Muscles

Skeletal Muscle 11-2

11-3

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 11-6

11-7 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 11-9

Connective Tissue Wrappings of Skeletal Muscle 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 11-15

11-16 Fascia

Structure of a Skeletal Muscle 11-17

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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)

11-20 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 Myofibrils: Long organelles inside muscle cell made of myofilaments 1. Actin 2. Myosin

Myofilaments Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bare zone Thin 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

11-23 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 Head Tail (b) Thick filament Myosin head Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

11-24 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 complexG actinTropomyosin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 11.3c

Actin & Myosin Complex 11-25

11-26 The organization of these myofibrils produces striations.

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Figure 6.3c Z disc Sarcomere M line Z 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 filamentBare zoneThin filament (d) Myofilament structure (within one sarcomere)

Sarcomeres Sarcomere—segment from Z disc to Z disc Functional 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

11-32 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 complexG actinTropomyosin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 11.3b,c