Properties of skeletal muscle lab report guidelines 

I. Analysis and results

1. Introduction

During the last session you collected the data that you will use for your formal report. 

At this point you are ready to start analyzing your data – basically, you’ll start making sense of it, organizing it in tabular or graph form, beginning to question its meaning in the context of body of knowledge of skeletal muscle contraction.

2.  Conversion of your recordings to grams of tension -- calibration

Recall that the semi-isometric force-displacement transducer you used to collect data converts a mechanical action (displacement) into an electrical signal that is quantified in millivolts.  However, in order to investigate the properties of skeletal muscle you will need to quantify the tension produced by the muscle in various conditions in terms of grams.  Thus your first analytical task is to appropriately convert the data collected in each experimental procedure to grams of tension – to do so you will use the calibration procedure you performed prior to each experimental procedure.

a.  Go to the square wave you recorded for calibration prior to the procedure of interest – we will use calibration of your record of threshold and recruitment of fibers as an example.  Using the mouse highlight the whole square wave.  Go to the WINDOWS menu and select ZOOM WINDOW.  The ZOOM window shown in Figure 1 appears, with the selected waveform enlarged.

Figure 1:  The ZOOM window

b.  Single-hold click on the MARKER (M) icon shown on the bottom left of the screen.  Drag the M icon to the baseline immediately prior to the upward deflection of the square wave and let go of the mouse button.  The M-icon will mark a segment of the baseline – both on the X-axis (t = ..) and the Y-axis (Y = ..) this point is now considered 0.  Check with your instructor or TA if you need clarification on this procedure. 

c.  Place the cursor on the square wave tracing on top of the M-icon.  On the top of the active window the coordinates t = 0.000 sec and Y = 0.000 mV will appear.  As you slowly move the cursor along the square wave tracing away from the M-icon, the value of the t and Y coordinates will change.

d.  Move the cursor to the highest point on the square wave and record the Y-axis coordinate (mV) in your notebook.  Next, return the M-icon to its “home” in the bottom left of your screen by either dragging it there or by clicking in the empty space in the bottom left of the screen (Figure 1).

e.  Now click on the Channel 1 icon and select the UNITS CONVERSION option.  The UNITS CONVERSION dialog box will appear (Figure 2). The UNITS CONVERSION lets you change your data from millivolts to some other unit of measurement, grams in this case.

Figure 2:  UNITS CONVERSION dialog box

Click on the first empty field from the left and type in the mV Y-coordinate you obtained from above.  Hit the TAB key and the cursor will move to the next field – type in the weight you used for calibration (10 or 20) and select the appropriate unit from the pull-down menu (grams, or g).  Hit the TAB key again and the cursor will move to the left field on the next line – for this line enter values that are exactly double (2X) of the ones in the fields above.  The following is an example of how data in the dialog box can look like:

15 mV (use your value) = 10 g

30 mV (use your value) = 20 g

Once your data are entered in the dialog box click OK or hit the ENTER key. 

At this point the computer will convert ALL DATA IN THIS FILE to grams based on the proportion you entered in the UNITS CONVERSION dialog box.  In other words the t-coordinate is still quantified in seconds but the Y-coordinate is shown in grams.  However, this proportion  only applies to the set of data you collected next, in this case the data for threshold and recruitment.  You will need to perform this same unit conversion for each of the experimental procedures, using as calibration the square wave you recorded prior to each procedure.  If you recorded multiple calibration waves prior to each procedure use an average Y-coordinate value to set up the calibration proportion.  If you have questions please talk to your instructor or to a TA.

f.  You are now ready to measure the grams of tension produced by each twitch or series of twitches, the duration of a contraction or relaxation phase, etc.  The specific data you should collect for each procedure is indicated below in I.3. 

Recall that you mark your baseline (t = 0 sec, Y = 0 g) by dragging the M-icon to the desired location and reading either the t or Y coordinates relative to it.  In most cases you can get more accurate data by highlighting the twitch/twitches in question and using the ZOOM window to enlarge them and get your data as described above.

3.  Data collection and graphs

Collect the following data for each procedure.

a.  Threshold and recruitment

Identify the tension generated by application of each specific stimulus.  Using CricketGraph or Excel, make a graph of stimulus intensity (mV) vs. tension generation (grams).  Make sure to appropriately label axes and give the graph a title.  For an example see Figure 3A below.

Figure 3A: Sample graph of threshold and recruitment of fibers

Also produce a printout of your record of multiple motor unit summation and recruitment identifying subthreshold, threshold, submaximal, maximal, and supermaximal stimuli.  For an example see Figure 3B below.  You can print this record as a hard copy and “tape-paste it” into your report, or copy it into the clipboard and paste it onto a Word file for later use.

