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CHAPTER
9
Muscles and Muscle Tissue
Objectives
Overview of Muscle Tissues
2. List four important functions of muscle tissue.
Skeletal Muscle
5. Describe the sliding filament model of muscle contraction.
6. Explain how muscle fibers are stimulated to contract by describing events that occur at
the neuromuscular junction.
8. Follow the events of excitation-contraction coupling that lead to cross bridge activity.
9. Define motor unit and muscle twitch, and describe the events occurring during the three
phases of a muscle twitch.
14. Describe factors that influence the force, velocity, and duration of skeletal muscle
contraction.
15. Describe three types of skeletal muscle fibers and explain the relative value of each type.
Smooth Muscle
17. Compare the gross and microscopic anatomy of smooth muscle cells to that of skeletal
muscle cells.
19. Distinguish between unitary and multi unit smooth muscle structurally and functionally.
Developmental Aspects of Muscles
20. Describe embryonic development of muscle tissues and the changes that occur in skeletal
muscles with age.
Suggested Lecture Outline
I. Overview of Muscle Tissues (pp. 276–278; Table 9.1)
A. Types of Muscle Tissue (p. 277; Table 9.1)
1. Skeletal muscle is associated with the bony skeleton and consists of large cells that
bear striations and are under voluntary control.
B. Special Characteristics of Muscle Tissue (p. 277)
1. Excitability, or responsiveness, is the ability to receive and respond to a stimulus.
C. Muscle Functions (pp. 277–278; Table 9.1)
1. Muscles produce movement by acting on the bones of the skeleton, pumping blood, or
propelling substances throughout hollow organ systems.
2. Muscles aid in maintaining posture by adjusting the position of the body with respect
to gravity.
II. Skeletal Muscle (pp. 278–305; Figs. 9.1–9.24; Tables 9.1–9.3)
A. Gross Anatomy of Skeletal Muscle (pp. 278–279; Fig. 9.1; Tables 9.1, 9.3)
1. Each muscle has a nerve and blood supply that allows neural control and ensures
adequate nutrient delivery and waste removal.
2. Connective tissue sheaths are found at various structural levels of each muscle:
endomysium surrounds each muscle fiber, perimysium surrounds groups of muscle
fibers, and epimysium surrounds whole muscles.
B. Microscopic Anatomy of a Skeletal Muscle Fiber (pp. 279–285; Figs. 9.2–9.5; Tables
9.1, 9.3)
1. Skeletal muscle fibers are large, cylindrical cells with multiple nuclei beneath the
sarcolemma, or plasma membrane.
4. Striations are due to a repeating series of dark A bands and light I bands.
5. Myofilaments make up the myofibrils and consist of thick and thin filaments.
6. Striations, alternating dark A bands and light I bands, extend the length of each
myofibril.
a. Each A band has a lighter central region, the H zone, which is bisected vertically by
an M line.
7. There are two types of myofilaments in muscle cells: thick filaments composed of
bundles of myosin, and thin filaments composed of strands of actin.
8. Thin filaments also have a set of regulatory proteins: tropomyosin, that wrap around
actin filaments, stabilizing it and blocking myosin binding sites; and troponin, which
binds to both actin and tropomyosin, and binds calcium ions.
9. The sarcoplasmic reticulum, a smooth endoplasmic reticulum that regulates the avail-
ability of calcium ions, surrounds each myofibril, and forms terminal cisterns at the
C. The sliding filament model of muscle contraction states that during contraction, the thin
filaments slide past the thick filaments. Overlap between the myofilaments increases and
the sarcomere shortens (p. 285; Fig. 9.6).
D. Physiology of Skeletal Muscle Fibers (pp. 285–293; Figs. 9.7–9.12; Table 9.1)
1. The neuromuscular junction is a connection between an axon terminal of a somatic
5. Repolarization restores the resting polarity to the sarcolemma and is accomplished by
diffusion of potassium ions out of the cell.
a. During repolarization, the muscle cell is in a refractory period and may not be
depolarized until repolarization is complete.
6. Excitation-contraction coupling is the sequence of events by which an action potential
on the sarcolemma results in the sliding of the myofilaments.
a. A nerve impulse reaches the axon terminal, causing the release of ACh to the
synaptic cleft.
b. ACh binds to ACh receptors in the sarcolemma, and a net influx of sodium ions
causes the generation of an end plate potential.
E. Contraction of a Skeletal Muscle (pp. 293–298; Figs. 9.13–9.18)
1. A motor unit consists of a motor neuron and all the muscle fibers it innervates. It is
smaller in muscles that exhibit fine control.
