Muscles and Muscle Tissue

Overview of Muscle Tissue

  • One of four primary tissue types specialized for movement.

  • Types: Skeletal, Cardiac, Smooth muscle.

Types of Muscle Tissue

  • Skeletal Muscle:

    • Location: Attached to bones.

    • Structure: Long, cylindrical, multinucleate, striated.

    • Control: Voluntary.

    • Function: Voluntary movement, locomotion, manipulation of environment.

  • Cardiac Muscle:

    • Location: Walls of the heart.

    • Structure: Short, branched cells, single central nucleus, striated.

    • Control: Involuntary.

    • Function: Propels blood into circulation.

  • Smooth Muscle:

    • Location: Walls of hollow organs.

    • Structure: Spindle-shaped cells with single nucleus, no striations.

    • Control: Involuntary.

    • Function: Propels substances or objects along internal passageways.

Special Characteristics of Muscle Tissue

  • Excitability: Ability to respond to stimuli.

  • Contractility: Ability to shorten forcibly when stimulated.

  • Extensibility: Ability to be stretched.

  • Elasticity: Ability to return to original length after stretch.

Functions of Skeletal Muscle

  1. Produce movement.

  2. Maintain posture and body position.

  3. Stabilize joints.

  4. Generate heat.

Connective Tissue Layers

  • Epimysium: Surrounds entire muscle.

  • Perimysium: Surrounds fascicles (bundles of muscle fibers).

  • Endomysium: Surrounds individual muscle fibers.

Muscle Fibers and Myofibrils

  • Skeletal muscle fibers are long, cylindrical, multinucleate.

  • Sarcoplasmic Reticulum (SR): Stores and releases Ca2+.

  • Myofibrils: Composed of thick (myosin) and thin (actin) filaments, arranged in sarcomeres, the functional unit.

Sliding Filament Model of Contraction

  • Thick and thin filaments slide past each other, increasing overlap.

  • During contraction: Z discs move closer, I bands shorten, A bands remain unchanged.

Excitation-Contraction Coupling

  • Action potential along sarcolemma leads to Ca2+ release from SR.

  • Ca2+ binds to troponin, moving tropomyosin to uncover myosin-binding sites on actin.

Cross Bridge Cycling

  • Steps:

    1. Cross bridge formation - Myosin heads attach to actin.

    2. Power stroke - Myosin heads pivot, pulling actin filaments.

    3. Cross bridge detachment - ATP binds to myosin, detaching it from actin.

    4. Cocking of myosin head - ATP hydrolysis returns myosin to high-energy state.

Types of Muscle Contraction

  • Isotonic: Muscle changes length; tension remains constant.

  • Isometric: Muscle length remains unchanged while tension increases.

  • Muscle twitch: Response of a motor unit to a single action potential, includes latent, contraction, and relaxation phases.

Energy Sources for Contraction

  • ATP needed for contraction and recovery.

  • Regeneration methods:

    1. Direct phosphorylation by creatine phosphate.

    2. Anaerobic pathway (glycolysis).

    3. Aerobic pathway (aerobic respiration).

Muscle Fiber Types

  • Classified by contraction speed and metabolic pathways:

    • Slow oxidative: Endurance activities.

    • Fast oxidative: Moderate activities.

    • Fast glycolytic: Short, intense activities.