Chapter 10 I. Skeletal Muscle Tissue and the Muscular System
A. Three types of muscle
B. Skeletal muscle functions
II. Anatomy of Skeletal Muscle
A. Organization of connective tissues
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Epimysium surrounds muscle
Perimysium sheathes bundles of muscle fibers
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Epimysium and perimysium contain blood vessels and nerves
Endomysium covers individual muscle fibers
Tendons or aponeuroses attach muscle to bone or muscle
B. Skeletal muscle fibers/cells
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Sarcolemma (cell membrane)
Sarcoplasm (muscle cell cytoplasm)
Sarcoplasmic reticulum (modified ER)
Sarcomeres – regular arrangement of myofibrils
C. Myofibrils
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Thick and thin filaments
Organized regularly into sarcomeres
1) Thin filaments
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F-actin
Nebulin
Tropomyosin
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Covers active sites on G-actin
Troponin
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Binds to G-actin and holds tropomyosin in place
2) Thick filaments
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Bundles of myosin fibers around titin core
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Myosin molecules have elongate tail, globular head
Heads form cross-bridges during contraction
Interactions between G-actin and myosin prevented by tropomyosin during rest
D. Sliding filament theory
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Explains the relationship between thick and thin filaments as contraction proceeds
Cyclic process beginning with calcium release from SR
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Calcium binds to troponin
Troponin moves, moving tropomyosin and exposing actin active site
Myosin head forms cross bridge and bends toward H zone
ATP allows release of cross bridge
III. The Contraction of Skeletal Muscle
A. Control of skeletal muscle activity
The neuromuscular junction:
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Action potential arrives at synaptic terminal
ACh released into synaptic cleft
ACh binds to receptors on post-synaptic neuron
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Action potential in sarcolemma
B. Excitation/contraction coupling
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Action potential along T-tubule causes release of calcium from cisternae of SR
Initiates contraction cycle
• Attachment
• Pivot
• Detachment
• Return
D. Relaxation
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Acetylcholinesterase breaks down ACh
Limits the duration of contraction
IV. Tension Production
A. Tension production by muscle fibers
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All or none principle
Amount of tension depends on number of cross bridges formed
Skeletal muscle contracts most forcefully over a narrow ranges of resting lengths
B. From twitch…to Tetanus
C. Motor Units
• Motor units
• All the muscle fibers innervated by one neuron
• Precise control of movement determined by number and size of motor unit
• Muscle tone
• Stabilizes bones and joints
D. Contractions
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Isometric
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Tension rises, length of muscle remains constant
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Isotonic
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Return to resting lengths due to elastic components, contraction of opposing muscle groups, gravity
• Tension rises, length of muscle changes
V. Energy Use and Muscle Contraction
A. Creatine Phosphate and ATP
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Creatine phosphate releases stored energy to convert ADP to ATP
Aerobic metabolism provides most ATP needed for contraction
At peak activity, anaerobic glycolysis needed to generate ATP
B. Energy use and level of muscular activity
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Activity levels:
At rest:
Moderate activity:
Peak activity:
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Fatigued muscle no longer contracts
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Build up of lactic acid
Exhaustion of energy resources
C. Recovery period
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Begins immediately after activity ends
Oxygen debt (excess post-exercise oxygen consumption)
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Amount of oxygen required during resting period to restore muscle to normal conditions
VI. Cardiac Muscle Tissue
A. Structural characteristics of cardiac muscle
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Located only in heart
Cardiac muscle cells are small
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One centrally located nucleus
Short broad T-tubules
Dependent on aerobic metabolism
Intercalated discs where membranes contact one another
B. Functional characteristics of cardiac muscle tissue
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Autorhythmicity
Contractions last longer than skeletal muscle
Do not exhibit wave summation
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No tetanic contractions possible
VII. Smooth Muscle Tissue
A. Structural characteristics of smooth muscle
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Nonstriated
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Lack sarcomeres
Thin filaments anchored to dense bodies
Involuntary
B. Functional characteristics of smooth muscle
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Contract when calcium ions interact with calmodulin
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Activates myosin light chain kinase
Functions over a wide range of lengths
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Plasticity
Multi-unit smooth muscle cells are innervated by more than one motor neuron
Visceral smooth muscle cells are not always innervated by motor neurons
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Neurons that innervate smooth muscle are not under voluntary control