Transcript The Muscular System

The Muscular System
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Skeletal muscle consists of numerous muscle
cells called Muscle fibers.
Muscle fiber terminology and characteristics
 Sarcolemma = plasma membrane of the muscle
cell
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Highly invaginated by transverse tubules (T tubules) that
permeate the cell.
Sarcoplasma = cytoplasm of the muscle cell
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Contains calcium-storing sarcoplasmic reticulum
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Specialized endoplasmic reticulum of a muscle cell
The Muscular System
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Muscle fiber terminology and characteristics
 Muscle cells are multinucleated
 Nearly the entire volume of the cell is filled
with numerous, long myofibrils.
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Two types of filaments
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Actin
 Thin filaments consisting of two strands
arranged in a double helix
 Troponin and Tropomyosin = molecules that
cover special binding sites on actin.
The Muscular System
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Two types of filaments
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Myosin
 Thick filaments
 Each myosin filament forms a protruding head at one
end.
Actin and myosin are parallel and arranged side by side.
Striated appearance produced from overlapping
filaments
Each repeating unit of the pattern is called a
sarcomere.
Each sacromere is separted by a Z-line, to which the
actin filaments are attached
Myosin filaments are located between the actin,
unattached to the Z-line.
The Muscular System
Muscle contraction: sliding-filament model
ATP binds to a myosin head and forms
ADP + Pi
1.
1.
When ATP binds to a myosin head, it is
converted to ADP + Pi, which remains attached
to the myosin head.
Ca++ exposes the binding sites on the
actin filaments
2.
1.
Ca++ binds to the troponin molecule causing
tropomyosin to expose positions on the actin
filament for the attachment of myosin heads.
The Muscular System
Muscle contraction: sliding-filament model
3.
Cross bridges between myosin heads
and actin filaments form.
When attachment sites on the actin are exposed, the
myosin heads bind to actin to form cross bridges.
1.
4.
ADP and Pi are released and sliding
motion of actin results.
The attachment of cross bridges between myosin and actin
causes the release of ADP and Pi. This, in turn, causes a
change in shape of the myosin head, which generates a
sliding movement of the actin toward the center of the
sarcomere.
1.
2. This pulls the two Z-lines together, contracting the
muscle fiber.
The Muscular System
Muscle contraction: sliding-filament model
5.
ATP causes the cross bridges to
unbind.
1. When
a new ATP molecule attaches to the myosin
head, the cross bridge between the actin and myosin
breaks, returning the myosin head to its unattached
position
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Without the addition of a new ATP molecule, the cross
bridges remain attached to the actin filaments.
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This is why corpses are stiff.
The Muscular System
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Neuromuscular junctions
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Synapses between neurons and muscles
Muscle contraction is stimulated through the
following steps:
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Action potential generates release of
acetylcholine.
Action potential generated on sacrolemma and
throughout the T-tubules
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Receptors on the sacrolemma initiate a depolarization
event and action potential.
The action potential travels along the sacrolemma
throughout the transverse system of tubules.
The Muscular System
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Muscle contraction is stimulated through
the following steps:
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Sacroplasmic reticulum releases Ca++.
Myosin cross bridges form.
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The Ca++ released by the sacroplasmic reticulum
binds to troponin molecules on the actin helix,
promoting tropomyosin molecules to expose binding
sites for myosin cross-bridge formation.
If ATP is available, muscle contraction begins.
The Muscular System
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Humans and other vertebrates have three kinds of
muscles
 Skeletal muscle
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Smooth muscle
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Attached to bones and causes movement of body
Lines the walls of blood vessels and digestive tract
No striation
No T-tubules, therefore, contraction is controlled and relatively
slow.
Cardiac muscle
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Striated
Highly branched with cells connected by gap junctions
Generates its own action potential, which spreads rapidly throughout
muscle tissue by electrical synapses across gap junctions.