Anatomy and Physiology 2e is developed to meet the scope and sequence for a two-semester human anatomy and physiology course for life science and allied health majors. This chapter will examine the structure and function of the three types of muscle tissues.

1. MUSCLE TISSUE
Chapter 10
Unless otherwise noted, the images and text used in this PowerPoint are from:
Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O.,
Johnson, J. E., Womble, M., & DeSaix, P. (2022). Chapter 10: Muscle Tissue. In
Anatomy and Physiology 2e. OpenStax. https://openstax.org/books/anatomy-and-
physiology-2e/pages/10-introduction

2. Introduction of Muscle Tissue
• Muscle tissue is one of the
four primary tissue types of
the body along with
epithelial tissue, connective
tissue and nervous tissue.
• Muscle tissue accounts for
approximately 50% of an
individual’s body weight. Credit: Emmanuel Huybrechts/flickr

3. Chapter Objectives:
After this chapter, you will be able to:
▫ Explain the organization of muscle tissue
▫ Describe the function and structure of skeletal,
cardiac muscle, and smooth muscle
▫ Explain how muscles work with tendons to move the
body
▫ Describe how muscles contract and relax
▫ Define the process of muscle metabolism

4. Chapter Objectives Cont.:
After this chapter, you will be able to:
▫ Explain how the nervous system controls muscle
tension
▫ Relate the connections between exercise and muscle
performance
▫ Explain the development and regeneration of
muscle tissue

5. Section 10.1

6. Four Common
Characteristics of Muscle
•Excitability - ability to respond to a stimulus, which
may be delivered from a motor neuron or a
hormone
•Contractibility – can shorten as tension increases
•Extensibility – can be stretched or extended
•Elasticity – after being contracted or extended,
muscle can return to its original length

7. Types of Muscle Tissue
•Skeletal Muscle
•Smooth Muscle
•Cardiac Muscle
Credit: Regents of University of Michigan Medical School © 2012

8. Summary of Skeletal Muscle
•Found attached to
bones
•Cells are long and
cylindrical-shaped
•Voluntary
•Multi-nucleated cells
•Possesses striations but
no intercalated discs Credit: Regents of University of Michigan Medical School © 2012

9. Summary of Smooth Muscle
•Found in the walls of
hollow organs, blood
vessels, and in the
arrector pili muscles
•Cells are short and
spindle-shaped
•Single nucleus per cell
•Lack striations and
intercalated discs
Credit: Regents of University of Michigan Medical School © 2012

10. Summary of Cardiac Muscle
•Found only in the heart
•Cells are short and
branched
•Involuntary
•Usually a single nucleus
per cell
•Possesses striations and
intercalated discs Credit: Regents of University of Michigan Medical School © 2012

11. Summary of Muscle Tissue
Video Source: https://youtu.be/7t-DGxG09l8

12. Section 10.2

13. Functions of Skeletal Muscles
•Produces movements of skeleton
•Maintains body posture, position, and balance
•Supports soft tissues
•Protects entrances and exits
•Maintains body temperature
•Serves as nutrient reserve

14. Anatomy of Skeletal Muscles
•Highly vascularized
•Highly innervated
•Supported by
connective tissues
Credit:
https://commons.wikimedia.org/wiki/File:An
giogenesis_medical_animation_still.jpg

15. Connective Tissues
•Epimysium – around
entire muscle
•Perimysium – around
each muscle fascicle
•Endomysium – around
each muscle fiber
(a.k.a. muscle cell)
•Aponeurosis – a broad,
tendon-like sheet

