Muscle Tissue

Chapter 11: Muscle Tissue

Overview of Muscle Tissue

  • Muscle tissue (fibers) is divided into three primary types:
    • Skeletal Muscle
    • Cardiac Muscle
    • Smooth Muscle
  • Muscle tissue is closely associated with connective tissues, nerves, and blood vessels.

Muscle Tissue Structures

Components of Skeletal Muscles

  • Connective tissues in skeletal muscles include:
    1. Epimysium: Surrounds the entire muscle and connects to the deep fascia, separating the muscle from surrounding tissues.
    2. Perimysium: Surrounds muscle fiber bundles (fascicles), containing blood vessels and nerve supplies to fascicles.
    3. Endomysium: Surrounds individual muscle fibers (muscle cells), contains capillaries and nerve fibers, and holds satellite cells (stem cells for repair).

Functionality of Skeletal Muscles

Functions

  • Skeletal muscles serve various functions, such as:
    1. Producing skeletal movement
    2. Maintaining body position
    3. Supporting soft tissues
    4. Guarding body openings
    5. Maintaining body temperature
    6. Storing nutrient reserves

Organization of Muscle Fiber Components

  • Skeletal muscles consist of several layers:
    • Connective tissue layers (Epimysium, Perimysium, Endomysium) surround muscle fibers and clusters of fibers known as fascicles.

Muscle Attachments

  • The connective tissues (endomysium, perimysium, and epimysium) converge at muscle ends to form connections to bones:
    • Tendon: A bundle of connective tissue.
    • Aponeurosis: A flattened sheet of connective tissue.

Skeletal Muscle Fibers (Muscle Cells)

  • Characteristics:
    • Very long cells developed from myoblasts (involved in muscle fiber formation).
    • Each fiber has hundreds of nuclei.

Formation of Skeletal Muscle Fibers

  • Muscle fibers develop through the fusion of mesodermal cells called myoblasts, resulting in:
    • Immature muscle fibers evolving into mature muscle fibers with myofibrils, sarcolemmas, nuclei, and mitochondria.

Muscle Cell Membrane: Sarcolemma

  • Sarcolemma: The cell membrane of muscle cells that maintains a transmembrane potential and surrounds the sarcoplasm (cytoplasm of muscle cells).

Transverse Tubules (T Tubules)

  • Functionality: Transmit action potentials throughout muscle fibers, ensuring simultaneous contraction. They are filled with extracellular fluid and allow for efficient signal transmission.

Myofibrils

  • Structure: Subdivisions within muscle fibers, each containing thousands of myofilaments (bundles of protein filaments).
  • Types of Myofilaments:
    1. Thin filaments: Composed of the protein actin.
    2. Thick filaments: Composed of the protein myosin.

Sarcoplasmic Reticulum

  • A membranous structure that surrounds each myofibril, high in calcium ions, and forms chambers related to T tubules to help transmit action potentials to myofibrils.

Sarcomeres: Contractile Units

  • Definition: Smallest functional units of muscle serving as contractile units, containing:
    • Thick filaments (myosin)
    • Thin filaments (actin)
    • Stabilizing proteins
    • Regulatory proteins
  • Characteristics:
    • Approximately 10,000 sarcomeres per myofibril.

Sarcomere Structure

Components

  • A Band: Dark center of sarcomere equal to the length of thick filaments.
    • M Line: Center of the A band, where thick filaments meet.
    • H Zone: Region where thick filaments are present without thin filaments.
    • Zone of Overlap: Area featuring overlapping thick and thin filaments.

I Band

  • Definition: Light band containing only thin filaments.
  • Components:
    • Z lines: Mark boundaries between adjacent sarcomeres.
    • Titin: Elastic protein ensuring alignment of thick and thin filaments.

Sarcomere Functionality

  • Mechanism: Transverse tubules encircle sarcomeres near overlap zones, triggering interactions between thin and thick filaments via calcium release from the sarcoplasmic reticulum (SR).

Levels of Skeletal Muscle Organization

Level 1: Skeletal Muscle

  • Surrounded by Epimysium.
  • Contains muscle fascicles.

