Skeletal Muscle Study Guide

Lecture 9: Skeletal Muscle

Overview

  • Chapter Reference: Chapter 9, Section A, pages 257-261, 265.

  • Key Topics: Types of muscle tissue, microstructure of skeletal muscle, sarcomere structure, actin and myosin, sliding filament mechanism, role of calcium ions (Ca2+), troponin, and tropomyosin.

Types of Muscle Tissue

  1. Skeletal Muscle

    • Definition: Voluntary muscle type that is attached to bones and enables movement.

    • Structure: Striated and multinucleated.

    • Characteristics: Largest tissue in the body, composed of muscle fibers (adult cells do not divide). Satellite cells assist in repair and growth.

    • Contraction: Individual fibers exhibit an "all-or-none" principle, meaning they contract fully or not at all.

  2. Smooth Muscle

    • Definition: Involuntary muscle type found in walls of hollow organs.

    • Structure: Non-striated, consists of spindle-shaped cells with a single nucleus.

    • Functionality: Controls various involuntary movements such as digestion and circulation.

  3. Cardiac Muscle

    • Definition: Involuntary muscle type specific to the heart.

    • Structure: Striated with intercalated disks that facilitate synchronized contractions.

    • Functionality: Responsible for pumping blood throughout the body.

Microstructure of Skeletal Muscle

  • Muscle Fibers: Composed of myofibrils that contain the contractile elements of muscle.

  • Sarcomere: The smallest unit of muscle tissue, functioning as the basic contractile unit within a myofibril.

    • Components:

    • Thick Filaments: Composed primarily of myosin.

    • Thin Filaments: Composed primarily of actin, along with accessory proteins.

Sarcomere Structure

  • Major Components:

    • A Band: Region containing thick filaments and overlapping thin filaments.

    • I Band: Region containing only thin filaments.

    • H Zone: Area within the A band where only thick filaments are present (may reduce during contraction).

    • Z Line: Boundaries of sarcomeres where thin filaments attach.

    • M Line: Central region of sarcomere where thick filaments are anchored.

    • Titin: A protein that stabilizes the thick filaments and contributes to elasticity.

Sarcomere in Different States

  • Relaxed Sarcomere:

    • Thick and thin filaments partially overlap.

    • A band remains unchanged.

    • I band and H zone may be wider.

  • Contracted Sarcomere:

    • Thick and thin filaments slide past each other, resulting in decreased I band and H zone.

    • Z lines are closer to each other, indicating contraction.

Actin and Myosin Mechanism

  1. Actin

    • Thin filament protein that forms the backbone of the sarcomere.

    • Provides binding sites for myosin cross-bridges.

  2. Myosin

    • Thick filament protein characterized by tails and heads (cross-bridges).

    • Contains ATP binding sites and actin binding sites crucial for muscle contraction.

    • Cross-bridge bending mechanism facilitates force generation during contraction.

  3. Cross-Bridge Formation:

    • Myosin heads attach to actin binding sites to form cross-bridges.

    • Each myosin head can operate independently, allowing coordinated contractions across the muscle fiber.

    • Requires ATP for energy and activity.

Sliding Filament Mechanism

  • Key Players:

    • Ca2+ (Calcium ions):

    • Initiates muscle contraction by binding to troponin, causing a conformational change.

    • In presence of high cytosolic Ca2+, cross-bridge binding sites on actin are exposed.

    • Troponin:

    • Binds to calcium, shifting tropomyosin away from actin’s binding sites.

    • Tropomyosin:

    • In a relaxed state, it covers actin binding sites, preventing cross-bridge formation.

    • When Ca2+ is present, it shifts away from the binding sites allowing myosin to bind and generate force.

  • Contraction Cycle:

    1. Myosin heads bind to actin (cross-bridge formation).

    2. Myosin heads pivot, pulling actin filaments closer (power stroke).

    3. ATP binds to myosin, causing it to detach from actin.

    4. ATP is hydrolyzed, re-cocking the myosin head for the next cycle.

Summary of Ca2+ Activity

  • Low Cytosolic Ca2+ Levels:

    • Troponin remains unbound, and tropomyosin obstructs actin binding sites, preventing muscle contraction.

  • High Cytosolic Ca2+ Levels:

    • Troponin binds Ca2+, revealing actin binding sites, facilitating muscle contraction via cross-bridge cycling.

Figures and Illustrations

  • Various figures illustrate the organizational structure of the sarcomere, comparing contracted and relaxed states, and detailing the molecular interactions between actin and myosin.

    • Figures Noted: Fig. 9.1, Fig. 9.2, Fig. 9.3, Fig. 9.4, Fig. 9.11, and Fig. 9.13, each demonstrating different structural components and functions within skeletal muscle.