16. Arrangement of a Muscle Fiber
•Sarcolemma
•Sarcoplasm
•Myofibril
•Myofilament
▫ Actin
▫ Myosin
•Sarcomere

17. Other Anatomical Structures
•Sarcoplasmic
reticulum
•Terminal cisternae
•T-tubule
•Triad

18. Summary of Skeletal Muscle Anatomy
Video Source: https://youtu.be/SCznFaTwTPE

19. Arrangement of a Sarcomere
•Myosin
•Actin
•Z disc
•I band
•A band
•H zone
•M line

20. Arrangement of Myosin
•Myosin molecule
•Tail
•Heads
▫ Actin-binding sites
•Flexible hinge region

21. Arrangement of Actin
•Globular actin
▫ Active site
•Filamentous actin
•Troponin
▫ Calcium ions
•Tropomyosin

22. The Neuromuscular Junction
•Because skeletal muscle cells are voluntary, they
cannot contract unless they receive a nerve
impulse from a motor neuron which excites the
muscle cell membrane.
•The area where a motor neuron innervates the
sarcolemma is called the neuromuscular
junction.

23. The Neuromuscular Junction Continued (1)

24. The Neuromuscular Junction Continued (2)
•Axon terminal
▫ Synaptic vesicles
▫ Acetylcholine (ACh)
•Synaptic cleft
•Motor end plate
•Chemical-gated ion
channels
•Voltage-gated ion
channels

25. The Neuromuscular Junction Continued (3)
• A nerve impulse triggers
the exocytosis of ACh
• ACh diffuses across the
synaptic cleft and binds
to receptors on chemical-
gated ion channels
• Sodium ions enter the
sarcoplasm producing
depolarization

26. Excitation-Contraction Coupling

27. Excitation-Contraction Coupling Continued (1)
• An action potential arrives
at axon terminal
• ACh is released, binds to
receptors, opens ion
channels, leading to an
action potential
• The action potential
travels down the T-tubules
which triggers the release
of calcium from the SR

28. Excitation-Contraction Coupling Continued (2)
• The release of calcium ions
from the terminal cisterna of
the SR allows calcium ions to
bind to troponin which
changes shape causing the
tropomyosin to swivel and
reveal the active sites.
• The myosin heads cross-
bridge with actin and
produce a power stroke.

29. Excitation-Contraction Coupling Continued (3)
• A cross-bridge forms
between the myosin heads
and actin triggering sliding
of the filaments and a build
up of tension in the muscle.
• As long as Ca++ ions remain
in the sarcoplasm, and as
long as ATP is available, the
muscle fiber will continue to
shorten (contract).

30. Summary of Excitation
Video Source: https://youtu.be/NfEJUPnqxk0

31. Summary of Excitation Continued (2)
•Resting membrane potential
▫ Sarcolemma is polarized
 Relatively high levels of Na+ outside the sarcolemma and
high levels of K+ inside the sarcolemma
 Negative charge inside (-70 mV) compared to the charge
outside the membrane
▫ All chemical-gated and voltage-gated ion channels are
closed and the membrane is essentially impermeable
to Na+ and K+ ions.

32. Summary of Excitation Continued (3)
•Step 1: Depolarization
▫ An action potential arrives at the axon terminal
triggering the exocytosis of ACh from the synaptic
vesicles into the synaptic cleft.
▫ Ach diffuses across the cleft and binds to receptors
on the chemical-gated ion channels causing the
channels to open.
▫ Na+ ions flood into the muscle cell causing the charge
at the motor end plate to move from -70 mV moves
toward -60 mV. This switch in charge is called
depolarization.

33. Summary of Excitation Continued (4)
•Step 2: Propagation of an Action Potential
▫ Depolarization at the motor end plate causes nearby
voltage-gated ion channels on the sarcolemma to
open, allowing for more influx of Na+ (-60 mV to +30
mV)
▫ The wave of depolarization begins to travel down the
sarcolemma away from the motor end plate and down
the T tubules. This is called an action potential.
▫ As the action potential travels down the T-tubule, it
triggers the release of Ca++ from the terminal cisterna.
▫ Ca++ binds to troponin triggering contraction of the
muscle by the sliding filament process (more later).

