Mechanical Properties of Single Muscle Fiber Contraction

Experimental Techniques

1.  Click the “Go” button to do a run using the default setting for the stimulation frequency ( = 1 Hz).  Note the appearance of the resulting twitch tension graph.  Particularly note the height and shape of the curve.  Measure the maximum amplitude of the curve.  Record the data on the attached data sheet.

2.  Increase the frequency in ‘jumps’ of 1 – 2 Hz until you reach 15 Hz, each time noting carefully the appearance of the tracing produced by the contracting muscle fiber and recording your data.  Clear the display as necessary.

3.  Using ‘jumps’ in stimulus frequency of  5 – 10 Hz between runs, continue increasing the stimulus frequency until you reach the maximum allowed value of 100 Hz.

4.  Measure the length of the 0 – 700 interval along the y-axis and record this value on the data sheet. 

Tips & Hints

1.  For the most part, you can change the stimulus frequency by 5 – 10 Hz between runs, but be sure to obtain tracings for stimulation frequencies of 9, 10, 11, & 12 Hz.

 

Questions:

1.  Does the muscle fiber generate tension immediately after each stimulus is applied, or is there a delay between the time of the stimulus and the time that tension is first generated?  If you noted a delay, what do you think could account for it?

2.  Notice the shape of the tension-versus-time plot for a single contraction.  Is it a symmetrical curve?  In other words, during each twitch contraction, does tension rise and fall at the same rate?  Can you suggest an explanation for the shape of the curve?

3.  Graph the relationship between Stimulus Frequency and Maximum Tension.  Describe the appearance of the graph.  Is there a smooth, gradual transition between ‘non-summating’ and summating contractions?  Of, is the transition abrupt?  What does the graph suggest about the mechanism underlying the tetanus phenomenon?   

4.  Once you’ve increased the stimulus frequency to about 15 Hz, tension during a run should rapidly increase to a ‘plateau’ and ,depending on the monitor & browser you’re using, you will be unable to detect any fluctuation in tension (this is not an artifact; you’d see the same thing if you were working with a real muscle fiber and recording device)  Yet, by increasing the stimulus frequency you can achieve greater and greater maximal tension values.  What is your explanation of this phenomenon?

5.  What was the minimum stimulus frequency that resulted in any summation of the contractions?  At what frequency did you first see significant summation?  How do these values correspond to the length of time required for a complete contraction-relaxation cycle of the fiber?

6.  When the muscle fiber is being stimulated at a frequency greater than 11Hz, do all individual twitch contractions cause the same increase in tension?  Suggest an explanation for your observation.

7.  Starting with a stimulus frequency of 10 Hz, do a series of runs in which you increase the stimulus frequency 1 – 2 Hz between each run.  Determine the incremental tension of the fifth twitch in each tracing (i.e., the difference between the maximum tension generated during the fifth and the tension present in the muscle fiber at the start of the fifth twitch).  (Depending on the monitor’s size, you’ll be able to increase the stimulus frequency to 25 – 35 Hz before you can no longer detect the fifth twitch’s tracing).  Convert your measurements into relative tension units and construct a graph of incremental tension vs. stimulus frequency.  Describe the appearance of the graph.  Does the appearance of the graph suggest anything about what might be going on in a muscle fiber subjected to stimulation at a tetanizing frequency?

8.  Had we used an entire muscle in this experiment instead of an individual muscle fiber, we would have obtained qualitatively similar results, with the principal difference being in the amount of tension generated during twitch contractions and tetanus.  Can you think of examples of tetanus in an entire muscle?  Given what you learned as a result of performing these exercises, try to think of some natural causes of tetanus.

 

Data Sheet For Muscle Tetanus Simulation

 

 

 

Run #

Stimulus

Frequency

( Hz )

 

Summating?

( Yes/No )

Maximum

 Tension

( mm )

Maximum

Tension

( N )

 

 

Notes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Length of 0 – 700 interval:  ______________