Chapter 9, Section 1 Muscular System
Muscle is derived from Musculus, for “Mouse�
Imagine a mouse running beneath the skin.
Functions of Muscles: 1. Body movement 2. Maintain posture 3. Produces heat 4. Propel substances through body 5. Heartbeat
Types of muscles: 1. Smooth muscle 2. Cardiac muscle 3. Skeletal muscle
Smooth Muscle Fibers Characteristics of smooth muscles • Involuntary control • Tapered cells with a single, central nucleus • Lack striations
• Multi-unit Smooth Muscle • unorganized cells that contract as individual cells •Located within the iris of eye and the walls of blood vessels
• Visceral Smooth Muscle • Form sheets of muscle • Cells are connected by gap junctions • Muscle fibers contract as a group • Rhythmic contractions • Within walls of most hollow organs (viscera)
Cardiac Muscle
•Located only in the heart •Striated cells •Intercalated discs • Muscle fibers branch •Muscle fibers contract as a unit • Self-exciting and rhythmic
Skeletal Muscle
• Usually attached to bone • Voluntary control • Striated (light & dark bands) • Muscle fibers form bundles • Several peripheral nuclei
Coverings of Skeletal Muscle Fascia • Dense connective tissue surrounding skeletal muscles Tendons • Dense connective tissue that attaches muscle to bones • Continuation of muscle facia and bone periosteum Aponeurosis • Broad sheet of connective tissue attaching muscles to bone, or to other muscles.
Coverings of Skeletal Muscle
Epimysium • Connective tissue closely surrounding a muscle • Lies deep to fascia Perimysium • Surrounds organized bundles of muscle fibers, called fascicles Endomysium • Connective tissue that surrounds individual muscle fibers (cells)
Figure 9.3 Scanning electron micrograph of a fascicle surrounded by its perimysium. Muscle fibers within the fascicle are surrounded by endomysium.
Organization of Skeletal Muscle Fascicle • Organized bundle of muscle fibers Muscle Fiber • Single muscle cell • Collection of myofibrils Myofibrils • Collection of myofilaments Myofilaments • Actin filament • Myosin filament Figure 9.2 Skeletal muscle organization
Skeletal Muscle Fibers Sarcolemma • Cell membrane of muscle fibers Sarcoplasm • Cytoplasm of muscle fibers Sarcoplasmic Reticulum • Modified Endoplasmic Reticulum • Store large deposits of Calicium sarcolemma
Figure 9.2 c. A single muscle fiber composed of several myofibrils. A sarcolemma (membrane) surrounds the cell, and an extensive sarcoplasmic reticulum runs along the cell.
Skeletal Muscle Fibers (Transverse)T-tubules: • invaginations of sarcolemma, extending into the sarcoplasm. Cisternae: • Enlarged region of sarcoplasmic reticulum, adjacent to t-tubules Triad • T-tubule + adjacent cisternae
Openings into t-tubules
Myofibrils
Myofibrils • Actin – thin filaments • Myosin – thick filaments Striations • appear from the organization of actin and myosin
Figure 9.4 Organization of actin and myosin filaments
Sarcomere
Sarcomere • Functional unit of skeletal muscle •Area between adjacent Z-lines •During contraction Z-lines approach together and sarcomeres shorten
Striation Pattern of Skeletal Muscle • I Bands (light): actin filaments • Z Line = attaches to actin filaments (center of I bands) • A Band (dark) : Myosin filaments and overlapping actin filaments
Figure 9.5 thin and thick filaments in a sarcomere.
Myofilaments • Thick myofilaments • Myosin proteins • Cross-bridges (heads) on myosin • Cross-bridge attaches to actin during contraction
• Thin myofilaments • Actin proteins • Associated with troponin and tropomyosin proteins
Cross-Bridges • Myosin cross-bridges are extended when muscles are at rest “cocked” position. •During a contraction, cross-bridges bind to actin and “spring” forward. (Power Stroke) • Cross-bridges pull on actin as they spring forward. • ATP is required to “recock” the cross-bridges. (Recovery stroke) Actin binding site
Myosin cross-bridge In the “cocked” position
Power stroke
Myosin cross-bridge In the “sprung” position