Adv Physiol Educ 33: 297–301, 2009;
doi:10.1152/advan.00036.2009.
How We Teach
Hand-held model of a sarcomere to illustrate the sliding filament mechanism
in muscle contraction
Karnyupha Jittivadhna, Pintip Ruenwongsa, and Bhinyo Panijpan
Institute for Innovative Learning, Mahidol University, Bangkok, Thailand
Submitted 4 May 2009; accepted in final form 25 August 2009
myofilament lattice; contraction mechanism; actin; myosin; learning
tool
on muscle physiology are presented to
students in many different forms, such as in textbooks, with
CD-ROMs, and by tutorials on the internet. Three-dimensional
(3-D) anatomic models have also been developed to demonstrate the workings of the musculoskeletal system. However, to
our knowledge, there are no publications in science educational
journals on 3-D pedagogical models that illustrate the mechanism operating at the cellular level. Thus, the majority of basic
structural illustrations of muscle contraction are two-dimensional (2-D) schematic representations, static computer models, and simple graphic animations.
In our teaching of first- and second-year undergraduates in
biology and life sciences, we have found that most of them
harbor misconceptions that are attributable to previous exposures to 2-D illustrations. These schematic representations
usually show the typical view of the sarcomere in longitudinal
sections with or without the transverse view, making a large
number of students consider the muscle sarcomere as a thin flat
sheet. Moreover, some students even see muscle contraction as
that of exactly one sarcomere. Another alarming finding is that
in drawing the sliding-filament model, sarcomere shortening is
quite commonly portrayed as a decrease of the volume of the
whole sarcomere because the transverse thickening is ignored.
The latter representation cannot capture the more correct sarcomere bulging upon its shortening. Morton and others (10)
EDUCATIONAL MEDIA
Address for reprint requests and other correspondence: B. Panijpan, Institute
for Innovative Learning, Mahidol Univ., 272 Rama 6 Rd., Rajathevi, Bangkok
10400, Thailand (e-mail: [email protected]).
have recently reported some common misconceptions, e.g., the
mechanic of muscle contraction among sport exercise science
students. The prevalence of misconceptions in students from
primary to tertiary levels indicates that the generally available
classroom teaching materials do not appear to achieve the
desired outcomes. Thus, it would be desirable to have learning
tools to remedy the situation.
As part of an effort to introduce beginning students to
fundamental concepts in muscle contraction, we developed a
simple, inexpensive, movable 3-D model of the sarcomere to
facilitate the understanding of the sliding-filament mechanism
and sarcomere transverse expansion. Here, we describe how to
assemble the model using common and inexpensive materials
and ways to show its molecular physiological details for
different target audience students. The educational efficacy of
the model is also discussed.
METHODS
Assembling the model. A 3-D model for teaching the mechanics of
muscle contraction (Fig. 1A) was designed to overcome the limitations
of 2-D textbook or graphic illustrations in terms of the end-on
arrangement of the filament lattice of the sarcomere, the striation
change accompanying the change in sarcomere length, and the relatively constant volume of a sarcomere throughout this change in
length. To build a model of one sarcomeric unit, the following
materials are needed: 3 circle-cut clear acrylic plates of 11.5 cm in
diameter, 48 balloon sticks of 11 cm long each, 48 pieces of 1-in.
screws that fit the balloon sticks, 14 balloon sticks of 8 cm long, 14
stiff wires that snugly fit the 8-cm balloon sticks (12 cm long), 7
plastic pipes (15 mm ⫻ 18 cm), 14 pieces of foam rubber (5 ⫻ 5 cm),
14 plastic spring key chains, and 6 strips of clear plastic sheetings (6
⫻ 33 cm). These materials are shown in Fig. 1C.
The construction of the model starts with the two end plates
representing the Z disk. Figure 2 shows a filament lattice of the
vertebrate striated muscle in the overlap region of the sarcomere (9).
Thick filaments are represented by solid circles and thin filaments are
represented by smaller open circles (Fig. 2). To prepare the Z disk
structure, the arrangement of the two filaments is marked on two
circle-cut clear plastic plates together with seven extra dots laid
closely to the thick filament marks. Holes are drilled through these
marks on the 2 end plates, giving rise to 7 holes for the stiff wires and
7 spring holes at which the coil-spring structures representing titin are
inserted and 24 screw holes at which the balloon sticks of 11 cm long
representing the thin filaments are finally secured. To prepare the M
line structure, the hexagonal arrangement of the thick filament is
marked on the third circle-cut clear plastic plate. Drilling through the
marked positions provides seven holes through which the plastic pipes
representing the thick filaments are inserted and fixed in place with a
light coat of Super Glue.
