3. Introduction
ο Human body contains over 400 skeletal muscles
ο 40-50% of total body weight
Functions of skeletal muscle
ο Body movement (Locomotion)
ο Maintenance of posture
ο Respiration
ο Diaphragm and intercostal contractions
ο Communication (Verbal and Facial)
ο Constriction of organs and vessels
ο Peristalsis of intestinal tract
ο Vasoconstriction of b.v. and other structures (pupils)
ο Production of body heat (Thermogenesis)
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9. Skeletal muscle structure
ο Composed of muscle cells (fibers),
connective tissue, blood vessels,
nerves
ο Fibers are long, cylindrical, and
multinucleated
ο Tend to be smaller diameter in small
muscles and larger in large muscles. 1
mm- 4 cm in length
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10. β’Develop from myoblasts;
numbers remain constant
β’Striated appearance
β’Nuclei are peripherally
located
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12. Muscle fiber anatomy
ο Sarcolemma - cell membrane
ο Surrounds the sarcoplasm (cytoplasm of fiber)
ο Contains many of the same organelles seen in other cells
ο An abundance of the oxygen-binding protein myoglobin
ο Punctuated by openings called the transverse tubules (T-tubules)
ο Narrow tubes that extend into the sarcoplasm at right angles to
the surface
ο Filled with extracellular fluid
ο Myofibrils -cylindrical structures within muscle fiber
ο Are bundles of protein filaments (=myofilaments)
ο Two types of myofilaments
1. Actin filaments (thin filaments)
2. Myosin filaments (thick filaments)
β At each end of the fiber, myofibrils are anchored to the inner surface
of the sarcolemma
β When myofibril shortens, muscle shortens (contracts)
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17. ο Thin Filament: composed of 3 major
proteins
1. F (fibrous) actin
2. Tropomyosin Actin (Thin)
3. Troponin
ο Two strands of fibrous (F) actin form
a double helix extending the length
Myofilaments
of the myofilament; attached at
either end at sarcomere.
ο Composed of G actin monomers
each of which has a myosin-
binding site
ο Actin site can bind myosin during
muscle contraction.
ο Tropomyosin: an elongated protein
winds along the groove of the F actin
double helix.
ο Troponin is composed of three
subunits:
ο Tn-A : binds to actin
ο Tn-T :binds to tropomyosin,
ο Tn-C :binds to calcium ions.
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19. ο Many elongated myosin molecules
shaped like golf clubs.
ο Single filament contains roughly 300
Myosin (Thick) ο
myosin molecules
Molecule consists of two heavy myosin
molecules wound together to form a rod
Myofilament portion lying parallel to the myosin
myofilament and two heads that extend
laterally.
ο Myosin heads
1. Can bind to active sites on the actin
molecules to form cross-bridges.
(Actin binding site)
2. Attached to the rod portion by a
hinge region that can bend and
straighten during contraction.
3. Have ATPase activity: activity that
breaks down adenosine
triphosphate (ATP), releasing
energy. Part of the energy is used to
bend the hinge region of the myosin
molecule during contraction
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21. ο Sarcomere - repeating functional units of
a myofibril
ο About 10,000 sarcomeres per
Sarcomeres: Z Disk myofibril, end to end
ο Each is about 2 Β΅m long
to Z Disk ο Differences in size, density, and
distribution of thick and thin filaments
gives the muscle fiber a banded or striated
appearance.
ο A bands: a dark band; full length of thick
(myosin) filament
ο M line - protein to which myosins attach
ο H zone - thick but NO thin filaments
ο I bands: a light band; from Z disks to ends of
thick filaments
ο Thin but NO thick filaments
ο Extends from A band of one sarcomere to A
band of the next sarcomere
ο Z disk: filamentous network of protein.
Serves as attachment for actin myofilaments
ο Titin filaments: elastic chains of amino
acids; keep thick and thin filaments in
proper alignment
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24. Sarcoplasmic Reticulum (SR)
ο SR is an elaborate, smooth endoplasmic
reticulum
ο runs longitudinally and surrounds each myofibril
ο Form chambers called terminal cisternae on either
side of the T-tubules
ο A single T-tubule and the 2 terminal cisternae
form a triad
ο SR stores Ca++ when muscle not contracting
ο When stimulated, calcium released into sarcoplasm
ο SR membrane has Ca++ pumps that function to pump
Ca++ out of the sarcoplasm back into the SR after
contraction
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28. Muscular Contraction
ο The sliding filament model
ο Muscle shortening occurs due to the movement of the
actin filament over the myosin filament
ο Formation of cross-bridges between actin and myosin
filaments
ο Reduction in the distance between Z-lines of the
sarcomere
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29. Sliding Filament Theory
ο Rest β uncharged ATP cross-bridge complex
ο Excitation-coupling β charged ATP cross-bridge
complex, βturned onβ
ο Contraction β actomyosin β ATP > ADP & Pi +
energy
ο Recharging β reload cross-bridge with ATP
ο Relaxation β cross-bridges βturned offβ
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30. Sliding Filament Model of
Contraction
ο Thin filaments slide past the thick ones so that the
actin and myosin filaments overlap to a greater degree
ο In the relaxed state, thin and thick filaments overlap
only slightly
ο Upon stimulation, myosin heads bind to actin and
sliding begins
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36. Excitation-Contraction Coupling
ο Mechanism where an
action potential causes
muscle fiber
contraction
ο Involves
ο Sarcolemma
ο Transverse or T tubules
ο Terminal cisternae
ο Sarcoplasmic reticulum
ο Ca2+
ο Troponin
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37. Sources of ATP for Muscle
Contraction
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38. Energy Sources
ο ATP provides immediate energy for muscle
contractions from 3 sources
ο Creatine phosphate
ο During resting conditions stores energy to synthesize ATP
ο Anaerobic respiration
ο Occurs in absence of oxygen and results in breakdown of
glucose to yield ATP and lactic acid
ο Aerobic respiration
ο Requires oxygen and breaks down glucose to produce
ATP, carbon dioxide and water
ο More efficient than anaerobic
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