Mechanical Properties of Single Muscle Fiber Contraction

Introduction

In a previous simulation, the Recruitment of Motor Units, you studied the mechanical properties of the gastrocnemius muscle of a cat. However, as with other organs in the body, the functional capabilities of the muscles depend on the cells of which they’re comprised. Physiologists often use the term functional unit to refer to the cells whose functional attributes mirror the functioning of the organ itself. When attempting to understand how and why a particular organ functions the way it does, It’s almost always extremely useful to begin with study of the organ’s functional unit. Thus, the neurophysiology simulations you performed earlier focused on the neuron as the functional unit of the nervous system.

The functional unit of the muscles is the sarcomere. However, we can’t look at the properties of individual sarcomeres, so we’ll do the next best thing…..study the muscle fiber, which is basically a large number of sarcomeres ‘connected’ in series, end-to-end. It’s a reasonable expectation that many or most of the functional attributes of entire muscles should reflect the properties of their constituent muscle fibers. Therefore, in this exercise, you will look at the mechanical properties of a single muscle fiber.

This particular simulation is based on the mechanical properties of a fiber taken from a frog gastrocnemius muscle, but your results will be applicable to skeletal muscle fibers taken from nearly any vertebrate.

What Does The Simulation Allow You To Do?

To prepare for this experiment, an individual muscle fiber is freed from a gastrocnemius muscle and attached to a myograph, which is a type of transducer that converts tension ( = force) into an electric signal. Using a stimulator, we will electrically stimulate the muscle fiber to undergo a maximal contraction, which physiologists refer to as a twitch contraction, or sometimes simply a twitch. You will first apply a single stimulus and observe the mechanical properties and temporal characteristics of the resulting twitch. Then, you will increase the frequency of stimulation and observe the effect on the mechanical (tension-generating) behavior of the muscle.

The only parameter you can vary in this simulation is the frequency with which the muscle fiber is being stimulated.

What Does The Simulation’s Display Look Like?

The display is similar to that seen in the other simulation exercises you’ve worked with in this sequence:

In addition to the usual control buttons (Go, Clear, and Reset), whose functions are self-explanatory, there is only one other control, a slider/textfield combination that allows you to vary the frequency of stimulation (in Hz) being applied to the muscle fiber.

Short color-coded vertical lines will be displayed below the x-axis to indicate the times that the muscle fiber is receiving a stimulus. This will facilitate your interpretation of the graphs.

The units of the y-axis are percentages of the maximum tension generated by a single twitch contraction of the muscle fiber. Can you suggest why these units were chosen?