Each coil-spring structure representing titin is made from a
plastic coil, an 8-cm-long balloon stick, thick and stiff wires that
snugly fit inside the hollow balloon stick, and foam rubber.
Fabrication of the titin begins by wrapping the foam rubber around
one end of the balloon stick associated with an end of the plastic
1043-4046/09 $8.00 Copyright © 2009 The American Physiological Society
297
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Jittivadhna K, Ruenwongsa P, Panijpan B. Hand-held model
of a sarcomere to illustrate the sliding filament mechanism in
muscle contraction. Adv Physiol Educ 33: 297–301, 2009; doi:
10.1152/advan.00036.2009.—From our teaching of the contractile
unit of the striated muscle, we have found limitations in using
textbook illustrations of sarcomere structure and its related dynamic
molecular physiological details. A hand-held model of a striated
muscle sarcomere made from common items has thus been made by
us to enhance students’ understanding of the sliding filament mechanism as well as their appreciation of the spatial arrangements of the
thick and thin filaments. The model proves to be quite efficacious in
dispelling some alternative conceptions held by students exposed
previously only to two-dimensional textbook illustrations and computer graphic displays. More importantly, after being taught by this
hand-held device, electronmicrographic features of the A and I bands,
H zone, and Z disk can be easily correlated by the students to the
positions of the thick and thin elements relatively sliding past one
another. The transverse expansion of the sarcomere and the constancy
of its volume upon contraction are also demonstrable by the model.
How We Teach
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MOVABLE THREE-DIMENSIONAL MODEL OF A SARCOMERE
spring and then inserting the wrapping into the pipe so that the
balloon stick is embedded and the spring projects outward. The
stiff wire as the inner core of the coil spring structure is inserted
into the hollow of the balloon stick and fixed to the end plate at the
drilled holes marked for the thick filament by flattening its outer
end. The spring responsible for the elasticity of the model extends
from the end of the thick filament and is inserted into its hole on
the end plate. It is important that the balloon stick embedded in the
pipe should be sized to closely accommodate the stiff wires so as
to keep the stiff wires moving straight when the sliding of myofilaments is demonstrated. Fixing and inserting all components
should be made from the inner part to the outer part of model.
Strips of clear plastic sheetings representing the side wall of each
sarcomere are screw fixed to the end plates and used to demon-
Fig. 2. Diagram of the filament lattice of vertebrate striated muscle in the
overlap region of the sarcomeres (9). Thick filaments are represented by solid
circles; thin filaments are represented by smaller open circles.
strate the transverse dimensional change of the sarcomere upon a
change in length.
Student use of the model. Our sarcomere model has been used for
instructing students from secondary to tertiary levels. Apart from
classroom usage, the model has been displayed in the university’s
Open House exhibitions on science and technology, giving secondary students the opportunity to participate in the model handling activity. All participants were organized into groups of two
to four students during 45 min of the learning period. Generally,
students were first engaged with the idea that all skeletal muscles
operate by shortening. Students were then encouraged to discuss
other ways that the muscle can perform work. Verbal answers were
in the form of arguments and defenses. After the discussion group,
student volunteers, having previously been exposed to textbook
presentations of sarcomere contraction during their earlier school
years, were checked for their prior knowledge before starting the
model handling activity (pretest). All students were then allowed to
manipulate the model themselves in groups under the instructor’s
guidance, gave reasoned arguments about some structural attributes of the model, and corrected their misunderstandings. At
this stage, only students who took the pretest survey were again
given another survey (posttest). During the following instructional
period, students were made to realize that the contractile units of
the muscle scale from the sarcomere level up to the whole muscle
fiber level and again up to the level of an entire recruitment group
of motor units. All of the sarcomeres in such a group are assumed
to operate homogeneously, with similar activation, lengths, and
velocity (13). A single sarcomere can thus be construed to represent groups of a similar motor unit type.
Evaluating the model. To assess the educational potential of the
model, a seven-item multiple-choice questionnaire, incorporating
some common student misconceptions from the authors’ experiences,
was administered as the pretest and posttest, with both containing the
same questions. These are questions on the contractile machinery of a
sarcomere, the occurrence in a sarcomere during a normal contraction
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Fig. 1. The sarcomere model. A: longitudinal features. B: end-on view. C: component parts for fabricating the model.
How We Teach
MOVABLE THREE-DIMENSIONAL MODEL OF A SARCOMERE
and relaxation of the muscle, and the volume change in sarcomere
shortening (Fig. 3). Students participated in this study voluntarily and
anonymously, and each student was provided with a written consent
form. Each student answered the questions individually, but they were
allowed to discuss the material as a group of two to four students.
