This document discusses the structure and function of skeletal muscle. It begins with an introduction to skeletal muscle and describes the characteristics of muscle fibers, including that they are multinucleated and striated. It then details the structure of the sarcomere, the basic contractile unit of skeletal muscle, including the thin actin filaments and thick myosin filaments. The document also describes excitation-contraction coupling and the sliding filament model of muscle contraction in which myosin cross-bridges attach to actin and generate force through an ATP-fueled cycling process.

1. Dr. Dinesh T,
Junior resident.
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2. Discussion topics
 Introduction
 Myocyte- structure, development
 Muscle proteins
 Sarcomere
 Sarcotubular system
 Excitation- contraction coupling
 Molecular basis of muscle contraction
 Summary
 Muscle relaxation
 Conclusion
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3. Introduction
 Human body contains over 400 skeletal muscles
 40-50% of total body weight
Functions of skeletal muscle
 Body movement (Locomotion)
 Maintenance of posture
 Respiration
 Diaphragm and intercostal contractions
 Communication (Verbal and Facial)
 Constriction of organs and vessels
 Peristalsis of intestinal tract
 Vasoconstriction of b.v. and other structures (pupils)
 Production of body heat (Thermogenesis)
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4. Skeletal Muscle Characteristics
Slide 6.3Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Most are attached by tendons to bones
Cells are multinucleate
Striated – have visible banding
Voluntary – subject to conscious control
Cells are surrounded and bundled by
connective tissue = great force, but tires
easily
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5. Terms
Sarcolemma = Cell membrane
Sarcoplasm = Cytoplasm
Sarcoplasmic Reticulum =
Endoplasmic Reticulum
Sarcosomes = Mitochontria
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6. Structure of skeletal muscle:
connective tissue covering
 Epimysium
 Surrounds entire muscle
 Perimysium
 Surrounds bundles of muscle fibers
 Endomysium
 Surrounds individual muscle fibers
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9. Skeletal muscle structure
 Composed of muscle cells (fibers),
connective tissue, blood vessels,
nerves
 Fibers are long, cylindrical, and
multinucleated
 Tend to be smaller diameter in small
muscles and larger in large muscles. 1
mm- 4 cm in length
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10. •Develop from myoblasts;
numbers remain constant
•Striated appearance
•Nuclei are peripherally
located
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11. Embryologic origin:
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12. Muscle fiber anatomy
 Sarcolemma - cell membrane
 Surrounds the sarcoplasm (cytoplasm of fiber)
 Contains many of the same organelles seen in other cells
 An abundance of the oxygen-binding protein myoglobin
 Punctuated by openings called the transverse tubules (T-tubules)
 Narrow tubes that extend into the sarcoplasm at right angles to
the surface
 Filled with extracellular fluid
 Myofibrils -cylindrical structures within muscle fiber
 Are bundles of protein filaments (=myofilaments)
 Two types of myofilaments
1. Actin filaments (thin filaments)
2. Myosin filaments (thick filaments)
– At each end of the fiber, myofibrils are anchored to the inner surface
of the sarcolemma
– When myofibril shortens, muscle shortens (contracts)
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15. Muscle proteins
Contractile proteins
 Actin- thin myofiliment
 Myosin- thick filament
Regulatory proteins
 Tropomyosin
 Troponin
Attachment proteins
 Titin, nebulin, alpha actinin, dystrophin
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16. Structure of Actin and Myosin
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17. Actin (Thin)
Myofilaments
 Thin Filament: composed of 3 major
proteins
1. F (fibrous) actin
2. Tropomyosin
3. Troponin
 Two strands of fibrous (F) actin form
a double helix extending the length
of the myofilament; attached at
either end at sarcomere.
 Composed of G actin monomers
each of which has a myosin-
binding site
 Actin site can bind myosin during
muscle contraction.
 Tropomyosin: an elongated protein
winds along the groove of the F actin
double helix.
 Troponin is composed of three
subunits:
 Tn-A : binds to actin
 Tn-T :binds to tropomyosin,
 Tn-C :binds to calcium ions.
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19. Myosin (Thick)
Myofilament
 Many elongated myosin molecules
shaped like golf clubs.
 Single filament contains roughly 300
myosin molecules
 Molecule consists of two heavy myosin
molecules wound together to form a rod
portion lying parallel to the myosin
myofilament and two heads that extend
laterally.