34. Summary of Excitation Continued (5)
•Step 3: Repolarization
▫ Almost as quickly as the contraction is triggered within
the cell, acetylcholinesterase (an enzyme) decomposes
the ACh at the NMJ.
▫ Chemical-gated ion channels close and influx of Na+
stops. However, the passive leakage of K+ out of the
sarcolemma continues and results in the switch of the
charge on the motor end plate back to resting
conditions (+30 mV toward -70 mV).
▫ Unfortunately, the Na+ and K+ ions are in the wrong
places so membrane potential becomes even more
negatively charged.

35. Summary of Excitation Continued (6)
•Step 4: Hyperpolarization
▫ The continued leakage of K+ ions out of the membrane
results in the interior becoming exceedingly negatively
charged (-70 mV to -90 mV).
▫ In this hyperpolarized state, the sodium-potassium
pump is turned on and the ions are pumped back to
their original locations (3 Na+ pumped out for every 2
K+ pumped in).
▫ The final outcome is the resting membrane potential is
re-established and the muscle can be stimulated again.

36. Section 10.3

37. Summary of Contraction
• The excitation of the
membrane results in the
release of calcium into
the sarcoplasm and onto
the sarcomere.
• The presence of calcium
on the sarcomere causes
myosin to bind to actin in
a process commonly
referred to as the sliding
filament mechanism.

38. Summary of Contraction Continued (2)
•As calcium binds to troponin, troponin changes shape
and moves the tropomyosin. The active site on actin
becomes exposed.
•The myosin head is attracted to the active site on actin,
and myosin binds actin forming the cross-bridge.

39. Summary of Contraction Continued (3)
• During the power stroke, the phosphate generated in the
previous contraction cycle is released.
• This results in the myosin head pivoting toward the center of the
sarcomere, after which the attached ADP and phosphate group
are released.

40. Summary of Contraction Continued (4)
• A new molecule of ATP attaches to the myosin head, causing
the cross-bridge to detach.

41. Summary of Contraction Continued (5)
• The ATPase of the myosin head hydrolyzes ATP to ADP and
phosphate, which returns the myosin to the cocked position.

42. Summary of Sliding Filament
Video Source: https://youtu.be/nTZnBdeIb5c

43. ATP and Muscle Contraction
•Each thick filament, composed of roughly 300 myosin
molecules, has multiple myosin heads, and many
cross-bridges form and break continuously during
muscle contraction.
•Multiply this by all of the sarcomeres in one
myofibril, all the myofibrils in one muscle fiber, and
all of the muscle fibers in one skeletal muscle, and
you can understand why so much energy (ATP) is
needed to keep skeletal muscles working.

44. Sources of ATP for Muscles
•Therefore multiple sources of ATP are required.
▫ Stored ATP and creatine phosphate
▫ Glycogen (metabolized anaerobically by glycolysis)
▫ Glycogen (metabolized aerobically by citric acid
cycle and the electron transport chain)

45. Sources of ATP for Muscles Continued (2)
• Some ATP is stored in a resting muscle. As contraction starts, it
is used up in seconds (approximately 2 seconds). More ATP is
generated from creatine phosphate for about 15 additional
seconds.

46. Sources of ATP for Muscles Continued (3)
• Each glucose molecule produces two ATP and two molecules of
pyruvic acid. If oxygen is not available, pyruvic acid is converted
to lactic acid, which may contribute to muscle fatigue.
• This occurs during strenuous exercise when high amounts of
energy are needed but oxygen cannot be sufficiently delivered to
muscle.

47. Sources of ATP for Muscles Continued (4)
• Aerobic respiration is the breakdown of glucose in the presence
of oxygen (O2) to produce carbon dioxide, water, and ATP.
• Approximately 95 percent of the ATP required for resting or
moderately active muscles is provided by aerobic respiration,
which takes place in mitochondria.

