SKELETAL MUSCLE TISSUE-PAWLINA

Muscle Tissue Overview

  • Muscle tissue is essential for body movement and the functionality of internal organs.

  • Characterized by elongated cells with contraction as the primary function.

  • Muscle contraction is governed by myofilament interactions, primarily involving actin and myosin.

Classification of Muscle Types

Principal Types of Muscle

  • Striated Muscle: Exhibits cross-striations, includes:

    • Skeletal Muscle: Attached to bones, responsible for voluntary movements, posture, and eye movements. Contains multinucleated syncytia.

    • Cardiac Muscle: Found in the heart, involuntary contraction, cells connected by intercalated discs.

  • Smooth Muscle: Lacks striations, found in visceral organs and blood vessels, involuntary control.

Myofilament Types

  • Thin Filaments: (6-8 nm diameter)

    • Composed mainly of actin.

    • Each thin filament consists of fibrous actin (F-actin) formed from globular actin (G-actin) molecules.

  • Thick Filaments: (~15 nm diameter)

    • Made of myosin II (200-300 molecules per filament).

    • Myosin molecules arranged in a staggered array with heads projecting outward.

Connective Tissue in Muscle

  • Endomysium: Surrounds individual muscle fibers; contains blood vessels and nerves.

  • Perimysium: Thicker layer surrounding bundles of fibers (fascicles);

  • Epimysium: Dense connective tissue surrounding whole muscle and compartments.

Skeletal Muscle Fiber Composition

  • Multinucleated muscle fibers with a polygonal shape (10-100 μm in diameter).

  • Cross-sectional area shows a variety of thicknesses based on fascicle orientation.

  • Fiber nuclei are located near the plasma membrane (sarcolemma).

Fiber Types in Skeletal Muscle

Classification Based on Speed and Metabolism

  • Type I (Slow Oxidative):

    • Small, red fibers, high myoglobin, fatigue-resistant.

    • Adapted for endurance, prevalent in postural muscles.

  • Type IIa (Fast Oxidative Glycolytic):

    • Intermediate size, moderate resistance to fatigue, adaptable.

    • Efficient for both aerobic and anaerobic energy use.

  • Type IIb (Fast Glycolytic):

    • Larger, light pink, high anaerobic enzyme levels, fatigue-prone.

    • Suitable for short bursts of powerful movement (e.g., sprinting).

Myofibrils and Myofilaments

  • Myofibrils: Structural subunits of muscle fibers filled with myofilaments.

    • Myofilaments: Contractile components, thin (actin) and thick (myosin), organized in patterns that result in cross-striation.

  • Sarcomeres:

    • Basic functional units between Z lines, measuring 2-3 μm.

    • Comprised of alternating A bands (thick filaments) and I bands (thin filaments).

Actomyosin Cross-Bridge Cycle

  • Initiation: Myosin heads interact with actin when binding sites are exposed post-calcium increase.

  • Stages of Cross-Bridge Cycle:

    1. Attachment: Myosin binds actin tightly (rigor state).

    2. Release: ATP binding reduces myosin’s affinity for actin.

    3. Bending: Myosin head power stroke is initiated.

    4. Force Generation: Myosin head straightens, moving actin.

    5. Reattachment: Cycle repeats with new actin binding.

Regulation of Muscle Contraction

  • Calcium Availability:

    • Calcium ions (from the sarcoplasmic reticulum) are crucial for contraction initiation.

    • Involves voltage-gated channels and T-tubule system connecting to muscle contraction signaling pathways.

  • Motor Innervation:

    • Motor neurons release neurotransmitters at the neuromuscular junction causing muscle fiber depolarization and contraction.

Muscle Development and Repair

  • Myogenic Stem Cells: Derive from embryonic mesoderm; essential for muscle repair and regeneration.

  • Satellite Cells: Divide and differentiate into myoblasts in response to muscle injury, allowing muscle fiber regeneration.

Cardiac Muscle

  • Structure similar to skeletal muscle, but features intercalated discs for intercellular communication.

  • Contains a single or binucleated central nucleus; rich in mitochondria and glycogen for energy production.

  • Specialized for rhythmic, involuntary contractions.