to the SR. The tropomyosin covers the active actin sites and the contraction ends. The myosin cross-bridges are left in their high-energy state and wait for the next opportunity to bind with actin.

If time permits, a critique of the demonstration is useful: In what way might you improve this model to better represent what happens during muscle contraction? 1) The power strokes of myosin molecules of the thick filament are actually asynchronous. For simplicity, they are represented here as though they all happened at once. 2) The troponin and tropomyosin should be closer together (the heads of the thin-filament students acted as troponin merely for convenience as a symbolic site of Ca²⁺ attachment). 3) The ATP or ADP + P_i should be attached to the myosin heads (bananas), but that would also clutter up the demonstration.

View larger version (87K): [in this window] [in a new window]   FIG. 2 A: orientation of each student in the thin-filament during the set-up of the demonstration. Tropomyosin (left forearm and hand) covers the actin- active sites (apples) during the set-up phase of the demonstration. Troponin (head of student) will bind Ca2+ in the interactive part of the demonstration, causing tropomyosin (left forearm and hand) to move, uncovering the actin-active sites (apples). B: students enacting myosin cross-bridge formation following Ca2+ binding.

1. Marieb EN. Human Anatomy and Physiology (5th ed.). San Francisco: Benjamin Cummings, 2000.

This Article Full Text (PDF) Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Meeking, J. Articles by Hoehn, K. Search for Related Content PubMed PubMed Citation Articles by Meeking, J. Articles by Hoehn, K.