Level 2: Muscle Fascicle

  • Surrounded by Perimysium.
  • Contains muscle fibers.

Level 3: Muscle Fiber

  • Surrounded by Endomysium.
  • Contains myofibrils.

Level 4: Myofibril

  • Surrounded by sarcoplasmic reticulum.
  • Composed of sarcomeres (Z line to Z line).

Level 5: Sarcomere

  • Composed of thick and thin filaments.
  • Contains Z line, M line, A band, H zone, and titin.

Muscle Contraction

  • Trigger: Interactions between thick and thin filaments.
  • Outcome: Caused by the specific structures of protein molecules, determining how they interact effectively.

Thin Filament Proteins (Actin)

  1. F actin/G actin: The structural foundation of thin filaments, forming double-twisted rows.
  2. Nebulin: Protein that holds F actin strands together.
  3. Tropomyosin: A double strand that covers G actin, preventing actin–myosin interaction.
  4. Troponin: A globular protein that binds tropomyosin to G actin and interacts with calcium ions (Ca²+) to unlock active sites on actin.

Thick Filaments (Myosin)

  • Composed of twisted subunits and characterized by:
    • Tail: Binds to other myosin molecules, pointing toward the M-line.
    • Head: Contains globular protein subunits that project toward thin filaments (actin) and are involved in cross-bridge formation.

Myosin Action Mechanisms

  • Hinge: Allows the head to pivot during the power stroke.
  • Cross-bridges: Occur when myosin heads connect with active sites on G actin.

Initiating Contraction

  • Key Element: Calcium binds to troponin, causing a conformational change in the troponin-tropomyosin complex that exposes the active sites on G actin.

Sliding Filament Theory

  • Involves:
    1. H zones and I bands: Decrease in size during contraction.
    2. Zone of overlap: Increases as filaments slide over one another.
    3. Z lines: Draw closer together; width of A band remains constant.

Skeletal Muscle Control

Excitation-Contraction Coupling

  • Process involves the release of calcium ions, neural stimulation at the sarcolemma, leading to thick-thin filament interaction and tension production.

The Neuromuscular Junction

  • Definition: The connection between nerve and muscle fibers that enables signal transmission to initiate contraction.
  • Components:
    • Synaptic terminal: Releases neurotransmitter acetylcholine (ACh) into the synaptic cleft.
    • Motor end plate: Membrane receiving ACh from the synaptic cleft, facilitating signal transmission to the muscle fiber.

Action Potential and Muscle Stimulation

  • Definition: A sudden change in transmembrane potential that is influenced by sodium ions influx through the sarcolemma.
  • Process Sequence:
    1. Action potential arrives; ACh released.
    2. ACh binds at the motor end plate; initiates action potential in the sarcolemma.
    3. Repolarization occurs via AChE breaking down ACh.

Steps of Muscle Contraction Cycle

  1. Exposure of active sites.
  2. Formation of cross-bridges.
  3. Pivoting of myosin heads.
  4. Detachment of cross-bridges.
  5. Reactivation of myosin heads.

Rigor Mortis

  • Definition: A condition resulting in fixed muscular contraction post-mortem due to lack of ATP for cross-bridge release.
  • Duration: Lasts until autolysis destroys muscle filaments (approx. 15-25 hours).

Summary of Muscle Contraction Process

Steps Initiating Contraction:

  1. ACh released, binding to receptors.
  2. Action potential reaches T tubules.
  3. ACh removed by AChE.
  4. Calcium released from SR.
  5. Exposure of active sites, leading to cross-bridge formation.
  6. Contraction begins.

Final Steps of Contraction:

  1. Recapture of Calcium by SR.
  2. Active sites covered, preventing further interaction.
  3. Contraction ends.
  4. Muscle returns to initial length passively.

Key Concept

  • Skeletal muscle fibers shorten as thin filaments slide between thick filaments, driven by free Ca²⁺ in the sarcoplasm. Contraction requires energy (ATP), while relaxation allows passive return to resting length.