48. ATP and Skeletal Muscles
•Muscle fatigue occurs when a muscle can no longer
contract in response to signals from the nervous
system. The exact causes of muscle fatigue are not
fully known, although several hypotheses have been
generated:
▫ ATP reserves are reduced, muscle function may decline.
▫ Lactic acid buildup may lower intracellular pH, affecting
enzyme and protein activity.
▫ Imbalances in Na+ and K+ levels as a result of membrane
depolarization may disrupt Ca++ flow out of the SR.
▫ Long periods of sustained exercise may damage the SR and
the sarcolemma, resulting in impaired Ca++ regulation.

49. ATP and Skeletal Muscles Continued
•Intense muscle activity results in an oxygen debt,
which is the amount of oxygen needed to compensate
for ATP produced without oxygen during muscle
contraction.
▫ Oxygen is required to restore ATP and creatine phosphate
levels, convert lactic acid to pyruvic acid, and, in the liver, to
convert lactic acid into glucose or glycogen.
▫ Other systems used during exercise also require oxygen, and
all of these combined processes result in the increased
breathing rate that occurs after exercise.
▫ Until the oxygen debt has been met, oxygen intake is
elevated, even after exercise has stopped.

50. Section 10.4

51. Muscle Tension
•To move an object, referred to as load, the
sarcomeres in the muscle fibers of the skeletal
muscle must shorten.
•The force generated by the contraction of the
muscle (or shortening of the sarcomeres) is called
muscle tension.
•However, muscle tension also is generated when the
muscle is contracting against a load that does not
move, resulting in two main types of skeletal muscle
contractions: isotonic contractions and isometric
contractions.

52. Isotonic (concentric) Contraction
During isotonic concentric contractions, muscle length
changes (shortens) to move a load.

53. Isotonic (eccentric) Contraction
During isotonic contractions, muscle length changes
(lengthens specifically) to move a load.

54. Isometric Contraction
During isometric contractions, muscle length does not change
because the load exceeds the tension the muscle can
generate.

55. Motor Units
•Every skeletal muscle fiber must be innervated by an
axon terminal of a motor neuron in order to contract.
•Each muscle fiber is innervated by only one motor
neuron. The collection of all muscle fibers in a muscle
innervated by a single motor neuron is called a motor
unit.
•As more strength is needed, larger motor units, with
bigger, higher-threshold motor neurons are enlisted to
activate larger muscle fibers. This increasing activation
of motor units produces an increase in muscle
contraction known as recruitment.

56. Motor Units Continued (2)
Credit: https://commons.wikimedia.org/wiki/File:Motor_unit.png

57. Motor Units Continued (3)
Credit: https://commons.wikimedia.org/wiki/File:Motor_unit_recruitment.png

58. Summary of Motor Units
Video Source: https://youtu.be/UnNGGD4-IHU

59. Twitch and Myogram
•A twitch is a single stimulus-contraction-relaxation
sequence in a muscle fiber and can vary in duration
depending on muscle type, location, and internal and
external environmental conditions.
•The tension produced by a single twitch can be
measured by a myogram, an instrument that
measures the amount of tension produced over time.

60. Events of a Twitch

61. Frequency of the Stimulus
If the fibers are stimulated
while a previous twitch is
still occurring, the second
twitch will be stronger.
This response is called
wave summation,
because the excitation-
contraction coupling
effects of successive
motor neuron signaling is
summed, or added
together.

62. Frequency of the Stimulus Continued (2)
If the stimulus frequency
is so high that the
relaxation phase
disappears completely,
contractions become
continuous in a process
called complete tetanus.

63. Frequency of the Stimulus Continued (3)
When muscle tension
increases in a graded
manner that looks
like a set of stairs, it is
called treppe.

