The Muscular System

1
The Muscular System
2
Objectives

• Describe the three types of muscle tissue
  smooth, skeletal, and cardiac
• Discuss the functions of skeletal muscle
• Describe the macrostructure of skeletal muscle
• Describe the microstructure of skeletal muscle
• Discuss the three different types of muscular
  actions concentric, eccentric, and isometric
• (continued)

3
Objectives

• Describe the roles that muscles can assume
• Discuss the factors that influence the force
  developed during muscular activity
• Understand the relationship between power output
  and velocity of contraction of a muscle

4
Types of muscles

• Smooth
• Involuntary muscle controlled unconsciously
• In the walls of blood vessels and internal organs
• Cardiac
• Controls itself with help from nervous and
  endocrine systems
• Only in the heart
• Skeletal
• Voluntary muscle controlled consciously
• Over 600 throughout the body

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Structure of muscle
6
Structure of a single muscle fiber
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Structure of a single muscle fiber

• Sarcolemma thin membrane that encloses the
  muscle fiber
• Sarcoplasm the cytoplasm of a muscle fiber
• T tubules an interconnected network that passes
  laterally
• Sarcoplasm reticulum longitudinal network that
  stores calcium

8
Structure of myofibrils
9
Structure of a sarcomere
10
Structure of a myosin filaments

• Composed of two protein strands twisted together
• One end of each strand is folded into a globular
  head myosin head

11
Structure of actin filaments

• Composed of three protein molecules actin,
  tropomyosin troponin
• Has binding sites that serve as a point of
  contact with myosin heads

12
Key points

• An individual muscle cell is called a muscle
  fiber.
• A muscle fiber is enclosed by a plasma membrane
  called the sarcolemma.
• The cytoplasm of a muscle fiber is called a
  sarcoplasm.
• Within the sarcoplasm, the T tubules allow
  transport of substances throughout the muscle
  fiber and the sarcoplasmic reticulum stores
  calcium.

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Muscle fiber action

• A motor neuron releases acetylcholine (ACh).
• ACh binds to receptors on the sarcolemma.

14
Muscle fiber action

• 3. The action potential triggers release of Ca2.

15
Muscle fiber action

• 4. The Ca2 binds to troponin on the actin
  filament, and the troponin pulls tropomyosin off
  the active sites, allowing myosin heads to attach
  to the actin filament.

16
Sliding filament theory

• 1. The muscle fiber is initially in a relaxed
  state

17
Sliding filament theory

• 2. When the myosin crossbridges are activated,
  they bind with actin.

18
Sliding filament theory

• 3. The myosin head then tilts toward the arm of
  the cross-bridge and drags the actin and myosin
  filaments in opposite direction.

19
Key points

• Muscle action is initiated by a nerve impulse..
• The nerve releases ACh, which allows sodium to
  enter and depolarize the cell. If the cell is
  sufficiently depolarized, an action potential
  occurs which releases stored Ca2 ions.
• Ca2 ions bind with troponin, which lifts the
  tropomyosin molecules off the active sites on the
  actin filament. These open sites allow the myosin
  heads to bind to them.
• (continued)

20
Key points

• Once myosin binds with actin, the myosin head
  tilts and pulls the actin filament so they slide
  across each other.
• Muscle action ends when calcium is pumped out of
  the sarcoplasm to the sarcoplasmic reticulum for
  storage.
• Energy for muscle action is provided when the
  myosin head binds to ATP. ATPase on the myosin
  head splits the ATP into a usable energy source.

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Muscle fiber types
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Slow-Twitch (ST) Muscle Fibers
w High aerobic (oxidative) capacity and fatigue
resistance
w Low anaerobic (glycolytic) capacity and motor
unit strength
w Slow contractile speed (110 ms) and myosin
ATPase
w 10180 fibers per motor neuron
w Low sarcoplasmic reticulum development
23
Slow-Twitch (ST) Muscle Fibers
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Fast-Twitch (FTa) Muscle Fibers
w Moderate aerobic (oxidative) capacity and
fatigue resistance
w High anaerobic (glycolytic) capacity and motor
unit strength
w Fast contractile speed (50 ms) and myosin ATPase
w 300800 fibers per motor neuron
w High sarcoplasmic reticulum development
25
Fast-Twitch (FTa) Muscle Fibers
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Fast-Twitch (FTb) Muscle Fibers
w Low aerobic (oxidative) capacity and fatigue
resistance
w High anaerobic (glycolytic) capacity and motor
unit strength
w Fast contractile speed (50 ms) and myosin ATPase
w 300800 fibers per motor neuron
w High sarcoplasmic reticulum development
27
Fast-Twitch (FTb) Muscle Fibers
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Determination of fiber type

• Genetics determine which motor neurons innervate
  our individual muscle fibers.
• Endurance/strength training and muscular
  inactivity may result in small lt10 changes in
  the percentage of FT and ST fibers.
• Aging may result in changes in the percentage of
  FT to ST fibers.

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Muscle fiber recruitment

• For a motor unit to be recruited into activity
  the motor nerve impulse must meet or exceed the
  threshold.
• If the threshold is not met no fibers in that
  unit act.

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Muscle fiber recruitment

• When this occurs, all muscle fibers in the motor
  unit act maximally.

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Muscle fiber recruitment

• More force is produced by activating more motor
  units.

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Muscle fiber recruitment
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Fiber type athletic success
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35
Muscle fiber architecture

• Parallel
• Pennate

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Roles of muscles

• Agonists prime movers responsible for the
  movement
• Antagonists oppose the agonists to prevent
  overstretching of them
• Synergists assist the agonists and sometimes
  fine-tune the direction of movement

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Roles of muscles
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Types of muscle action
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Muscle contraction force

• Factors that contribute to force production

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Muscle contraction force

• Cross-sectional area

41
Muscle contraction force

• Joint Angle

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Muscle contraction force

• Speed of Contraction

43
Muscle contraction force

• What factor ???

44
Muscle contraction force

• Prestretch

45
Muscle contraction force

• Others
• Fatigue
• Fiber type

46
Power-Velocity relationship
Force
O
O
O
O
Velocity
47
Further Reading
Chapter 1 from Wilmore, J. H., Costill, D. L.
(2004). Physiology of sport and exercise 3rd ed.
Champaign, IL Human Kinetics. Chapter 11
from McGinnis, P. M. (2005) Biomechanics of
sport and exercise. 2nd ed. Champaign Human
Kinetics.