Human Physiology, 8e

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Chapters 9 10 Muscular System Outline

• Muscle tissues
• General characteristics
• Types
• Muscle structure
• Excitation-contraction coupling
• Resting potentials
• Action potentials
• Sliding filament theory of contraction

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Figure Acknowledgements

• Widmaier et. al., Vander's Human Physiology,
  11th ed.

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Muscular System (cont.)

• The sliding filament mechanism, in which myosin
  filaments bind to and move actin filaments, is
  the basis for shortening of stimulated skeletal,
  smooth, and cardiac muscles.

• In all three types of muscle, myosin and actin
  interactions are regulated by the availability
  of calcium ions.

• Changes in the membrane potential of muscles are
  linked to internal changes in calcium release
  (and contraction).

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Muscular System (cont.)

• Neuronal influences on the contraction of muscles
  are affected when neural activity causes changes
  in themembrane potential of muscles.

• Smooth muscles operate in a wide variety of
  involuntaryfunctions such as regulation of blood
  pressure andmovement of materials in the gut.

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Chapters 9 10 Muscular System Outline

• Muscle tissues
• General characteristics
• Types
• Muscle structure
• Excitation-contraction coupling
• Resting potentials
• Action potentials
• Sliding filament theory of contraction

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Figure 9-1
Diameter 10 100 mm Length up to 20 cm
Diameter 15 25 mm Length 50 70 mm
Diameter 2 10 mm Length 50 400 mm
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Contractile cells
Skeletal muscle fibers are multinucleated, and
arise only during embryonic development.
Cardiac-muscle fibers are single cells, but they
possess specialized junctions that can only be
formed during embryonic development.
Smooth muscle cells are true cells, and thus
are the only muscle type that undergoes mitosis
during adulthood.
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• Locations of the 3 muscle types
• Skeletal muscle is attached to bones and moves
  and supports the skeleton. Requires innervation
  for contraction.
• Smooth muscle surrounds hollow cavities and
  tubes. Some types (multi-unit) require
  innervation while others (single-unit) do not.
• Cardiac muscle is the muscle of the heart. Does
  not require innervation for contraction
  sympathetic and parasympathetic nerves merely
  modify the heart rate (rate of contraction).

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Chapters 9 10 Muscular System Outline

• Muscle tissues
• General characteristics
• Types
• Muscle structure
• Excitation-contraction coupling
• Resting potentials
• Action potentials
• Sliding filament theory of contraction

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I. Skeletal MuscleA. Gross Anatomy 1)
3 connective tissue wrappings 1)
epimysium surrounds a whole muscle. 2)
perimysium surrounds a collection of muscle
fibers (a muscle bundle). 3) endomysium
surrounds an individual muscle fiber
(cell).
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Skeletal muscles are attached to the skeleton
by tendons.
Skeletal muscles typically contain many, many
muscle fibers.
Figure 9-2
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2) Nerve and blood supply generally, 1 nerve, 1
artery and 1 or more veins supply 1
muscle.3) Attachments direct
(muscle to bone) or
indirect (muscle to rope-like tendon to bone,
or muscle to sheet-like tendon to
bone). Tendons are bundles of
collagen fibers.
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B. Microscopic Anatomy 1) Skeletal
muscle fibers are cylindrical and striated, with
pericentric multiple nuclei. 2) Each
muscle fiber is composed of many parallel,
rod-like myofibrils each myofibril is composed
of many sarcomeres placed end-to-end. 3)
The sarcomere is the contractile unit of the
muscle cell. A sleeve of sarcoplasmic
(endoplasmic) reticulum surrounds each sarcomere
the SR is the intracellular storage compartment
for Ca2.
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Figure 9-3b
The sarcomere is composed of thick filaments
called myosin, anchored in place by titin fibers,
and thin filaments called actin, anchored to
Z-lines .
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4) Striations result from repeating dark A
bands and light I bands. A bands are due to
thick (myosin) myofilaments, and I bands are due
to thin (actin) myofilaments, within each
sarcomere. Actin and myosin filaments also
overlap, to some extent, within the A bands
during shortening or lengthening, these 2 types
of myofilaments slide past each other. The
muscle fiber cytoplasm (sarcoplasm) surrounds the
myofilaments.The H zone is a lighter area in
the middle of the A band where there is no
overlap of actin and myosin. The M line is a
dark line, in the middle of the H zone, which
connects myosin filaments.
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Figure 9-3
Sarcomere structures in an electron micrograph.
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Chapters 9 10 Muscular System Outline