64. Muscle Tone
A variable number of
motor units is always
active, even when the
entire muscle is not
contracting. This
creates a resting tension
called muscle tone.
Credit: www.surestep.net

65. Section 10.5

66. Types of Skeletal Muscle Fibers
•There are many criteria to consider when classifying
the types of muscle fibers including vascularity,
resistance to fatigue, color, ATP sources, and more.
Using these criteria, there are three main types of
skeletal muscle fibers:
▫ Slow-oxidative fibers (SO)
▫ Fast-glycolytic fibers (FG)
▫ Fast-oxidative fibers (FO)

67. Types of Skeletal Muscle Fibers Continued
Property Slow-Oxidative Fast-Glycolytic Fast-Oxidative
Cross-sectional
diameter
Small Intermediate Large
Time to peak tension Prolonged Intermediate Rapid
Contraction speed Slow Fast Fast
Fatigue resistance High Intermediate Low
Color Red Pink White
Myoglobin content High Low Low
Capillary supply Dense Intermediate Scarce
Mitochondria Many Intermediate Few
Glycolytic enzyme
concentration
Low High High
Substrate used for ATP
production
Lipids, carbs, and
amino acids
(aerobically)
Primarily carbs
(aerobically)
Carbs only
(anaerobically)

68. Section 10.6

69. Hypertrophy versus Atrophy
•Hypertrophy – increase
in muscle size from
physical training
•Atrophy - reduced
muscle size resulting
from lack of use. Age-
related atrophy is
sarcopenia
(credit: Lin Mei/flickr)

70. Endurance Exercise
•Predominant performed
by SO fibers with high
resistance to fatigue.
•Endurance exercise
increases myoglobin
content, increases
number of mitochondria
per cell, and triggers
angiogenesis
Credit: “Tseo2”/Wikimedia Commons

71. Resistance Exercise
•Requires large number of
FG fibers
•Resistance exercise
affects muscles by
increasing the formation
of myofibrils, thereby
increasing the thickness
of muscle fibers. This
added structure causes
hypertrophy

72. Performance-Enhancing Substances
•Anabolic steroids
•Erythropoietin
•Human growth
hormone
Credit:
https://commons.wikimedia.org/wiki/File:Rawdeal
steroids4.jpg

73. Section 10.7

74. Cardiac Muscle Tissue
•Found only in the
heart
•Cells are short and
branched
•Involuntary
•Usually a single
nucleus per cell
•Possesses striations
and intercalated discs

75. Cardiac Muscle Tissue Continued
Intercalated discs are part of the cardiac muscle sarcolemma and
they contain gap junctions and desmosomes.

76. Functional Syncytium
Video Source: https://youtu.be/IMkHo11reWg

77. Section 10.8

78. Smooth Muscle Tissue
•Found in the walls of
hollow organs, blood
vessels, and in the
arrector pili muscles
•Cells are short and
spindle-shaped
•Single nucleus per cell
•Lack striations and
intercalated discs

79. Smooth Muscle Tissue Continued (2)
The dense bodies and intermediate filaments are networked
through the sarcoplasm, which cause the muscle fiber to
contract.

80. Smooth Muscle Tissue Continued (3)
When the thin filaments slide past the thick filaments, they
pull on the dense bodies, structures tethered to the
sarcolemma, which then pull on the intermediate filaments
networks throughout the sarcoplasm. This arrangement
causes the entire muscle fiber to contract in a manner
whereby the ends are pulled toward the center, causing the
midsection to bulge in a corkscrew.

81. Smooth Muscle Tissue Continued (4)
A varicosity releases neurotransmitters into the synaptic
cleft. Also, visceral muscle in the walls of the hollow
organs (except the heart) contains pacesetter cells. A
pacesetter cell can spontaneously trigger action potentials
and contractions in the muscle.

82. Smooth Muscle Tissue Continued (5)
Video Source: https://youtu.be/Q2GchCG-KHI

83. Section 10.9

84. Myogenesis
Image Source:
https://en.wikipedia.org/wiki/File:Myoblast_Fusion_-_Myogenesis.jpg

85. Citation
Unless otherwise noted, the images and text used in this
PowerPoint are from:
Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse,
D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2022).
Chapter 10: Muscle Tissue. In Anatomy and Physiology 2e.
OpenStax. https://openstax.org/books/anatomy-and-
physiology-2e/pages/10-introduction