Virtual Lab: Muscle Stimulation (overview)
Open the Virtual Lab entitled “Muscle Stimulation”. Please read the “Information” window that opens; click the “X” to close the
window when you are finished. You can reopen this information in the future by clicking the “Information” button found at the
bottom of the laboratory area.
The left side of the page contains background information and instructions in the “Question” area. On the bottom of the page are
the following icons for your use: “Journal” (these are questions that may be answered/submitted to your instructor), “Calculator”
(working calculator function), “Data Table” (where you may be submitting your collected data from the exercise), “Audio” (turns
sound on/off) and “Print” (prints the screen).
Read the background information found under the “Question” area first, and then continue on to reading the procedure information
posted there as well.
At this point, you are ready to begin the lab exercise. You will need to first open and/or print the “Post-Lab Quiz and Lab Report”,
which can be found on the previous page. When you are ready, select the muscle type you would first like to test on the “Muscle”
pull down menu. Remember to follow the directions given to you in the “Question” as well as in the “Post-Lab Quiz and Lab Report”
areas, and refer back to your notes in the “Information” area if necessary.
Complete the exercise as directed, recording any data or information needed in your “Data Table” (icon on the bottom of the page)
and/or your “Lab Report”.
When you are finished, please answer all of the questions found at the end of the “Post-Lab Quiz and Lab Report”.
BACKGROUND INFORMATION
Three types of muscle tissue exist in the human body: cardiac, smooth, and skeletal.
Both cardiac and smooth muscles are called “involuntary” because their contractions cannot be consciously controlled. Cardiac
muscle exists only in the heart and contracts approximately 70 times per minute to pump blood throughout a body's circulatory
system. Smooth muscles line many internal organs such as the trachea, intestines, and bladder. Smooth muscles contract and relax
slowly, exerting and releasing pressure on the organs they surround.
Skeletal muscles are the muscles that are used for movement. They are called “voluntary” muscles because their contractions are
consciously controlled. Skeletal muscles are attached to bones by bands of tissue called tendons and work in opposing pairs to move
the bones of the skeleton.
Skeletal muscle tissue is made up of individual cylindrical muscle cells, called fibers. When exposed to stimuli from the nervous
system, each individual muscle fiber will contract either completely or not at all. This is known as the “all-or-none” law of skeletal
muscle contraction. If enough individual fibers contract at one time, the entire muscle will contract. The strength of the muscle
contraction is determined by the number of individual muscle fibers that respond to the stimulus.
If a single, quick electrical stimulus is applied to a skeletal muscle, the muscle will respond by contracting and relaxing quickly. This
type of quick contraction is called a muscle twitch. The weakest electrical stimulation (measured in volts) required to cause a muscle
to twitch is known as the muscle's “threshold of stimulation.” This value has been found to vary among different types of skeletal
muscles and for a single muscle, under varying workloads.
As the workload on a muscle increases, the muscle will eventually reach a point at which it will no longer be able to contract, no
matter how much electrical stimulation it receives. This is known as the “point of muscle overload.”
Much of what biologists have learned about how skeletal muscles work has been discovered in the laboratory. The classic
experiment modeled in this Investigation involves isolating frog muscles and stimulating the muscles to observe their responses. The
isolated muscle is attached to a ring stand that is wired to an oscilloscope. A stimulating electrode is clamped to the stand as well,
with its tip resting on the muscle tissue. The oscilloscope is used both to generate stimuli of various voltages and to record the
muscle's response to a stimulus (a twitch), indicated by a spike on the oscilloscope trace. To vary the amount of load on a muscle,
small laboratory weights can be attached with a hook to the base of the muscle. To determine the muscle's threshold of stimulation
under a specific workload, increasingly stronger stimuli are applied to the muscle via the stimulating electrode until a twitch is
registered on the oscilloscope trace.
NAME: ____________________
LAB: MUSCLE STIMULATION
How does increased workload affect a skeletal muscle's threshold of stimulation?
Go to rrhodeskhs.pbworks.com click link to the Virtual Lab: “muscle stimulation”
Click on “laboratory exercise”. Read the information in the INFORMATION box. If you come upon terms that are
unfamiliar to you, please refer to your textbook for further explanation or search the word here:
http://encarta.msn.com/encnet/features/dictionary/dictionaryhome.aspx
Close the window when you are finished. You can reopen this information later by clicking the “information” button
found at the bottom of the laboratory area.
The left side of the page contains background information and instructions in the “Question” area. On the bottom of the
page are the following icons for your use: “Journal” (these are questions that may be answered/submitted to your
instructor), “Calculator” (working calculator function), “Data Table” (where you may be submitting your collected data
from the exercise), “Audio” (turns sound on/off) and “Print” (prints the screen). However, unless otherwise directed by
your teacher, your data and analysis will be recorded on paper.