2. The muscle twitch is the response of a muscle to a single action potential on its motor
neuron, and has three phases: the latent period, corresponding to the lag between
contraction to become sustained, a condition called tetanus.
b. Multiple motor unit summation (recruitment) involves the response of a muscle to
increasing stimulus voltage: smaller stimuli result in contraction of the smallest
motor units, and as voltage increases, larger, more forceful motor units respond,
leading to progressively greater contractile force.
6. Isometric contractions result in increases in muscle tension, but no lengthening or
shortening of the muscle occurs, and often are used to maintain posture or joint
stability while movement occurs at other joints.
F. Muscle Metabolism (pp. 298–301; Figs. 9.19–9.20)
1. Muscles contain very little stored ATP, and consumed ATP is replenished rapidly
through phosphorylation by creatine phosphate, anaerobic glycolysis, and aerobic
respiration.
2. As muscle metabolism transitions to meet higher demand during vigorous exercise,
3. As stored ATP and creatine phosphate are consumed, ATP is produced by breaking
down blood glucose or stored glycogen in glycolysis, an anaerobic pathway that pre-
cedes both aerobic and anaerobic respiration. If adequate oxygen is not available to
4. Aerobic respiration provides most of the ATP during light to moderate activity,
includes glycolysis, along with reactions that occur within the mitochondria, and
produces 32 ATP per glucose, as well as water, and CO2, which will be lost from the
body in the lungs.
6. Muscle fatigue is the physiological inability to contract, and results from ionic
imbalances that interfere with normal excitation-contraction coupling.
7. Excess postexercise oxygen consumption (EPOC) is the extra oxygen the body
8. Muscle activity produces excess energy that is lost from the body as heat: excess
body heat can be lost through sweating and radiant heat loss from skin, while heat
production through shivering can be used to warm the body when it is too cold.
G. Force of Muscle Contraction (pp. 301–302; Figs. 9.21–9.22)
1. As the number of muscle fibers stimulated increases, force of contraction increases.
4. The length-tension relationship optimizes the overlap between the thick and thin
filaments that produces optimal contraction.
H. Velocity and Duration of Contraction (pp. 302–304; Figs. 9.23–9.24; Tables
9.2–9.3)
1. There are three muscle fiber types: slow oxidative fibers, fast glycolytic fibers, and
fast oxidative fibers.
a. Slow oxidative fibers contract slowly, rely mostly on aerobic respiration, and are
highly fatigue resistant.
2. All muscles have varying amounts of all fiber types and, while the proportion of each
type is a genetically influenced trait, that proportion can be modified by specific types
of exercise.
3. As the load on a muscle increases, velocity and duration of contraction decreases.
4. Recruitment of additional motor units increases velocity and duration of contraction.
I. Adaptations to Exercise (pp. 304–305)
1. Aerobic exercise promotes an increase in capillary penetration, the number of mito-
chondria, and synthesis of myoglobin, leading to higher efficiency and endurance,
III. Smooth Muscle (pp. 305–311; Figs. 9.25–9.28; Table 9.3)
A. Microscopic Structure of Smooth Muscle Fibers (pp. 305–307; Figs. 9.25–9.27;
Table 9.3)
3. Contraction of the opposing layers of muscle leads to a rhythmic form of contraction,
called peristalsis, which propels substances through the organs.
4. Smooth muscle lacks neuromuscular junctions, but has varicosities: numerous
bulbous swellings that release neurotransmitters to a wide synaptic cleft.
5. Smooth muscle cells have a less developed sarcoplasmic reticulum, sequestering large
amounts of calcium in extracellular fluid within caveolae in the cell membrane.
B. Contraction of Smooth Muscle (pp. 307–309; Fig. 9.28; Table 9.3)
1. Mechanism of Contraction
a. Smooth muscle fibers exhibit slow, synchronized contractions due to electrical
coupling by gap junctions.
2. Regulation of Contraction
a. Autonomic nerve endings release either acetylcholine or norepinephrine, which
may result in excitation of certain groups of smooth muscle cells, and inhibition of
others.
3. Special Features of Smooth Muscle Contraction
a. Smooth muscle initially contracts when stretched, but contraction is brief, and then
the cells relax to accommodate the stretch.
b. Because the muscle filaments have an irregular overlapping pattern, smooth muscle
C. Types of Smooth Muscle (p. 309)
1. Unitary smooth muscle, called visceral muscle, is the most common type of smooth
muscle. It contracts rhythmically as a unit, is electrically coupled by gap junctions,
and exhibits spontaneous action potentials.
contractions.
