Frog Skeletal Muscle Lab I


updated 16 Oct 2013 at 3:21 pm

The objective of this exercise is for students to
1) determine the stimulus parameters necessary to demonstrate threshold, twitch, recruitment, multiple summation, incomplete tetanus, fused tetanus, fatigue, and the latent period,
2) gain practice with experimental design as a group effort,
3) become more experienced with the use of computers for data acquisition and analysis,
4) observe various types of muscle contractions,
5) compare the properties of muscles from various parts of the body, and
6) predict the fiber type composition of muscles from various part of the body.

 

During this lab, each group of 3 students will be provided with a frog gastrocnemius muscle or a wrist extensor muscle, a PowerLab computer setup including a force transducer (also known as a strain gauge), ringstand, clamps, micromanipulators, dissecting tools, pin electrodes for stimulation, and 0.9% saline solution or Ringer's solution.

Consult with your labmates and design experiments to obtain recording that demonstrate threshold*, twitch* contraction, recruitment*, multiple summation (also called wave summation), incomplete tetanus, complete (fused ) tetanus, the latent period* of muscle contraction, and fatigue. (* = defined in glossary of text.) You'll also measure the contraction time and twitch duration for a single twitch and peak twitch tension. You'll also measure peak tetanic tension. Consult Chapter 9 of your Widmaier text and the Figures 9-16, 9-20, and 9-21 for more information. The text on the pages associated with those figures will be helpful.

You will be instructed on the use and calibration of the force transducer.

Near the end of the lab, be prepared to show examples of each phenomenon on your computer screen. To facilitate this, add comments to the pages of computer data you record.

You will enter your data in a datasheet and the computer. A spreadsheet with the experimental results from each of the groups in all labs will be available on the website on Friday.



Discussion among group members at the beginning of lab.


What factors must be considered when choosing which muscles from a frog would be suitable for this study? Why have we elected to study the wrist extensor and gastrocnemius?

Each muscle will be removed from the body for study. How will its survival be affected once it no longer is attached to the circulatory system? What can we do to ensure its longevity, at least for the duration of the lab period?

The muscle can be made to contract by using pin electrodes to stimulate either the nerve that innervates that muscle or by direct stimulation of the muscle itself. What are the advantages and disadvantages of each method? (Hint: consider how many myfibers might be affected by each of the modes of stimuli and how technically challenging it would be stimulate in each location.)

How long does a muscle twitch last? How long must the recordings be in order to see twitches and summation?

How will the response be measured? What will the units be? (grams, volts, millimeters?)

In which order should the experiments be completed? Before beginning your experiments, have the instructor approve your sequence. Indicate the proposed sequence by re-ordering the following terms:


Fatigue
Recruitment
Twitch
Threshold
Complete tetanus
Incomplete tetanus
Latent period
Wave summation


The Scope program is used to record events that last a second or less. The Chart program is useful to record events that are longer than a second and can be minutes in duration. Which program will be needed for today's recordings? Might it be necessary to use both programs?


Execution of the Experiments


Once your group has formulated a plan and these plans have been reviewed with the class and approved by the instructor, the instructor will demonstrate the set-up of the computer software and distribute muscles to each lab group. Proceed with your experiments and obtain values for the following table.


Complete this table based on measurements of your experimental records. Be sure to include the appropriate units.

 

Parameter Measured

value

Units

Threshold Voltage

 

 

Twitch duration

 

 

Contraction time

 

 

Latent period

 

 

Tetanic Interval (1)

 

ms

Tetanic Frequency (2)

 

pulses per second

Time to fatigue (3)

 

 

Maximal Twitch tension

 

 

Maximal Tetanic tension

 

 


(1) the lowest stimulus frequency at which complete tetanus occurs.
(2) calculated based on Tetanic Interval. Units are pulses per second (equivalent to Hertz)
(3) time required for tension to be reduced to one-half the peak tension (our convention)



Example of converting from Tetanic Interval to Tetanic Frequency:

Suppose your Tetanic Interval was 100ms (not a realistic value).

1 pulse/100ms x 1000ms/sec = 10 pulses per second.

 


Before leaving lab

Review the data provided by your classmates in the spreadsheet and consider whether the data fits your projections regarding the types of myofibers that predominate in each muscle given the normal role of that muscle in the frog's body.