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