Muscles
Skeletal Muscle
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Introduction
• Movement is a fundamental characteristic of all living organisms
• Three types of muscular tissue—skeletal, cardiac, and smooth
• Important to understand muscle at the molecular, cellular, and tissue levels of organization
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Universal Characteristics of Muscle• Responsiveness (excitability)
– To chemical signals, stretch, and electrical changes across the plasma membrane
• Conductivity– Local electrical change triggers a wave of excitation that travels
along the muscle fiber
• Contractility – Shortens when stimulated
• Extensibility – Capable of being stretched between contractions
• Elasticity – Returns to its original resting length after being stretched
Characteristics of Muscles
• Skeletal and smooth muscle cells are elongated (muscle cell = muscle fiber)
• Contraction and shortening of muscles is due to the movement of microfilaments
• All muscles share some terminology– Prefixes myo and mys refer to “muscle”– Prefix sarco refers to “flesh”
• Composes almost 50% of body mass
Structure of a Skeletal Muscle
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Connective Tissue Wrappings of Skeletal Muscle
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Fascia: a band or sheet of connective tissue, primarily collagen, beneath the skin that attaches, stabilizes, encloses, and separates muscles and other internal organs
Tendons & Aponeurosis• Tendon: fascia that projects beyond end of
muscle forming chordlike structure to attach muscle to bone
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Tendon Ruptures
Connective Tissue Wrappings of Skeletal Muscle• Cells are surrounded and bundled by connective tissue
– Endomysium—encloses a single muscle fiber (cell)– Perimysium—wraps around a fascicle (bundle) of
muscle fibers– Epimysium—covers the entire skeletal muscle
Tendons & Aponeurosis
• Aponeurosis: connective tissue that forms broad fibrous sheets which may attach to bone or to covering a adjacent muscles
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Fascia
Structure of a Skeletal Muscle
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Figure 6.3a
Sarcolemma
Myofibril
Dark(A) band
Light(I) band
Nucleus
(a) Segment of a muscle fiber (cell)
Muscle fibers (cells) have many nuclei and lots of mitochondria
Muscle cell (myofiber)
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The Muscle Fiber• Sarcolemma—plasma membrane of a muscle fiber• Sarcoplasm—cytoplasm of a muscle fiber• Sarcoplasmic Reticulum—specialized smooth
endoplasmic reticulum• Transverse tubules (T-tubules)— pass electric
impulse to Sarcoplasmic reticulum
Fig 8.2 21
Myofibrils: Long organelles inside muscle cell made of myofilaments
1. Actin2. Myosin
MyofilamentsCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Bare zoneThin filament (Actin)
Thick filament (Myosin)
• Thick filaments are made up of the protein myosin.• Thin filaments are mainly composed of the protein
actin, along with troponin and tropomyosin•
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Myofilaments
• Thick filaments—made of several hundred myosin molecules– Shaped like a golf club
• Two chains intertwined to form a shaftlike tail
• Double globular head
(a) Myosin molecule
HeadTail
(b) Thick filament
Myosin head
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Myofilaments• Thin filaments
– Actin: two intertwined strands • String of globular (G) actin subunits each with an active site that
can bind to head of myosin molecule
– Tropomyosin molecules— block active site– Troponin molecule: small, calcium-binding protein on
each tropomyosin molecule
(c) Thin filament
Troponin complex G actinTropomyosin
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 11.3c
Actin & Myosin Complex
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The organization of these myofibrils produces striations.
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Figure 6.3c
Z disc
Sarcomere
M lineZ disc
Thin (actin) filament
Thick (myosin) filament
(c) Sarcomere (segment of a myofibril)
• Myofibrils are made up of many sarcomeres, joined end-to-end
Sarcomeres
Figure 6.3d
Thick filament Bare zone Thin filament
(d) Myofilament structure (within one sarcomere)
Sarcomeres
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• Sarcomere—segment from Z disc to Z discFunctional contractile unit of muscle fiber
• Muscle cells shorten because their individual sarcomeres shorten – Z disc (Z lines) are pulled closer together as thick and thin filaments slide past each other
• Neither thick nor thin filaments change length during shortening—Only the amount of overlap changes
Microscopic Anatomy of Skeletal Muscle• Myofibrils are aligned to give distinct bands
– I band = light band, Contains only thin filaments– A band = dark band, Contains the entire length
of the thick filaments– H zone (bare zone)= only thick filaments
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Myofilaments
• Contractile proteins—myosin and actin do the work • Regulatory proteins—tropomyosin and troponin
– Like a switch that determines when the fiber can contract and when it cannot
– Contraction activated by release of calcium into sarcoplasm and its binding to troponin
– Troponin changes shape and moves tropomyosin off the active sites on actin
(b) Thick filament
Myosin head
(c) Thin filament
Troponin complex G actinTropomyosin
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 11.3b,c
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