 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. Sarcomeres: Z Disk
to Z Disk
 Sarcomere - repeating functional units of
a myofibril
 About 10,000 sarcomeres per
myofibril, end to end
 Each is about 2 µm long
 Differences in size, density, and
distribution of thick and thin filaments
gives the muscle fiber a banded or striated
appearance.
 A bands: a dark band; full length of thick
(myosin) filament
 M line - protein to which myosins attach
 H zone - thick but NO thin filaments
 I bands: a light band; from Z disks to ends of
thick filaments
 Thin but NO thick filaments
 Extends from A band of one sarcomere to A
band of the next sarcomere
 Z disk: filamentous network of protein.
Serves as attachment for actin myofilaments
 Titin filaments: elastic chains of amino
acids; keep thick and thin filaments in
proper alignment
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24. Sarcoplasmic Reticulum (SR)
 SR is an elaborate, smooth endoplasmic
reticulum
 runs longitudinally and surrounds each myofibril
 Form chambers called terminal cisternae on either
side of the T-tubules
 A single T-tubule and the 2 terminal cisternae
form a triad
 SR stores Ca++ when muscle not contracting
 When stimulated, calcium released into sarcoplasm
 SR membrane has Ca++ pumps that function to pump
Ca++ out of the sarcoplasm back into the SR after
contraction
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25. Sarcoplasmic Reticulum (SR)
Figure 9.5
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28. Muscular Contraction
 The sliding filament model
 Muscle shortening occurs due to the movement of the
actin filament over the myosin filament
 Formation of cross-bridges between actin and myosin
filaments
 Reduction in the distance between Z-lines of the
sarcomere
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29. Sliding Filament Theory
 Rest – uncharged ATP cross-bridge complex
 Excitation-coupling – charged ATP cross-bridge
complex, “turned on”
 Contraction – actomyosin – ATP > ADP & Pi +
energy
 Recharging – reload cross-bridge with ATP
 Relaxation – cross-bridges “turned off”
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30. Sliding Filament Model of
Contraction
 Thin filaments slide past the thick ones so that the
actin and myosin filaments overlap to a greater degree
 In the relaxed state, thin and thick filaments overlap
only slightly
 Upon stimulation, myosin heads bind to actin and
sliding begins
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31. Cross-Bridge Formation in Muscle
Contraction
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32. Myosin ATPase Cycle
From: Stryer 1988. Biochemistry. Freeman eds. Based on:
Lymn and Taylor. 1971. Biochemistry 10: 4617
Pi
ADP
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34. EM Shows Different TM positions along the Actin Filament
Relaxed Ca-Activated Rigor Ca-Activated
X-bridge
From: Craig & Lehman.. 2001. J. Mol. Biol 311: 1027.
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36. Excitation-Contraction Coupling
 Mechanism where an
action potential causes
muscle fiber
contraction
 Involves
 Sarcolemma
 Transverse or T tubules
 Terminal cisternae
 Sarcoplasmic reticulum
 Ca2+
 Troponin
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37. Sources of ATP for Muscle
Contraction
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38. Energy Sources
 ATP provides immediate energy for muscle
contractions from 3 sources
 Creatine phosphate
 During resting conditions stores energy to synthesize ATP
 Anaerobic respiration
 Occurs in absence of oxygen and results in breakdown of
glucose to yield ATP and lactic acid
 Aerobic respiration
 Requires oxygen and breaks down glucose to produce
ATP, carbon dioxide and water
 More efficient than anaerobic
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39. Energy for Muscle Contraction
Slide 6.24Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Direct phosphorylation
Muscle cells contain creatine
phosphate (CP)
CP is a high-energy
molecule
After ATP is depleted, ADP is
left
CP transfers energy to
ADP, to regenerate ATP
CP supplies are exhausted in
about 20 seconds
Figure 6.10a11/19/2011 39Jipmer Physiologist

40. Energy for Muscle Contraction
SlideCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Anaerobic glycolysis
Reaction that breaks
down glucose without
oxygen
Glucose is broken down
to pyruvic acid to
produce some ATP
Pyruvic acid is
converted to lactic acid
Figure 6.10b11/19/2011 40Jipmer Physiologist

41. Energy for Muscle Contraction
Slide 6.25Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Aerobic Respiration
Series of metabolic
pathways that occur in
the mitochondria
Glucose is broken down
to carbon dioxide and
water, releasing energy
This is a slower reaction
that requires continuous
oxygen
Figure 6.10c
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