IV. Developmental Aspects of Muscles (pp. 312–313, 315; Fig. 9.29)
A. Nearly all muscle tissue develops from specialized mesodermal cells called myoblasts
(p. 312).
B. Skeletal muscle fibers form through the fusion of several myoblasts, and are actively
contracting by week 7 of fetal development (p. 312; Fig. 9.29).
F. Muscular dystrophy is characterized by atrophy and degeneration of muscle tissue.
Enlargement of muscles is due to fat and connective tissue deposit (pp. 312, 315).
Cross References
Additional information on topics covered in Chapter 9 can be found in the chapters listed below.
1. Chapter 2: ATP; ions
2. Chapter 3: General cellular structural components; membrane transport; microfilaments;
gap junctions; membrane potentials
6. Chapter 10: Skeletal muscles of the body; interaction between muscle and bones
7. Chapter 11: General structure and function of synapses and neurotransmitters
8. Chapter 13: Sensory receptors located in skeletal muscle; motor neurons of the peripheral
nervous system and the neuromuscular junction
Lecture Hints
1. Point out that extend is a root of the word extensibility, indicating lengthening, or stretch-
2. Because the prefixes endo-, epi-, and peri- are often used in anatomical terminology,
emphasize the meanings and indicate that students will see these again.
3. Clearly describe the stepwise progression through related terms myofilament, myofibril,
4. In the description of the sliding filament mechanism, present an animation showing
6. Emphasize that graded muscle contraction is achieved by increasing the frequency of
stimulation of motor units or increasing the number of motor units activated.
7. As a way of introducing isometric and isotonic contractions, ask the class if muscle
contraction always results in movement. Whatever their answer, illustrate the point by
8. To illustrate length-tension relationships, ask the class to comment on the amount of
9. When explaining the differences between slow and fast (oxidative) fibers, it is helpful
10. Explain that all muscle types contain actin and myosin myofilaments, but that the
arrangement (in part) accounts for the structural and functional differences.
11. Be sure to inform students that the terms striated and skeletal muscle are interchangeable,
and that although cardiac muscle is striated, the term striated should not be used as a
name for cardiac muscle.
Activities/Demonstrations
1. Audiovisual materials are listed in the Multimedia in the Classroom and Lab section of
this Instructor Guide (p. 387).
2. Demonstrate muscle contraction using a simple myograph or kymograph apparatus and
3. Ask students to demonstrate examples of isometric and isotonic contractions and to
4. Set up a microscope with a slide of a motor unit for class viewing.
6. Use an articulated skeleton to point out various origins and insertions; then ask students
to specify the resulting movement.
7. Pick apart a piece of cooked chicken breast to demonstrate individual fascicles.
8. Obtain a 3-D model of a sarcomere to exhibit tubules and myofibrils.
10. Stress the importance of extracellular calcium ions to smooth muscle contraction, and
distinguish this from skeletal muscle, which is more reliant on stored intracellular
calcium ions.
Critical Thinking/Discussion Topics
1. Why is it more beneficial for a person stranded in the cold to keep moving and exercising
rather than remaining inactive?
2. Muscles that are immobilized for long periods of time, as with a cast, frequently get
smaller. Why? What is necessary to revitalize them?
6. Why do athletes “warm up” before a competitive event? Would you expect the warm-up
period to make contraction more or less efficient? Why?
7. Visit a local gym frequented by body builders. Obtain information on the procedures
used to build muscle mass and an explanation of how those procedures accomplish that
goal.
8. If the number of myosin heads were doubled, what would be the effect on force
production? ATP consumption?
Library Research Topics
1. Why have muscle cells “lost” their ability to regenerate? What current research is being
done in this area?
5. Explore the current theories for the etiology of muscular dystrophy.
6. What is the current status of the sliding filament model of muscle contraction? Do we
know all there is to know?
8. Describe several metabolic diseases of muscle (usually due to an enzyme or enzyme
group deficiency).
10. Define the term myositis. What causative agents could result in this form of muscle
disease?
List of Figures and Tables
All of the figures in the main text are available in JPEG format, PPT, and labeled & unlabeled
format on the Instructor Resource DVD. All of the figures and tables will also be available in
Transparency Acetate format. For more information, go to www.pearsonhighered.com/educator.
Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium,
perimysium, and endomysium.
Figure 9.2 Microscopic anatomy of a skeletal muscle fiber.
Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to
myofibrils of skeletal muscle.
Figure 9.6 Sliding filament model of contraction.