During the pretest, students were also allowed to consult the textbooks
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provided for them. To avoid the problem of interference between the
pretest and posttest, students had to return the pretest sheet to the
instructor before model handling activities, and no corrections were
made in the meantime. Feedback from the students was collected at
the end of the activity session by means of informal interviews to
learn about the usefulness of the model.
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Fig. 3. Questions and responses to the pretest and posttest.
How We Teach
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RESULTS
Fig. 4. In the constant-volume sarcomere contraction, the redistributed volume
is shown by the widening of the midpoint radius of a bulge of the side wall.
Fig. 5. Models showing a repeating linear array of sarcomeres. A: two adjacent
sarcomeres connected through elastic cross-links. B: simultaneous shortening
of the sarcomere showing stretching of the elastic cross-links.
A repeating linear array of sarcomeres along a single
myofibril is presented to the students as a linear arrangement
of individual models connected through a zigzag pattern of
a spring that simulates the simplified elastic Z filaments
(Fig. 5A). The linker between two models is constructed
based on the ultrastructure of the Z disk featuring the
cross-link filaments, ␣-actinin Z filaments that maintain
diametrically opposed thin filaments (7).
Simultaneous shortening of the sarcomere can be demonstrated by having some students pushing together a series of
spring-linked model of sarcomeres. In this activity, students
will learn that during muscle contraction, the Z disks still form
the fixed points at the ends of the sarcomere to which the actin
filaments are attached, and this occurs by the stretching of
elastic Z filament during force production (Fig. 5B).
This model, used by us as a teaching tool for various groups
of large number of students, has proved to be durable since it
has been used for ⬎5 yr with ⬃600 students and does not
require elaborate maintenance. The most important aspect of
ensuring the physical durability of our model is the strength of
the plastic spring that is responsible for connecting the thick
filament with the Z disk structures and conferring much of the
elasticity.
Student perceptions of the model. Secondary and tertiary
students that participated in the model-handling activity responded positively to the model. The majority of students
remarked during the activity that they preferred the 3-D model
to static 2-D textbook presentations for the following three
reasons. The first is that the model, in a more concrete manner,
helps the students learn the relative longitudinal arrangement
of the myofilaments when the sarcomere is in the relaxed or
contracted position. The second is that when handled, even if
it is not exactly the same as the process in the muscle, it is
much easier to visualize the sliding filament and to appreciate
that the sarcomere maintains a constant volume throughout
changes in sarcomere length. Finally, this model of the sarcomere is novel to the students and facilitates learning in an
interesting and realistic way.
Efficacy of the model as a teaching tool. The model evaluation has so far involved 343 students (278 high school
students from public and private schools and 65 undergradu-
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Features and durability of the model. The 3-D model of the
sarcomere so designed is roughly cylindrical when stretched as
the side wall (made from strips of clear plastic sheetings)
straightens (Fig. 1A). This model has ordered arrangements of
the thick and thin filaments in a functional contractile unit of
the muscle between two Z disks. The order in contractile
assemblies is brought about by the sarcomeric Z disks and M
line structures. The M line in the middle is seen as a transverse
band that hemisects the sarcomere and is responsible for the
packing of the thick filaments, as described in the literature
(12). The thick filaments are arranged in register in the center
of the sarcomere, forming an A band and having twofold
rotation axes: flipping the picture of a sarcomere around an axis
by 180° yields an identical picture. The actin-containing thin
filaments are attached to the Z disks at either end of the
sarcomere organizing radially around each thick filament (4).
The end-on view of the sarcomere model shows one thick
filament surrounded by six thin filaments hexagonally arranged and one thin filament surrounded by three equidistant
thick filaments triangularly arranged (9) (Fig. 1B). Thus, the
thin filaments order themselves side by side on their own in
the I bands and then overlap with the thick filaments in
peripheral regions of the A band and terminate just before
the center of the sarcomere, leaving a gap of the H zone (4).
The model also includes an additional coil-spring structure,
simulating the giant protein titin (8), which provides the
backbone to the sarcomere. Titins in the A band region are
integral with the thick filament, whereas the titin part
between the end of the thick filament and Z disk is attached
to Z disks at the two ends of the model.
Sarcomere contraction can be demonstrated by pushing the
two Z disks inward (Fig. 4). During sarcomere shortening, the
thin filaments of the I band move toward the center of the thick
filaments of the A band. This leads to a reduction of the I band
and H zone, whereas the A band remains relatively the same.
The distance between the Z disk and the edge of the H zone
also remains constant at all stages during the process. Displaced volume during sarcomere shortening is represented by
the widening of the midpoint radius of a bulge of the side wall.
