Construction of a Length-Tension Diagram

Introduction

This exercise builds on the experience you gained with the previous exercises in this series of simulations based on the properties of vertebrate skeletal muscles. In the ‘wet’ lab version of this exercise, one would remove a whole gastrocnemius muscle from a frog and connect it to a myograph (lit. “muscle writer”), a transducer that allows us to determine very precisely the tension generated by a muscle when it contracts.

Part of the myograph setup is a sliding bar that allows you to change the length of the muscle, making it shorter or longer than its normal in vivo length (i.e., its length in the living frog). Then, by varying the length of the muscle, you will observe the effect on passive tension in the stretched muscle, and on the active tension generated by the muscle when you stimulate it to contract.

Note that while this is a simple experiment, it is not a trivial one. When investigators first gathered these data, they achieved some significant insights into muscle function, insights that ultimately led to our current understanding of the mechanism by which muscles to contract.

What Does The Simulation Allow You To Do?

The only parameter you can control in this simulation is the length of the gastrocnemius muscle. You change the muscle’s length by means of a slider/text field combination. The possible range of muscle lengths is 50 – 150% of its normal in vivo length.

What Does The Simulation’s Display Look Like?

When you click the “Run The Simulation” link, you will see a display that is similar in many ways to those of other simulation exercises you’ve worked with in this series:

Most of the display area is taken up by a set of axes on which the output of the myograph will be displayed. To correspond with the experience you’d get doing this experiment as a wet lab, the x-axis is not labeled. However, as the y-axis’ label indicates, an upward deflection of a line along the y-axis represents an increase in the tension being transduced by the myograph.

Near the right end of the x-axis is a line whose length represents the deflection in the Tension graph that would be produced by suspending a 50 g mass from the myograph’s hook. You will use this scale to calibrate the data you gather.

A slider/text field combination is provided that allows you to change the length of the gastrocnemius muscle. You may do this either by using the mouse to move the slider or by typing a number in the text field (review the “Preface and Introduction” if you need a refresher on this). The usual control buttons (Go, Clear, and Reset) are provided and function similarly to those you’ve used in previous simulations in this series.

In the right part of the display is a schematic diagram of the experimental apparatus, which consists of the myograph, a myograph support (the vertical black bar), and a moveable length-adjusting rod (the horizontal blue bar). A complete gastrocnemius muscle has been dissected free from a frog and placed in the apparatus with one end tied to the myograph’s hook, the other to the blue length-adjusting rod. A yellow stimulus electrode will supply a maximal stimulus to the muscle when you click the Go button.

Try moving the slider with your mouse and note that moving the slider upward will, by causing the blue adjustment bar to move upward, lead to shortening of the muscle, while moving the adjustment bar downward will stretch the muscle. (If you happen to click the Go button at this time, simply click the Reset button to return parameters to their default value and clear the display)

Any tension in the muscle, whether generated passively by stretching or actively during a contraction, will be registered by the myograph. The amount of stretch in the muscle – and therefore the passive tension – is varied by moving the blue adjusting rod up or down until the desired degree of stretch is achieved. When you start the simulation, the length of the muscle is set to 100% of its in vivo length ( = its normal length when in place in the frog’s calf).

Clicking the “Go” button will cause the stimulus electrode to flicker, indicating that a maximal stimulus has been applied to the gastrocnemius muscle. The resulting action potential on the sarcolemma (not displayed) stimulates the gastrocnemius muscle to undergo a twitch contraction. Note that the stimulating electrode is applied directly to the gastrocnemius muscle itself. Why do you think this was done?

After the muscle has finished its twitch contraction, a short horizontal red line will appear on the display, followed by a vertical red line of varying height. The horizontal line simply serves to facilitate data collection by providing separation between adjacent vertical lines, but its elevation above the x-axis represents the passive tension generated in the gastrocnemius muscle that you induced by stretching it to the desired length. The length of the vertical line segment represents the amount of active tension that was generated by the gastrocnemius muscle when it contracted.