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Notes to Instructors
Chapter 50 Sensory and Motor Mechanisms
What is the focus of these activities?
Skeletal muscles are made up of bundles of muscle fibers (muscle cells). Each muscle
What are the particular activities designed to do?
Activity 50.1 How does sarcomere structure affect muscle function?
Activity 50.2 What would happen if you modified particular aspects of muscle
function?
These questions are designed to have students test their understanding of muscle function
Answers
Activity 50.1 How does sarcomere structure affect muscle
function?
Working in groups of three or four, use playdough or cutout pieces of paper to construct a
Notes to Instructors 331
When developing and explaining your model, be sure to include definitions or
descriptions of the following terms and structures.
332 Activity 50.1
thick filament
thin filament
actin
myosin
Z line
H zone
cross-bridges
sarcomere
contracted length
extended length
troponin complex
tropomyosin
Ca⫹⫹
Alternatively, assign at least three people to model different parts of the sarcomere and
demonstrate its action in muscle contraction. Assign a different role to each person, and
assign specific actions to each role.
When you have completed your model, explain it to another student or to your instructor.
Use your understanding of the model to answer the questions.
1. How are sarcomeres arranged within muscle fibers (and therefore within muscles)?
Sarcomeres are arranged linearly in myofibrils. Myofibrils are arranged linearly in muscle
2. Describe the sliding-filament model of muscle action. For example, what
interactions power the movement of filaments in association with each other?
Include troponin complex, Ca⫹⫹ binding sites, tropomyosin, myosin binding site,
and actin in your discussion.
Refer to Figures 50.27, 50.28, and 50.29. In the sarcomere, thick myosin filaments
3. How do your answers to questions 1 and 2 help explain why, at maximum
contraction, muscle measures 70% of its extended (uncontracted) length?
During contraction, the actin filaments are pulled toward the center of the sarcomere
4. Review Figure 50.30 and the associated text in Campbell Biology, 9th edition.
Then answer the next questions.
a. Acetylcholine, an excitatory neurotransmitter, is the neurotransmitter released
into the neuromuscular junction. This release can trigger an action potential
along the length of the muscle membrane. Describe the process involved.
A motor neuron synapses with a muscle fiber at a neuromuscular junction. If an
b. If acetylcholine always produces an excitatory response in the neuromuscular
junction, how can we regulate which muscles (in the arm, for example) are
contracted and which are extended at any given time? For example, how can we
bend the arm only partially?
Muscles are arranged into motor units. The contraction of each motor unit is
c. Severe calcium deficiency can lead to a reduction in bone mass. It can also have
serious effects on the functioning of the nervous system and on the action of
muscles. Explain what role(s) calcium plays in the activity of muscles.
As noted, it is the release of Ca2⫹from the sarcoplasmic reticulum that ultimately
Activity 50.1 333
50.1 Test Your Understanding
The level of musculature in various organs is directly related to the amount of work done
by that organ or organ part.
334 Activity 50.1
A few examples are noted below.
a. Give two examples of different parts
of organ systems where this is
evident. (Use two different organ
systems.)
b. For each example, explain the
advantage of the difference in
musculature for each system.
Examples in the circulatory system include:
This is advantageous because:
An example in the digestive system:
• The stomach wall is more muscular
than the esophagus.
This is advantageous because the stomach
acts to churn the ingested food, which
assists in breaking it into smaller pieces
(aided by action of stomach acid). The
Activity 50.2 335
Activity 50.2 What would happen if you modified particular
aspects of muscle function?
Test your understanding of muscle function by asking yourself what would happen if a
specific part was damaged. What would the system still be able to do? What would it be
unable to do?
In each of the following situations, indicate whether the proposed answers are true or
false. If false, indicate why it is false.
1. A toxin in a newly discovered bacterial strain causes irreversible inactivation of the
acetylcholine receptor. If you were infected with this organism, the symptoms would
include:
2. In an experiment, a physiologist was able to destroy some but not all of the motor
neurons to a specific skeletal muscle. Given what you know about motor units, what
is the result of this action?
T/Fa. There is no effect because motor neurons are redundant; multiple neurons
innervate each muscle fiber as backup.
Explain:
False—Each motor unit is innervated by a specific motor neuron (and its
branches). As a result, if you destroyed its motor neuron, the motor unit would
be unable to respond.
