Muscles & Muscle Tissue

Muscle Tissue Overview

Definition and Importance

  • Muscle: The term derives from the Latin word "mus," which means mouse, as flexing muscles resemble mice beneath the skin. Muscle is a dominant tissue type found in the heart and walls of hollow organs and constitutes nearly 50% of the body’s mass.
  • The main characteristic of muscle tissue is its ability to transform chemical energy (ATP) into mechanical energy.

Muscle Terminology

  • Muscle fibers: Used to refer to skeletal and smooth muscle cells; the term reflects their elongated structure.
  • Prefix ‘myo’ or ‘mys’: Refers to muscle (e.g., myocyte = muscle cell).
  • Prefix ‘sarco’: Means flesh (e.g., sarcoplasm = cytoplasm of a muscle cell).

Special Characteristics of Muscle Tissue

  1. Excitability (Responsiveness): Ability to receive and respond to stimuli, primarily through chemical signals (e.g., neurotransmitters). The response typically results in an electrical impulse, leading to contraction.
  2. Contractility: Unique capability of muscle cells to shorten.
  3. Extensibility: Muscles can stretch beyond their resting length.
  4. Elasticity: After contracting or stretching, muscles return to their resting length.

Major Functions of Muscle Tissue

  • Movement: Enables locomotion and movement of materials through organs and vessels.
  • Posture & Body Position: Works with the skeletal system to maintain body position against gravity.
  • Stabilizing Joints: Pulls on bones to facilitate movement and stabilize joints.
  • Generate Heat: Especially skeletal muscle, important for maintaining body temperature (typically around 37°C).
  • Protection: Safeguards internal organs.

Classifications of Muscle Tissue

  1. Skeletal Muscle

    • Location: Attached to the skeleton
    • Control: Voluntary (except for reflexes)
    • Appearance: Striated
    • Function: Body mobility
    • Pace: Can exert huge forces quickly but tires easily.

  2. Cardiac Muscle

    • Location: Only in the heart (myocardium)
    • Control: Involuntary but bpm can increase with physical activity
    • Appearance: Striated
    • Function: Pumps heart
    • Pace: Contracts at a steady, baseline pace.

  3. Smooth Muscle

    • Location: Walls of hollow organs (excluding heart)
    • Control: Involuntary
    • Appearance: Spindle-shaped
    • Function: Movement of materials through internal organs
    • Pace: Contractions are slow and sustained.

Structure and Organizational Levels of Skeletal Muscle

  • Muscle (Organ): Composed of numerous muscle cells, connective tissues, blood vessels, and nerve fibers.
  • Connective Tissue Wrappings:
    • Epimysium: Outer covering of muscle.
    • Perimysium: Surrounds fascicles – bundles of muscle cells.
    • Endomysium: Covers individual muscle fibers.
  • Fascicle: Discrete bundle of muscle cells segregated by a connective tissue sheath.
  • Muscle Fiber (Cell): Elongated multinucleate with striated appearance.
  • Myofibrils: Rodlike contractile elements occupying much of the muscle cell volume, made of repeating sarcomeres.
  • Sarcomere: Contractile unit composed of thick (myosin) and thin (actin) filaments.

Muscle Attachments

  • Every skeletal muscle connects to bone or connective tissue at a minimum of two points:
    • Origin: Attachment to the less movable bone.
    • Insertion: Attachment to the movable bone.
  • Attachment Types:
    • Direct (Fleshy): Epimysium fused to periosteum of bone or perichondrium of cartilage.
    • Indirect: Connective tissues forming tendons or aponeuroses extend beyond the muscle; more common than direct attachments.

Connective Tissues Associated with Skeletal Muscle

  • Epimysium: Dense irregular connective tissue covering the muscle.
  • Perimysium: Fibrous connective tissue around muscle fascicles.
  • Endomysium: Sheath of connective tissue surrounding each muscle fiber.

Muscle Fiber Structure

  • Muscle Fiber: Single cylindrical cell with multiple nuclei and unique organelles:
    • Sarcoplasm: Cytoplasm of muscle fiber containing glycogen and myoglobin (oxygen storage).
    • Sarcoplasmic Reticulum (SR): Smooth ER regulating calcium release during contraction.
    • T-tubules: Extensions of the sarcolemma that penetrate muscle fibers, conducting impulses to deep regions.

Myofibrils and Sarcomeres

  • Myofibrils: Multiple per fiber; heavily packed (~80% of fiber volume), crucial for contraction.
  • Sarcomeres: Functional units with:
    • A Band: Comprised of thick (myosin) and thin (actin) filaments.
    • H Zone: Lighter area within A band (only thick filaments).
    • I Band: Contains only thin filaments.
    • Z Line: Anchors thin filaments, subdivides I Bands.

Cross-Bridge Formation and Contraction Cycle

  1. Cocking of Myosin Head: ATP hydrolysis energizes myosin head.
  2. Cross-Bridge Formation: Energized head binds to actin, forming a cross-bridge.
  3. Power Stroke: ADP and P are released; myosin head pivots, pulling actin.
  4. Cross-Bridge Detachment: New ATP binds to myosin, causing detachment from actin.

Sliding Filament Model of Contraction

  • When relaxed, thick and thin filaments overlap minimally. On contraction:
    • Thin filaments slide past thick filaments, increasing overlap.
    • Z Lines pulled toward the M Line, shortening I Bands and disappearing H Zone.

Sarcoplasmic Reticulum and T-tubules

  • Triad: Formed by SR cisterns and T-tubules, essential for calcium regulation during contraction.

Phases Leading to Muscle Contraction

Phase 1: Motor neuron stimulates muscle fiber:

  • Action potential generated, leading to release of acetylcholine.
  • Changes in ion permeability initiate depolarization, igniting action potential.
  • AP travels across the sarcolemma and into T-tubules.

Phase 2: Excitation-contraction coupling:

  • SR releases calcium, facilitating binding to troponin, which exposes myosin-binding sites on actin, initiating contraction.

The Motor Unit

  • A motor unit comprises one motor neuron and all muscle fibers it innervates.
  • Muscle control varies; fine control requires smaller motor units, while larger muscle groups use larger units.