This exercise is designed to demonstrate some mechanical and physiological properties of skeletal muscle using the gastrocnemius muscle of a frog.
At the end of this exercise the student should be able to:
1.Define minimal (threshold), subminimal, maximal, and supramaximal stimulus.
2.Explain what is meant by a “graded” response.
3.Draw a diagram of the setup used in this lab exercise.
4.Calculate the work that is done in a given muscle.
5.Describe the function of a force displacement transducer.
6.Describe what happens to muscle contraction when the load is increased.
7.Define twitch, treppe, tetanus and incomplete tetanus.
8.Explain the difference between a single and double pith of the frog.
9.Post all data to Google Docs.
The gastrocnemius muscle contains many muscle fibers. Each fiber has its own threshold and responds all-or-none when stimulated. It is known that all of the fibers in a muscle do not have the same threshold and that a stimulus applied to a muscle does not necessarily excite all the fiber in it. The threshold is that intensity (voltage) of stimulus which brings response. As the intensity (voltage) of the stimulus is increased above the threshold, more and more fibers are stimulated and the response becomes greater and greater. Eventually, however, a stimulus intensity (voltage) is reached beyond which the response is constant. This stimulus, called the maximal stimulus marks the point where all of the fibers in the muscle are stimulated and responding all-or-none. Stimuli above this maximal stimulus are called supramaximal stimuli. Stimuli below the threshold that do not initiate a mechanical response are called subthreshold stimuli.
The frog muscle is used in this laboratory exercise in place of mammalian muscle because of its tolerance to temperature change and handling. The results are similar to what would be seen in more carefully controlled mammalian experiments.
III. Preparation of the Frog
After the frog has been doubly pithed you are ready to remove a muscle for testing. One of the largest and easiest muscles to obtain is the gastrocnemius of the lower leg.
Place the frog on a clean dissecting pan. Be sure that neither the pan nor the dissecting instruments have been contaminated with preservatives such as formaldehyde. You will be provided with pans and tools reserved specifically for the dissection of fresh materials.
It is not necessary to tie down the frog when using the femur clamp preparation. As the muscle is dissected assign one of your lab partners the task of frequently flooding the surgical area with frog ringers using a small beaker and disposable pipet. The muscle must not dry out while dissecting or it will be useless.
With forceps, lift the skin from one thigh and cut the skin completely around the leg using scissors. Pull the cut end back and peel the skin off the leg. Minimize the touching of the muscle tissue with contaminated dissecting equipment. Keep the are moist with frog Ringers. With a blunt probe or the blunt end of the forceps separate the body of the gastrocnemius from the underlying bone. The distal end of the muscle is attached to the Achilles tendon ,a white to yellow strand of fibrous tissue. Loosen a portion of the Achilles tendon from the bone then insert a piece of thread about 10 inches long around the tendon and tie it securely. Cut the tendon distal to this knot. The proximal end of the gastrocnemius is attached to the femur. Do not remove it from the bone. Carefully remove all other muscles from the upper leg but leave the gastrocnemius attached. Then cut the lower leg bone (tibia) at or slightly distal to the “knee” joint with strong scissors or bone cutters (DO NOT USE A SCALPEL). Cut the proximal end of the femur leaving at least one half inch of...