• Muscle tissues
• General characteristics
• Types
• Muscle structure
• Excitation-contraction coupling
• Resting potentials
• Action potentials
• Sliding filament theory of contraction

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A thin shell of positive (outside) and negative
(inside) charge provides an electrical gradient
across the plasma membrane of all cells.
is attracted
is repulsed
Figure 4-6
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Figure 6-8
How is Vm measured?
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Figure 6-14
Overshoot refers to the development of a charge
reversal.
A cell is polarized because its interior is
more negative than its exterior.
Repolarization is movement back toward the
resting potential.
Depolarization occurs when ion movement
reduces the charge imbalance.
Hyperpolarization is the development of even
more negative charge inside the cell.
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An action potential is an all-or-none sequence
of changes in membrane potential resulting from
an all-or-none sequence of changes in ion
permeability due to the operation of
voltage-gated Na and K channels.
Figure 6-19
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Figure 9-10
The latent period between excitation and
development of tension in a skeletal muscle
includes the time needed to release Ca from
sarcoplasmic reticulum, move tropomyosin, and
cycle the cross-bridges.
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The neuromuscular junction is the point of
synaptic contact between the axon terminal of a
motor neuron and the muscle fiber it controls.
Action potentials in the motor neuron cause
acetylcholine release into the neuromuscular
junction.
Muscle contraction follows the delivery of
acetylcholine to the muscle fiber.
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Figure 10-12
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Figure 9-15
1. The exocytosis of acetylcholine from the axon
terminal occurs when the acetylcholine vesicles
merge into the membrane covering the terminal.
2. On the membrane of the muscle fiber, the
receptors for acetylcholine respond to its
binding by increasing Na entry into the fiber,
causing a graded depolarization.
3. The graded depolarization typically exceeds
threshold for the nearby voltage-gate Na and K
channels, so an action potential occurs on the
muscle fiber.
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Figure 9-10
The latent period between excitation and
development of tension in a skeletal muscle
includes the time needed to release Ca from
sarcoplasmic reticulum, move tropomyosin, and
cycle the cross-bridges.
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Chapters 9 10 Muscular System Outline

• Muscle tissues
• General characteristics
• Types
• Muscle structure
• Excitation-contraction coupling
• Resting potentials
• Action potentials
• Sliding filament theory of contraction

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How skeletal muscles (and all other muscles)
workmuscles always pull, never push!
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Figure 9-5
Contraction (shortening) myosin binds to actin,
and slides it, pulling the Z-lines closer
together, and reducing the width of the I-bands.
Note that filament lengths have not changed.
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Figure 9-6
Contraction myosins cross-bridges bind to
actin the crossbridges then flex to slide
actin.
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Figure 9-7
The thick filament called myosin is actually a
polymer of myosin molecules, each of which has
a flexible cross-bridge that binds ATP and actin.
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In relaxed skeletal muscle, tropomyosin
blocks the cross-bridge binding site on
actin. Contraction occurs when calcium ions bind
to troponin this complex then pulls tropomyosin
away from the cross-bridge binding site.
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Figure 9-8
The myosin-binding site on actin becomes
available, so the energized cross-bridge binds.
1.
The cross-bridge cycle requires ATP
The full hydrolysis and departure of ADP Pi
causes the flexing of the bound cross-bridge.
2.
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Resting myosin heads have previously
bound ATP, in order to detach from myosin-binding
sites on actin ATP hydrolysis by myosin ATPase
releases energy, which is stored in the
high-energy cocked position of myosin heads.
Therefore, at rest, myosin heads are cocked,
ready and waiting for the next opportunity to
bind to actin.
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If no additional action potentials
occur, the muscle relaxes and lengthens to its
original resting length, since myosin-binding
sites on actin are blocked by tropomyosin when
cytoplasmic Ca2 levels are low no
cross-bridges can attach.
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Know the sequence of E-C events as outlined in
this table
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The End.