How We Teach
MOVABLE THREE-DIMENSIONAL MODEL OF A SARCOMERE
DISCUSSION
Our low-cost, home-made tangible sarcomere model provides a mechanical 3-D model of the sarcomere for learning the
mechanism by which a sarcomere contracts and returns to a
normal relaxed position. Changes in the banding pattern of the
model that accompany the changes in sarcomere length fit the
sliding filament model proposed independently by Andrew
Huxley and Rolf Niedergerke and by Hugh Huxley and Jean
Hanson in 1954 (2, 6). Their model has formed the basis for a
hypothesis known as the sliding filament theory of muscle
contraction. A caveat to note here is that in this activity, the
model handler actively pushes the Z disks; in actual muscle
cells, the relative motion of the Z disks as a result of the sliding
between the thick and thin filaments is an internal energydriven process during muscle contraction (4).
Upon contraction, each striated muscle structure changes its
length but maintains its volume almost perfectly, as seen in the
bulging of the flexed biceps. Consistent with this fact, the myofibril sarcomere thickens as it shortens and also appears to maintain a constant volume accounting for axial tension (1). This
mechanism requires the presence of a flexible boundary at the
surface of muscle fiber, that is, the sarcolemma, to serve as a
container for maintaining the constant volume but not for transmitting active tension; this is accomplished by means of the
intermolecular forces of the sarcoplasmic fluid (11). The volume
redistribution in the sarcomere, in reality, is shown by the thickening of the longitudinal section (1) and the change in packing
density of the filament lattice (3, 5). Our model suffers from some
shortcomings, such as the change in transverse spacings between
the filaments during sarcomere contraction/relaxation; instead, the
displaced volume in sarcomere shortening can only be seen by the
bulging of the midpoint of the side wall. Also, the closer spacings
of boundary thick filaments cannot be shown. Limitations such as
these need to be explicitly discussed with students whenever this
movable model is used.
If one wishes, the volume (V) of a sarcomere can be
reasonably determined from this model by the following equation: V ⫽ ␲ ⫻ r2 ⫻ l (cross-sectional area times sarcomere
length), where r is the radius of the myofibril cylinder (midpoint radius of an arc formed by the side walls) and l is the
length of sarcomere. Any larger segment of the myofilament
lattice can also reasonably be determined by this formula.
Incidentally, the energetics of a pressure-volume work at a
constant volume would be simplified by having one of the
variable constants.
Our 3-D hand-held sarcomere model has proven to be quite
efficacious in dispelling some alternative conceptions held by
students previously exposed to only 2-D textbook illustrations and
computer graphic displays. A large majority of students no longer
consider the sarcomere as a thin flat sheet, view muscle contraction as that of exactly one sarcomere, or assume that the myofilaments shorten during a contraction. This physical representation
can also capture the more correct sarcomere bulging upon its
shortening. This model is appropriate for frequent interactive
teaching with various groups of students, since it is easy to handle,
robust, and does not require elaborate maintenance.
In addition to the model above, we have supported our
students with a Thai language multimedia CD, which provides
information and explanations on various aspects of muscle
physiology and other actin-based macromolecular movements.
The multimedia CD also contains a FLASH animation simulating graphically the dynamism of the sarcomere, especially
the relative movement of the filaments and their “3-D” arrangements. We also included several types of questions to which
students have to respond. Answers and feedbacks are provided
where appropriate.
ACKNOWLEDGMENTS
The authors thank Pitakpong Kompudsa and Jiraporn Thanpaew (Institute
for Innovative Learning, Mahidol University) for the generosity in providing
technical support.
GRANTS
This work was supported by a graduate fellowship from the Tertiary
Education Commission, Ministry of Education (to K. Jittivadhna).
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ates from public universities). Survey questions and responses
are shown in Fig. 3. The findings from the pretest survey
showed that 158 (46.1%), 75 (21.9%), 197 (57.4%), 187
(54.5%), 21 (6.1%), 117 (34.1%), and 23 (6.7%) participating
students provided the correct answers to questions 1–7, respectively. To investigate the enhanced understanding, the participating students again completed a posttest survey during the
manipulation of model. From the posttest survey, 301 (87.8%),
329 (95.9%), 343 (100%), 343 (100%), 331 (96.5%), 306
(89.2%), and 323 (94.2%) participating students provided the
correct responses for questions 1–7.
The students’ ability to correctly answer the questions on the
posttest showed a dramatic improvement over their responses
on the pretest, which contained the same questions on the
arrangement of sarcomeres within a single myofibril, the contractile machinery of a sarcomere, the spatial arrangement of
the thick and thin filaments, the occurrence in a sarcomere
during a normal contraction and relaxation of the muscle, and
the displaced volume in sarcomere shortening.
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