Read the background information found under the “Question” area first, and then continue on to reading the procedure
information posted there as well. The procedure is also described below.
PROCEDURE:
In this exercise, you will use an oscilloscope to examine the effect of varying load (weight) on frog skeletal muscle action.
To begin, click on the pull down menu of the “Muscle” button to select the type of muscle you would first like to test.
Making sure that the oscilloscope is set to “0V”, click the “Apply Stimulus” button. As you do so, pay close attention to
the trace readout on the screen; you are looking specifically for a spike in the straight line to be produced. If no spike is
observed, use the upward pointing arrow on the oscilloscope to increase the voltage to the next highest level and then
click “Apply Stimulus”. Keep repeating this procedure until you observe the production of a spike in the oscilloscope
trace readout. At that point, note the voltage in the “0g” load weight in your “Data Table” and/or in Table I below.
NOTE: If no spike is observed, be sure to click “Apply Stimulus” at least 3 times to make sure the muscle received the
stimulus properly.
When you are through, you will then need to repeat the above steps using the varying loads (weights). To do so, drag
the 5g weight to the hook attached to the frog muscle and then proceed as you did to test the muscle action before.
When you are through testing the 5g weight, repeat these steps using each of the four remaining loads.
When you are completely finished testing all variables for the muscle type selected, you will need to repeat this
procedure for ALL three remaining muscle types.
Remember, you can always click the “Reset” button to clear your experiment and start fresh.
Be sure to appropriately note all of your data in the “Data Table” or Table I below.
Please finish this exercise by opening the “Journal” link at the bottom of the page and answering the questions. Journal
Questions are included in the lab handout…so unless otherwise directed by your teacher, answer them here.
NAME: ____________________
LAB: MUSCLE STIMULATION
How does increased workload affect a skeletal muscle's threshold of stimulation?
Table I: Threshold Stimulus (V)
Load
Lower
Forelimb
Muscle
Upper Forelimb Calf Muscle
Muscle
Thigh Muscle
0g
5g
10g
20g
40g
80g
POST LAB:
1. Smooth muscle:
a. Is voluntary
b. Is found in the heart
c. Is found in lining the intestines
d. All of the above
_____
2.
_____
Skeletal muscle:
a. Is involuntary
b. Is made up of muscle fibers
c. Is attached to bone via ligaments
d. All of the above
3. In skeletal muscle:
a. The greater the number of muscle fibers responding to a stimulus,
the greater the strength of the contraction
b. the fewer the number of muscle fibers responding to a stimulus,
the greater the strength of the contraction
c. the actual number of muscle fibers responding to a stimulus has
no effect on the strength of the contraction
_____
4.
_____
An oscilloscope:
a. Provides stimulus to a tested muscle
b. Adds weight (load) to a tested muscle
c. Measures a twitch response of a tested muscle
d. A and C
e. All of the above
5. At the point of muscle overload:
a. Skeletal muscle contracts steadily (tetany)
b. Skeletal muscle contracts and relaxes repeatedly
c. Skeletal muscle does not contract at all
_____
6. A skeletal muscle’s “threshold of stimulation”:
a. Is determined by the lowest voltage stimulus needed to elicit a muscle twitch
b. Varies with muscle type
c. Is not altered by the application of muscle load
d. B and C
_____
7.
_____
In response to a stimulus, skeletal muscle fibers:
a. Always contract to 25% of their potential ability
b. Always contract to 50% of their potential ability
c. Always contract 100% of their potential ability
d. None of the above
8. Based upon your experimental data, which muscle type exhibited the lowest
Threshold of stimulation in all conditions tested?
a. Lower forelimb muscle
b. Upper forelimb muscle
c. Calf muscle
d. Thigh muscle
_____
9.
Based upon your experimental data, which muscle type(s) did not reach the
point of muscle overload?
a. Lower forelimb muscle
b. Upper forelimb muscle
c. Calf muscle
d. Thigh muscle
_____
10. Based upon your experimental data, which muscle tested appears most capable
to provide the contraction strength needed for a frog to jump?
a. Lower forelimb muscle
b. Upper forelimb muscle
c. Calf muscle
d. Thigh muscle
_____
JOURNAL QUESTIONS
1.
Based on the results of your investigation, what conclusions can you draw about the relationship between a
muscle's workload and its threshold of stimulation?
2. Why would a muscle's threshold of stimulation change as its workload changes?
3. Which muscles were able to contract under the greatest loads? What does this suggest about the role these
muscles play in frog movement?
4. Describe an experiment you might perform to determine which leg muscles of a frog are important for jumping
long distances.