Figure 3B:  Sample tracing of threshold and recruitment of fibers

NOTE:  To print, highlight the selection of interest.  Go to FILE menu and select PRINT SELECTION.  Do not print without highlighting selection of interest since otherwise your ENTIRE file will be printed, a waste of paper.  You can also ZOOM the tracing of interest and print the zoom window.

b.  The muscle twitch

Produce a printout of your high speed recording of a muscle twitch.  You can either print a window showing both Channels 1 and 2, or a window showing only Channel 1 as long as the M-icon identifies the point along the baseline where the stimulus was applied.  For an example see Figure 4A.  To highlight data in both Channel 1 and Channel 2, move the cursor to the bottom of the screen, below the t-axis – in this area the cursor will become a double arrow (ßà).  Now highlight the tracing of interest in both channels.

Figure 4A: Sample tracing of muscle twitch and table of durations of various phases of muscle twitch

Measure the duration of the latent phase, the contraction phase, and the relaxation phase (msec).  Present the data in tabular form as shown in Figure 4A.

c.  Twitch fusion and tetany

Produce a printout of muscle stimulation for each frequency you tested.  For an example see Figure 5A.

Figure 5A:  Sample tracing of twitch fusion records

For each frequency tested, identify the maximum muscle tension produced and record it in your notebook.  Present the data by graphing frequency of stimulation (pulses/sec) vs. maximum tension produced (grams).  You can also present the data in tabular form. 

d.  Length-tension relationships

Measure the respective tension generated by the muscle twitches at three initial resting lengths.  Produce a graph of “muscle length” (cm) vs. tension (grams).  See Figure 6 for an example.

Figure 6:  Sample graph of muscle length (cm) vs. tension

e.  Effects of afterload and preload

Use graphs to present muscle tension as a function of different preload and afterload. 

f.  Fatigue

Produce a printout of the muscle fatigue record:  using the cursor highlight your entire fatigue record; ZOOM on this selection, and PRINT SELECTION.

Measure the peak tension produced by the initial tetanic contraction and measure how long it took muscle to fatigue.  Identify the point at which you added Ringers saline to the muscle.

II.  The lab report

a.  Overview

Once a scientist has completed an experiment and analyzed his/her data, an important step remains--to relay the findings to the wider scientific community through publications in scholarly journals.  Scientific writing may differ from other styles with which you are familiar.  For example, a novelist might write,

  "The tiny honeybee flew far and wide gathering nectar and returned to the hive laden with the sweet ambrosia of colorful summer blossoms."

On the other hand, a biologist would state,

"A honeybee (Apis mellifera) gathered nectar from yellow daisies (Rudbeckia hirta) in a patch 200 m from the hive."

Evoking sights, sounds, smells and emotions is a novelist’s goal.  A scientist is interested in relaying factual information to build a knowledge base.   Succinct, accurate and precise prose is a universal feature of scientific writing.   Features shared by both styles are complete, grammatically correct sentences, correct punctuation and spelling, and logical progression throughout a paragraph.

The following sections outline our expectations for this lab report and give you suggestions on how to start putting it together.  All reports must be typed on a computer or word processor. They must be written individually, but collaboration in data analysis, reference-hunting, and interpretation is permitted and encouraged.

b.  Format:

Use the standard format followed by most scientific journals. You may, for example, use almost any paper in Physiological Zoology (CSB) or the Journal of Comparative Physiology (CSB), American Journal of Physiology (SJU) as a model for style and organization. Your report should include the following sections: Title and Introduction (15 points), Materials and Methods (15), Results (20), Discussion (40), and References (10).  When writing the first drafts of your lab report, you might find it easiest to complete the sections of a lab report non-sequentially.   It is often easiest to start with either your Methods section or the Results section.  Once warmed up with these two sections, the Discussion can be tackled, followed by the Introduction, Title and References.  In your final polished draft, place them in the proper sequence, as listed below, with the name of each section (except the title) preceding it.

i.  Title

The title concisely describes the subject of the lab report.  Include the following informational details in a concise, coherent title:

If you studied an organism, identify it using the scientific name.  Remember that both components (genus and specific epithets) are underlined or italicized.  Capitalize only the genus name (i.e., northern frog or Rana pipiens)

If you completed an experiment, what factor(s) did you manipulate?  What response(s) did you examine?

ii. Introduction

The introduction gives background information about the topic of the lab report and logically states the purpose or objective of the experiment, which you should state near the end.  Include only the information needed directly to instruct readers about the intent of the study you actually performed.  The test of a well-written introduction is if a reader (such as your roommate) can easily see why conducting this experiment would be logical and useful research activity.

