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Anatomy & Physiology: Muscle Tissue and Contraction

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  • Three types of muscle tissue

    Skeletal (voluntary, striated), cardiac (involuntary, striated), and smooth (involuntary, non-striated).

  • Functions of muscle tissue

    Produce movement, maintain posture, stabilize joints, and generate heat through contractions.

  • Skeletal muscle fiber composition

    Long cylindrical cells with multiple nuclei, containing myofibrils, sarcoplasm, sarcolemma, and specialized structures like T tubules and sarcoplasmic reticulum.

  • Sarcomere

    The smallest contractile unit of a muscle fiber, defined as the segment between two Z discs, containing thick (myosin) and thin (actin) filaments.

  • Sliding filament model of contraction

    Thin filaments slide past thick filaments, increasing overlap and shortening the sarcomere without changing filament length.

  • Role of calcium in muscle contraction

    Calcium binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin, enabling cross bridge formation.

  • Cross bridge cycle steps

    Formation, power stroke (myosin head pivots), detachment (ATP binds), and cocking (ATP hydrolysis) of myosin heads.

  • Neuromuscular junction function

    Motor neuron releases acetylcholine (ACh), which binds to receptors on the sarcolemma, triggering depolarization and muscle fiber excitation.

  • Muscle twitch phases

    Latent period (excitation-contraction coupling), contraction period (cross bridge activity), and relaxation period (calcium reuptake).

  • Temporal summation

    Increasing stimulation frequency causes twitches to combine, producing stronger, smoother contractions.

  • Motor unit recruitment

    Increasing stimulus strength activates more motor units, resulting in stronger muscle contractions.

  • Isotonic vs. isometric contractions

    Isotonic: muscle changes length to move load; isometric: muscle tension increases without length change.

  • ATP regeneration pathways in muscle

    Direct phosphorylation by creatine phosphate, anaerobic glycolysis, and aerobic respiration.

  • Anaerobic glycolysis

    Breakdown of glucose to lactic acid without oxygen, producing 2 ATP quickly but inefficiently.

  • Aerobic respiration

    Oxygen-dependent process in mitochondria producing about 32 ATP per glucose, slower but more efficient.

  • Muscle fatigue causes

    Ionic imbalances, depletion of ATP and glycogen, accumulation of metabolic byproducts, and contractile mechanism failure.

  • Muscle fiber types

    Slow oxidative (fatigue-resistant, aerobic), fast oxidative (intermediate), and fast glycolytic (fatigue quickly, anaerobic).

  • Factors affecting muscle contraction force

    Number of fibers recruited, fiber size, stimulation frequency, and muscle stretch degree.

  • Smooth muscle characteristics

    Non-striated, involuntary, spindle-shaped cells with single nucleus, found in hollow organs, contracting slowly and sustained.

  • Smooth muscle contraction regulation

    Calcium binds calmodulin, activating myosin light chain kinase, which phosphorylates myosin to enable contraction.

  • Differences between smooth and skeletal muscle

    Smooth muscle lacks sarcomeres and troponin, uses calmodulin for calcium binding, and contracts via gap junction electrical coupling.

  • Smooth muscle tone and energy efficiency

    Maintains moderate contraction with low energy cost due to slow ATPase activity and latch state of myosin heads.

  • Types of smooth muscle

    Unitary (visceral) smooth muscle contracts as a unit via gap junctions; multi-unit smooth muscle has independent fibers with neural control.

  • Muscle adaptation to aerobic exercise

    Increased capillaries, mitochondria, and myoglobin, enhancing endurance and fatigue resistance.

  • Muscle adaptation to resistance exercise

    Muscle hypertrophy via fiber enlargement, increased myofilaments, and conversion of fiber types to fast glycolytic.

  • Role of T tubules in muscle contraction

    Transmit action potentials deep into muscle fibers, triggering calcium release from sarcoplasmic reticulum.

  • Sarcoplasmic reticulum function

    Stores and releases calcium ions essential for muscle contraction.

  • Muscle tone

    Continuous, slight contraction of muscles maintained by spinal reflexes to stabilize joints and posture.