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Anatomy and Physiology
Chapter 12:
Physiology of the Muscular System
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General Functions
Movement of the body and its
parts
 Heat production
 Posture

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Function of Skeletal Muscle Tissue

Characteristics of skeletal muscle cells
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Excitability (irritability)—ability to be
stimulated
Contractility—to contract/shorten, &
produce movement
Extensibility—to extend/stretch, to return to
their resting length
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Function of Skeletal Muscle Tissue

Muscle cells are called fibers because of
their threadlike shape
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Contain many mitochondria and several
nuclei
Sarcomere
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Each myofibril consists of many sarcomeres
Segment of myofibril between two
successive Z disks
Contractile unit of muscle fibers
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Function of Skeletal Muscle Tissue

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Sarcolemma—plasma membrane of muscle
fibers
Sarcoplasmic reticulum (SR)


T tubules—network of tubules and sacs found
within muscle fibers
Continually pumps calcium and stores the ions for
later release
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Function of Skeletal Muscle Tissue

T tubules
 Transverse tubules extend
across the sarcoplasm
 Membrane has ion pumps that
continually transport Ca++ ions
 Allow electrical impulses
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Myofilaments
Each myofibril contains thousands of
thick and thin myofilaments

4 protein molecules make up
myofilaments
 Myosin
thick filament
 Myosin “heads” attracted to actin
 Form cross bridges when
attached to actin

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4 different kinds of protein cont..
2.) Actin—protein that forms thin
filament
3.) Tropomyosin—protein that blocks
the active sites on actin molecules
4.) Troponin—protein that holds
tropomyosin molecules in place
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Mechanism of contraction

Excitation and contraction
Skeletal muscle fiber remains at
rest
1.
Stimulated by a motor neuron
2.
•
•
Neuromuscular junction(NMJ)—motor
neurons connect to the sarcolemma
at the motor endplate
“NMJ” is a synapse where
neurotransmitter transmit signals
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
Excitation and contraction (cont)
Acetylcholine: neurotransmitter
released into the synaptic cleft
1.
Stimulates the receptors
2.
Initiates an impulse in sarcolemma
3.
Triggers the release of calcium ions
4.
Calcium binds to troponin, which
expose active sites on actin
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Sliding filament model/theory
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Active sites on actin are exposed,
myosin heads bind to them
Myosin heads bend and “pull” the
thin filaments past them
Each head releases, binds to the
next active site, and pulls again
The entire myofibril shortens
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Whole Muscle Contraction
A single muscle fiber has an “all or nothing”
response
But a whole muscle can vary its force of
contraction.
Two characteristics of a whole muscle allow this
• Motor unit
• Recruitment
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Function of Skeletal Muscle Organs
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Motor unit
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Motor unit= motor neuron + the muscle
fibers it attaches
Could be a couple of fibers or an entire
fascicle
• Recruitment:
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Smaller the number of fibers in a motor
unit, the more precise are the available
movements
Larger the number of fibers in a motor unit,
the more powerful the contraction available
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Energy sources for muscle
contraction

ATP yields the energy required for muscular
contraction
Muscle fibers resynthesize ATP from the
breakdown of creatine phosphate (CP)
Catabolic pathways
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•
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Aerobic pathway
Presence of O2 from blood
 Slower than anaerobic pathway
 Supplies energy for the long term rather than
the short term

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Catabolic pathways (cont)
Anaerobic pathway
Little to no O2 is available
 Very rapid, providing energy during
first minutes of maximal exercise
 Formation of lactic acid
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Requires oxygen to convert back to
glucose,
Soreness
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producing of an “oxygen debt”
excess post-exercise oxygen consumption
(EPOC)
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Responses of a Whole Muscle
• Twitch: Single muscle response in which
muscle contracts and then fully relaxes
• Tetanus: Sustained muscle contraction caused
by repeated stimulation
• Smooth, sustained contraction
• Tonus: Normal, continuous state of partial
muscle contraction
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Skeletal Muscle Fiber Types
Speeds of response depends on
which fibers are stimulated:
Type I: slow-twitch (weakest force,
longest time)
Type II: fast-twitch (2 divisions)
type IIa: intermediate speed
type IIb: fastest (greatest force, shortest
time)
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Isotonic vs. Isometric contractions
Isotonic contraction: “same tension”
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Contraction where tone/tension of
muscle is the same as the length
changes
Concentric—muscle shortens
as it contracts
 Eccentric—muscle lengthens
while contracting

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Isometric contraction
Isometric: “same length”
o
muscle length stays the same
while muscle tension increases
Most body movements occur as
a result of both types of
contractions
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