Address the following:

Leave details for the discussion, but describe the general role of the system or tissue here. (about 1 page).

iii.  Methods

The Methods section describes the procedure of the experiment in enough detail that another researcher with a similar degree of training could duplicate the experiment.  Use the past tense and keep this part very short and to the point (about 1-2 pages).

·         Details of standard techniques, and operating procedures for laboratory equipment do not need to be described – how to turn on MacLab, open software, etc.

·         Briefly describe how you set up apparatus -- pictures of apparatus are useful in relating the details of your setup; they also help you to be brief and concise.

·         Describe the steps of an experiment in paragraph format, do not list them (1, 2, etc.).

iii.  Results

The Results section should be just that--your data, presented as described above and neatly arranged in a coherent form. 

Tables should contain the following features:

  Figures should contain the following features:

The Results should be summarized in paragraph fashion -- a written description of data trends, average values, and changes you noted with experimental treatments. Data shared between lab partners can be copied and handed in with each report. The written summary must be individually composed. 

In this section do not interpret  your results, simply present, or describe them (2-4 pages with graphs and tables).

iv.  Discussion

The discussion is the most important section of a scientific paper.  This section is where you interpret your results.  In the introduction, you stated your purpose;  here you interpret your results, make inferences in light of the existing body of knowledge in the field, suggest further research, discuss applications of your findings to human systems.  In short, you explain, criticize, or support your results with other information. We expect you to draw on outside sources (textbooks, research papers, review articles) to support or broaden your conclusions, but be sure to credit the author for ideas or work not your own. [2-4 pgs.]

v.  References or literature cited

In a scientific paper, you give credit to other biologists when you use their ideas or results.

Credit must be given for ideas, though direct quotations are rarely used.  In other forms of writing, footnotes or endnotes are used to credit sources, but in scientific writing, citations are incorporated into the text.  The following is an example of a citation: Killer whales travel in stable groups called pods and cooperatively search for food (Hoelzel 1993).

At the end of the paper, full bibliographies are given for all the factual information cited in the paper.  List them in alphabetical order starting with the first author’s last name.  In the body of the paper, your citation need only list the first author’s name followed by et al, if there is more than one author.  However, in this section, include all the author’s names. 

For journals use the following format:  Author’s last name, first initials. Year of publication.  Title of article.  Title of the journal, volume number: page numbers.

For books use the following format: Author’s last name, first initials.  Year of publication.  Title.  Publisher, Publisher’s location.

For web sources use the following format:  author, year.  URL.

List the Literature Cited alphabetically by the primary authors' last names.

c.  Plagiarism will not be tolerated.

III. Questions to guide your discussion

Interpret your results using the information available in Marieb or the many other texts and references available in lab or in the libraries. These questions may get you started:

1. How do muscles work? Give a brief account of the mechanism of muscle contraction and excitation. How does the excised muscle differ from the nerve-muscle arrangement in vivo?

2. For multiple motor unit summation, classify all stimuli and responses as either subthreshold, threshold, submaximal, maximal, or supermaximal. Explain mechanism of multiple motor unit summation -- there is a lot of work done in this area, this is a good focus of library research.

3. How is multiple motor unit summation explained in the context of the All or None Law?

4. What is meant by a subthreshold response? Does this type of response obey the All or None Law?

5. What is a supermaximal stimulus? Why is it used to demonstrate wave summation in skeletal muscle?

6. Why does tension generation increase with increasing frequency of stimulation of the muscle?  Explain this in the context of the All or None Law. Also account for the fusion of twitches resulting from stimuli close together in time.

7. What is the ionic mechanism for wave summation? What is tetany?

8. Account for the graded nature of skeletal muscle contraction. How are smooth contractions of varying intensity obtained in the human body?

9.  What is the relationship between length of the muscle and peak tension? How does this relate to actual muscle rest length in your body or a frog's?

10.  How does varying preload affect tension generated by muscle contraction?

11.  How does varying afterload affect duration of muscle contraction, velocity of contraction, force of contraction, duration of latent period?

12.  What is fatigue and how does it occur?  Why does tension drop as muscle fatigues?

IV. References found in 314 SC

In addition to your text, there are many books in the library that will provide you with background information on skeletal muscle. We will keep a set of papers on reserve in 312 PENGL.