Muscular System
Chapter 11
Part 2
Copyright © 2006 Lippincott Williams & Wilkins.
Copyright © 2006 Lippincott Williams & Wilkins.
Gross Structure of
Skeletal Muscle
• Skeletal muscles is encased by epimysium:
fascia of fibrous connective tissue.
• Fasciculus, bundle of cylindrical muscle
fibers surrounded by perimysium.
• Muscle fibers is cylindrical with length of
few mm to many cm, contains nuclei.
• Endomysium surrounds muscle fiber.
• Sarcolemma beneath endomysium is thin,
elastic membrane.
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Copyright © 2006 Lippincott Williams & Wilkins.
• Sarcoplasm (cytosol)
is fluid portion in
muscle fiber
contains contractile
proteins, nuclei,
mitochondria,
sarcoplasmic
reticulum.
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Gross Structure of
Skeletal Muscle
• Contractile proteins of cell are myofibrils.
• Chemical composition: 75% water, 20%
protein, 5% other.
• Blood supply. Milking action of rhythmic
exercise; compressive force of resistive
exercise retards blood flow.
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Ultrastructure of
Skeletal Muscle
• Sarcomere is basic functional unit.
• Distinguished area between Z lines.
• Thicker filaments confined to A band, a
lighter middle region called the H zone.
• Thinner filaments arise in middle region of
I band, at the Z line.
• During contraction, neither thick nor thin
filaments change in length. They slide.
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Sarcomere
H zone
A band
I band
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• During contraction,
neither myosin nor
actin filaments
change in length.
• They slide past each
other.
• A band remains
same, I band
decreases.
• H zone decreases on
contraction.
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Ultrastructure of
Skeletal Muscle
Actin-Myosin Orientation
• Myosin filament (thick). Myosin molecules
have long rod-shaped tails with 2 globular
heads. The heads form cross bridges.
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Ultrastructure of
Skeletal Muscle
• Actin filament (thin). Actin molecules are
pear-shaped double helix.
• Tropomyosin is a rigid, rod-shaped protein
lies in groove on either side of actin.
• Troponin is complex of 3 globular proteins.
Copyright © 2006 Lippincott Williams & Wilkins.
Copyright © 2006 Lippincott Williams & Wilkins.
Copyright © 2006 Lippincott Williams & Wilkins.
Ultrastructure of
Skeletal Muscle
• Tropomyosin is rigid,
rod-shaped protein
which lies in groove on
either side of actin.
• Troponin is complex of
three polypeptides
embedded at regular
intervals along
tropomyosin. Binds
Ca++.
Copyright © 2006 Lippincott Williams & Wilkins.
Ultrastructure of
Skeletal Muscle
• Intracellular
tubule system
connects inner
myofibrils with
sarcolemma.
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•Sarcoplasm is the cytoplasm of a
muscle cell that contains the usual
subcellular elements along with the
Golgi apparatus, abundant myofibrils,
a modified endoplasmic reticulum
known as the sarcoplasmic reticulum
(SR), myoglobin and mitochondria.
•Sarcolemma has holes in it.
•Holes lead into tubes called
Transverse tubules or T tubules.
•T tubules pass down into muscle cells
and go around the myofibrils.
•Function of T tubules is to conduct
impulses from the surface of the cell
(sarcolemma) down into the cell to
the sarcoplasmic reticulum.
•Sarcoplasmic reticulum is a bit like
endoplasmic reticulum of other cells, it
is hollow.
•Function of sarcoplasmic reticulum is
to store calcium ions.
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Sliding-Filament Theory
• The sliding-filament theory proposes
that muscle fibers shorten or lengthen
because thick and thin myofilaments
slide past each other without the
filaments themselves changing length.
• See http://thepoint.lww.com Animation:
Sliding Filament Theory.
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Sliding-Filament Theory (cont’d)
• The myosin crossbridges, which
cyclically attach, rotate, and detach
from the actin filaments with energy
from ATP hydrolysis, provide the
molecular motor to drive fiber
shortening
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Excitation-Contraction Coupling
• Provides the physiologic mechanism
whereby an electrical discharge at the
muscle initiates the chemical events
that cause activation.
• When stimulated to contract, Ca++
released from SR.
• Rapid binding of Ca++ to troponin in
actin filaments releases troponin’s
inhibition of actin-myosin inhibition.
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• Actin combines with
myosin-ATP.
• Actin activates ATPase,
which splits ATP.
• Energy release
produces crossbridge
movement (power
stroke).
• New ATP attaches to
myosin crossbridge to
dissociate from actin.
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Relaxation
• When muscle
stimulation ceases,
intracellular Ca++
decreases as Ca++ is
pumped back in SR by
active transport.
• Ca++ removal restores
inhibitory action of
troponin-tropomyosin.
Copyright © 2006 Lippincott Williams & Wilkins.