Muscle Contraction
Name:___________________________________________ Per:______________ Date:________________
Use your Human Biology text book 232-235 and the info below to help you!
Background: Skeletal muscle is specialized for contraction. A muscle fiber that runs the entire length of
a skeletal muscle is actually a single cell. Each muscle fiber is filled with myofilaments made up of thick
(myosin) and thin (actin) filaments. The myofilaments are arranged into sarcomeres which shorten
during muscle contraction.
The shortening of the sarcomere occurs due to the interaction between actin and myosin. In a resting
muscle fiber, tropomyosin, a regulatory protein, blocks the myosin binding sites on actin. In a
contracting muscle fiber calcium binds to troponin which moves tropomyosin away from the myosin
binding sites on actin. Myosin can then bind to actin forming a cross bridge. Myosin uses ATP in order
to perform a power stroke and pull the thin filament towards the M line. Myosin then binds to a new
ATP molecule in order to break the cross bridge between itself and actin. If calcium is still present
myosin then forms a new cross bridge between itself and actin and contraction continues.
Draw a resting sarcomere and label the following:
thick filament (Myosin), thin filament(Actin), Z line, I band, A band, and H zone
Next draw a shortened sarcomere and describe how the shortening of the sarcomere affects the I
band, A band and H zone
Answer the questions on the back----------------------------------------------------
1. What is the importance of Ca2+ in muscle contraction?
2. What type of cell tells the muscle fibers to contract?
3. How is the release of Ca2+ into the Sarcomere like turning on a light?
4. Why is Muscle contraction sometimes called the “Sliding filament model”?
5. How can Myosin be compared to people in a Tug-0-War?
6. How can Actin be compared to the rope in a Tug-0-War?
7. Describe the functions of the following Muscle Micro structures ( see page 232 table 12.1):
A. Sarcoplasmic reticulum –
B. T tubule –
C. Myoglobin –
D. Myofibril –
E.
Myofilaments -
LAB: Observing muscle fiber contraction
Form groups of 3-4 students and obtain the following:
 Microscope
 Glass teasing needles
 6 slides and coverslips
 Ruler
 Muscle fibers
 Glass dish
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Solutions:
o ATP and K+ and Mg2+
o ATP only
o K+ and Mg2+ only
Deionized (DI) water
Marking pen
We will be using glycerinated muscle. Glycerol denatures troponin and tropomyosin so calcium is not
needed to initiate contraction. ATP is still needed for the power stroke and cross bridge cycling and
the ATPase (puts ADP & P together) on myosin needs K+ and Mg2+ salts to function properly.
Based on this information come up with hypotheses about what will occur in terms of contraction
and relaxation when the muscle fibers are incubated with ATP, K+ and Mg2+salts or both.
ATP only
ATP and salts
Salts only
1. Using the glass teasing needles tease the muscle segment you obtained from your TA into its
separate fibers. The goal is to get a single muscle fiber or cell. Transfer one of the fibers onto a
microscope slide and cover it with a coverslip.
2. Examine the fiber under the microscope starting on low magnification and move to a higher
magnification in order to observe the striations in the resting muscle fiber.
3. Rinse three slides in DI water and label them A, B and C. On each slide place 3 muscle fibers in
parallel on the slide and measure and their lengths by placing the slide on top of the ruler
under the microscope. Record the lengths of the resting fibers.
4. WHILE LOOKING UNDER A MICROSCOPE On slide A flood the fibers with solution A, on
slide B flood the fibers with solution B and on slide C flood the fibers with solution C. Watch
the reaction of the fibers after flooding them and then re-measure and re-record their lengths.
Calculate % contraction using the following formulas:
Degree of contraction (mm) = Initial length (mm) – Contracted length (mm)
% contraction = Degree of contraction (mm) X 100
Initial length (mm)
Muscle Fiber 1
Muscle Fiber 2
Muscle Fiber 3
AVERAGE
ATP and salts
Initial length (mm)
Contracted length (mm)
% contraction
ATP only
Initial length (mm)
Contracted length (mm)
% contraction
Salts only
Initial length (mm)
Contracted length (mm)
% contraction
Muscle Fiber without solutions
Muscle fiber after ATP & Salt solution
Explain what your results are showing you:
In our notes we said calcium is needed for a muscle to contract. Why is this not so in this lab?
Why don’t the muscle fibers relax in this lab situation?
What overall conclusion can you make of ATP and salts in the contraction process?