Skeletal muscle experiments

For muscles to contract, the muscle proteins called actin and myosin must interact with each other. This occurs when they are optimally aligned opposite each other. If they are too far away from each other, they can't interact optimally, and the same thing happens if they are too close.

Think of two trapeze artists. One of them will be caught by the other - but only if they jump at the right moment. If they jump when they are too far apart, then that artist will fall. If the artist jumps when they are too close, they are likely to collide.

You're going to explore this effect of overlap between actin and myosin in the muscle on the amount of useful force (active force) that the muscle can produce.

The purpose of this experiment is to examine how Passive and Active forces developed by the muscle change with muscle length. The video above explains some of the physiology involved in this process, and discusses expected results in the simulation and if you were doing a wet practical on this aspect of muscle function.

For this simulation, the voltage has been pre-set to achieve the peak muscle contraction response. This simulation looks at the effect of muscle length on contraction strength.

Please note that although this video demonstrates an older version of the simulation, it should function the same.

Instructions:

1. Begin by setting the muscle at its shortest length. (42.0mm)
2. Apply a single electrical stimulus and observe the passive (baseline) force, and the active tension (the difference between the peak force and the baseline).
3. Increase the muscle length by 0.5mm to 42.5mm. Stimulate the nerve supplying the muscle, observing the changes to both passive and active tensions.
4. Continue this until you get three successive recordings where you have increased voltage with no increase in muscle twitch size.
5. Systematically increase the muscle length by 0.5mm for the remaining values presented in the scroll box. Press stimulate, then observe the results.

Once you have finished, look at the second graph which plots the active, passive and total tensions for each muscle length. What trends do you observe? How does this relate to the physiology?

Full instructions can be found on the previous tab. In short:

• Muscle length can be selected from the scroll box on the left.
• First, stimulate the muscle's nerve with the muscle set at 42.0mm.
• Systematically increase the length of the muscle by 0.5mm at a time. Stimulate the nerve at each length.
• Observe the changes in active and passive tension.

The active tension produced by muscle is the force that can be used to do useful work. It is produced by the cross bridge interaction between the muscle contractile proteins, myosin and actin. During this cross-bridge cycle, actin combines with myosin and ATP to produce force, resulting in the ATP being broken down into adenosine diphosphate and inorganic phosphate.

In each muscle fibre (myofibril), these proteins are organized into repeating segments, called sarcomeres (one of which is shown in the visualization above). Each sarcomere consists of overlapping thick filaments (myosin; dark blue in the illustration above) and thin filaments (actin; red in the illustration).

Muscles contract by the thick and thin filaments interacting and sliding along each other. This interaction occurs when certain actin-binding-sites (the spiky "twigs" on the blue myosin filaments) bind to sites on the actin. Notice that these actin-binding heads are not found along the full length of the myosin filaments, but only at the ends. That means that there will a certain sarcomere length where these actin-binding heads will overlap optimally with the actin filaments - at longer lengths the overlap will be less, and at shorter sarcomere lengths the actin filaments will overlap with each other, so that there will less than optimal actin sites for the myosin actin-binding heads to bind to, to produce the cross bridge cycle.

This will become clearer when you play with the visualization in the previous tab.