Name: _______________________________________________
Date: __________ Day/Block: _____
Zig and Zag Dig Deeper into Muscles
Zig and Zag talk about what is going on when your muscles contract. Throughout this dialogue
you will have to stop and complete certain exercises relating to what you read. Don’t rush
through this. Take your time and re-read it if you have to.
Part I
Zig: (excitedly) Hey Zag. Why do you still have your nose in a textbook? It’s after school. Time
to reeee-lax and forget about all of our school related problems!
Zag: Sorry bud! It doesn’t work that way! Sometimes it takes a little work on our own time in
order to be successful in our classes…For example, Ms. Levendusky was talking about how
muscles contract today. I am still slightly confused so I’m going over the different parts she
covered today in class and how they work!
Zig: (eyes wide with disbelief) You mean there are MORE parts to the muscle…??? In ADDITION
to what we already covered? (said with exasperation) How complex does a muscle have to
be?!
Zag: It’s a GREAT thing that muscles are so complex! The complexity allows us to control how,
when and what part of the body we move at any given time. So it’s actually a good thing
that there are so many parts to study… So pop quiz! What do you remember from last class
about the organization of a muscle?! (excitedly) List the order big to small!
Zig: Fill in the blanks using your notes if you don’t remember. Well that’s easy! A
_______________(A) is composed of a _______________(B) is composed of a
_______________(C) is composed of a _______________.
On the diagram to the
right color (A) red,
(B) orange, (C)
yellow
© Levendusky 2014 -1
Name: _______________________________________________
Date: __________ Day/Block: _____
Zag: Great job Zig! Now we are going to start to dive into the structure of the myofibrils
because they are the key players in muscle contraction. To begin with, myofibrils are made
up of two kinds of protein filaments (strands) – myosin and actin.
Zig: (deep in thought) Myosin and actin huh…? Kind of like how I’m ACTIN out this dialogue?
(smiles and winks)
Zag: (rolling eyes) Oh brother….
Part II
Zig: But seriously I remember this! This is why muscles have that striated (striped)
look….because of
the way the actin
and myosin are
arranged in the
muscles. Actin
filaments are
thinner so they
create a lighter
stripe and Myosin
filaments are
thicker so they
create a darker
stripe. When you
group a few of these stripes together it’s called a sarcomere. (*specifically a sarcomere
begins at one “z disc” and ends at the next “z disc”*)
Label the following on the diagram above: Actin striation (stripe), Myosin striation (stripe),
muscle fiber, myofibril, sarcomere, endomysium
Part III
Zag: But we’ll come back to sarcomeres later. Let’s talk about how muscles contract which
allows us to move…This has to do with the nervous system!!!! They work together, in
beautiful harmony! There are these specific components of the nervous system called
motor neurons which are (make parentheses with fingers) “connected” to each muscle
fiber.
Zig: (looking confused) And do tell my friend what you mean by (making parentheses with
fingers) “connected”?
© Levendusky 2014 -2
Name: _______________________________________________
Date: __________ Day/Block: _____
Zag: (laughing) Well they aren’t actually connected physically. But they communicate with
each other and are “connected” in a sense. It’s kind of like when we talk on our cell
phones. We can talk and communicate with another person using the phone but we aren’t
actually connected to the phone or the other person.
Zig: (looking unimpressed) I’m not convinced… you’re telling me that our muscles can
communicate like the latest and greatest technology?
Zag: Yes! AND communicate better in fact! There are these chemicals called neurotransmitters
which are released across the space between a motor neuron and the muscle fiber. This
space is called a neuromuscular junction.
Zig: (face lighting up) Ohhhh! I get it! (dramatically) The motor neuron releases the
neurotransmitters…they valiantly cross the neuromuscular junction to reach the muscle
fiber and fulfill their destiny of causing that muscle fiber to contract!
On the diagram above, color the neuron
(A) green, the skeletal muscle fiber (B)
(B) blue, the neurotransmitters (C)
(C) purple, and the neuromuscular
(D) junction (D) grey.
© Levendusky 2014 -3
Name: _______________________________________________
Date: __________ Day/Block: _____
Zag: Awesome! NOW let’s look at what actually happens when the muscle contracts!
Zig: Alright! Let me recap what I remember! First we have these two proteins – myosin and
actin- which make up a myofibril which makes up a muscle fiber. These proteins are
stimulated by chemicals called neurotransmitters which are released by motor neurons!
Zag: Nice job Zig-meister! What happens is that the myosin and actin basically bind to each
other (grab onto one another) when the neurotransmitters stimulate them. When they
grab onto one another, they begin to pull each other closer.
Zig: (cringing) Oh dear…this sounds like a horrible soap opera drama…
Zag: (laughing) Ha ha…Focus Zig! This pulling action causes them to slide past one another and
results in the muscle fiber getting SHORTER. This explains how the muscle can PULL on the
bone or area of the body that it wants to move. (flexes his arm and pushes on his bicep)
This is also why when we contract or flex a muscle, it gets harder, because all of the
filaments are basically sliding or stacking on top of one another, causing them to bunch up.
(*Note: filaments DO NOT change in length, but sarcomeres DO change in length). This is
also called the sliding filament model.
Zig: Ooooo! Its like the cha-cha slide. (begins to dance) Sliiiiide to the left. Sliiiide to the right.
Contract right now.
Zag: Well I guess so…(smiling) Whatever helps you remember.
Zig: So then when the myofibril stops being stimulated, the filaments (sliding to the right)
sliiiiide back to their normal position and the entire muscle extends. Here! Let’s look at
what happens with this activity below!
© Levendusky 2014 -4
Name: _______________________________________________
Date: __________ Day/Block: _____
Muscle Contraction Activity
Diagram (a) shows a muscle fiber when it is relaxed. Diagram (b) shows a muscle fiber when it is contracted.
Using the diagrams above, record the following measurements and recorde the measurmenets in the tbale below.
Sarcomeres Section
Relaxed Sarcomeres
Measurement in cm
Sarcomere Length
Myosin filament length
Actin filament length
Sarcomeres length
Contracted Sarcomeres
Myosin filament length
Actin filament length
2. Now, graph the data on separate graph paper as a bar graph. Put length on the “Y” axis and labels on the “X” axis.
Make sure your data graph can TALK.
© Levendusky 2014 -5
Name: _______________________________________________
Date: __________ Day/Block: _____
Data analysis questions:
1. When the muscle contracts, do the actin and myosin filaments shorten? Support your answer with
data from the table above.
2. Explain how the sarcomere shortens when the parts that make it up don’t shorten.
© Levendusky 2014 -6
Name: _______________________________________________
Date: __________ Day/Block: _____
Teacher’s notes:
Student’s should first read through the dialogue as a pair. Then switch roles and read through the entire thing
without completing any exercises. Then, as an individual they should read through the dialogue a third
time, highlighting key information and completing the included activities.
Answer Keys:
Fill in the blank:
Muscle  Fascicle  Muscle Fiber  Myofibril
Supporting Material
Virtual Animation – Step by step animations of muscle contraction
http://www.wisc-online.com/Objects/ViewObject.aspx?ID=AP2904
© Levendusky 2014 -7