Figure 9.7 The phases leading to muscle fiber contraction.
Figure 9.8 Events at the Neuromuscular Junction.
Figure 9.9 Summary of events in the generation and propagation of an action
potential in a skeletal muscle fiber.
Figure 9.16 Relationship between stimulus intensity (graph at top) and muscle
tension (tracing below).
Figure 9.21 Factors that increase the force of skeletal muscle contraction.
Figure 9.22 Length-tension relationships of sarcomeres in skeletal muscles.
Figure 9.23 Factors influencing velocity and duration of skeletal muscle
contraction.
Figure 9.24 Influence of load on duration and velocity of muscle contraction.
Figure 9.25 Arrangement of smooth muscle in the walls of hollow organs.
Figure 9.26 Innervation of smooth muscle.
Figure 9.27 Intermediate filaments and dense bodies of smooth muscle fibers
harness the pull generated by myosin cross bridges.
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix H of the main text.
Short Answer Essay Questions
15. The functions are: excitability—the ability to receive and respond to a stimulus;
16. a. In direct attachment, the epimysium of the muscle is fused to the periosteum of a bone,
and in indirect attachment, the muscle connective tissue sheaths extend beyond the
17. a. A sarcomere is the region of a myofibril between two successive Z-lines and is the
smallest contractile unit of a muscle cell. The myofilaments are within the sarcomere.
18. AChE destroys the ACh after it is released. This prevents continued muscle fiber contrac-
tion in the absence of additional stimulation. (p. 286)
19. A slight (but smooth) contraction involves rapid stimulation of a few motor units and
20. Excitation-contraction coupling is the sequence of events by which an action potential trav-
eling along the sarcolemma leads to the contraction of a muscle fiber. (p. 290; Fig. 9.11)
21. A motor unit is the motor neuron and all the muscle fibers it controls. (p. 293; Fig. 9.13)
23. False, most body muscles contain a mixture of fiber types. This allows muscles to exhibit
24. Muscle fatigue is the physiological inability to contract. It occurs due to ATP deficit,
lactic acid buildup, and ionic imbalance. (p. 300)
25. EPOC is excess postexercise oxygen consumption, and is defined as the additional
26. Smooth muscle is located within the walls of hollow organs, including blood vessels. It is
highly fatigue resistant, and tolerates stretch. These characteristics are essential because
Critical Thinking and Clinical Application Questions
1. Regular resistance exercise leads to increased muscle strength by causing muscle cells to
hypertrophy, or increase in size. The number of myofilaments increases in these muscles.
(p. 305)
2. The reason for the tightness is rigor mortis. The myosin cross bridges are “locked on” to
the actin because of the lack of ATP necessary for release. Peak rigidity occurs at 12
3. Chemical A would be a better muscle relaxant. By blocking binding of ACh to the motor
end plate, neural stimulation of the cell is blocked, and the muscle cell cannot depolarize.
4. The calcium actually binds to troponin, which changes shape and moves the tropomyosin
to expose the myosin head binding sites. (p. 289)
Suggested Readings
Andersen, J. L., and P. Aagaard. “Effects of Strength Training on Muscle Fiber Types and
Size; Consequences for Athletes Training for High-Intensity Sport.” Scandinavian
Journal of Medicine & Science in Sports 20 (Suppl. 2) (Oct. 2010): 32–38.
Bers, D., and M. Fill. “Coordinated Feet and the Dance of Ryanodine Receptors.” Science
281 (5378) (Aug. 1998): 790–791.
Burniston, Jatin G., and Eric P. Hoffman. “Proteomic Responses of Skeletal and Cardiac
Muscle to Exercise.” Expert Review of Proteomics 8 (3) (June 2011): 361–377.
Kang, J. S., and R. S. Krauss. “Muscle Stem Cells in Developmental and Regenerative
Myogenesis.” Current Opinion in Clinical Nutrition and Metabolic Care 13 (3) (May
2010): 243–248.
Lin, Weichun, et al. “Distinct Roles of Nerve and Muscle in Postsynaptic Differentiation of
the Neuromuscular Synapse.” Nature 410 (6832) (April 2001): 1057–1064.
(2006): 93–97.
Somiyo, A. P., and A. V. Somiyo. “Signal Transduction and Regulation in Smooth Muscle.”
Nature 372 (6503) (Nov. 1994): 231–236.
Spudich, James A. “The Myosin Swinging Cross-Bridge Model.” Nature Reviews: Molecular
Cell Biology 2 (5) (May 2001): 387–392.
