Muscle Tissue

Overview of the Muscular System

  • Comprehensive study of muscle tissue and its functions in Anatomy and Physiology.

Types of Muscle Tissue

  • Skeletal Muscle: Voluntary control, striated, attached to bones.

  • Smooth Muscle: Involuntary control, non-striated, found in walls of hollow organs.

  • Cardiac Muscle: Involuntary, striated, found in the heart.

  • Muscle Properties:

    • Excitability: Ability to respond to stimuli.

    • Elasticity: Ability of muscle to return to original length after stretching.

    • Extensibility: Ability to be stretched.

    • Contractility: Ability to shorten and generate force.

Connective Tissue Layers Surrounding Muscles

  • Epimysium:

    • Covers the entire muscle.

    • Composed of dense irregular connective tissue.

  • Perimysium:

    • Surrounds fascicles (bundles of muscle fibers).

  • Endomysium:

    • Covers individual muscle fibers.

    • Contains collagen and reticular fibers, providing nutrients to fibers.

Structure of Muscle Fiber

  • Sarcomere:

    • Structural and functional unit of striated muscle.

    • Contains:

      • Dark A-band: Thick filaments (myosin).

      • Light I-band: Thin filaments (actin).

      • Z disc: Boundary between sarcomeres.

      • H zone: Zone with only thick filaments.

      • M line: Middle of the sarcomere.

Neuromuscular Junction

  • Action potential arrives at the synaptic end bulb.

  • Calcium Channels: Voltage-sensitive; open and allow calcium influx.

  • ACh Release:

    • Calcium triggers exocytosis of acetylcholine (ACh).

    • ACh binds to receptors, resulting in sodium ion influx and depolarization.

  • Motor End Plate: Charge changes from negative to positive.

Excitation-Contraction Coupling

  • Arrival of action potential leads to ACh release and Na+ entry.

  • Action potentials travel along T-tubules, causing calcium release from sarcoplasmic reticulum.

  • Interaction between thick (myosin) and thin (actin) filaments leads to muscle contraction.

  • Muscle shortens and tension is produced.

Muscle Relaxation

  • Calcium is resorbed; ATP is required for relaxation.

  • The thick and thin filament interaction ceases, allowing muscle lengthening.

Sliding Filament Model of Muscle Contraction

  • Interaction details:

    • Binding of calcium to troponin exposes actin binding sites.

    • Myosin heads attach, forming crossbridges and pulling actin filaments (power stroke).

    • ATP detaches myosin from actin, resetting the cycle.

Energy Sources for Muscle Contraction

  • ATP Sources:

    • Creatine phosphate: Rapid ATP regeneration.

    • Glycolysis and Aerobic Respiration: Generates ATP from glucose and fatty acids.

  • Byproducts: Heat, CO2, H2O from metabolism.

Types of Muscle Contractions

  • Isotonic (muscle shortens under load):

    • Concentric: Muscle shortens.

    • Eccentric: Muscle lengthens under load.

  • Isometric: Muscle develops tension without shortening.

Motor Units

  • Defined as a motor neuron and all the muscle fibers it innervates.

  • Small Motor Units: Precise control (e.g., extraocular muscles).

  • Large Motor Units: Gross movement (e.g., thigh muscles).

  • Recruitment: Increasing contraction strength by activating more motor units.

Length-Tension Relationship

  • Optimal muscle length for contraction correlates with the number of cross-bridges formed between actin and myosin.

Muscle Tone

  • State of partial contraction maintains posture and stabilizes joints.

  • Hypotonia: Reduced tone due to CNS damage.

  • Hypertonia: Excessive tone often due to neurological conditions.

Types of Muscle Fibers

  • Slow Oxidative Fibers: Aerobic, fatigue-resistant, rich blood supply, red.

  • Fast Oxidative Fibers: Intermediate characteristics.

  • Fast Glycolytic Fibers: Anaerobic, large, high tension, quick fatigue, white appearance.

Impact of Exercise on Muscle

  • Endurance Exercise: Increases oxidative capacity of muscles.

  • Resistance Exercise: Leads to muscle hypertrophy.

Cardiac Muscle Characteristics

  • Contains intercalated discs that provide mechanical and electrical connections between fibers.

  • Specialized for rhythmic contractions.

Smooth Muscle Characteristics

  • Involuntary contractions regulated by autonomic neurons.

  • Contractile proteins are organized in dense bodies and intermediate filaments.

Muscle Atrophy and Hypertrophy

  • Atrophy: Decrease in muscle size and strength, often due to disuse.

  • Hypertrophy: Increase in muscle size due to increased workload.