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Anatomy & Physiology: Muscle Physiology

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  • What is a skeletal muscle cell also called?
    Muscle fiber, a multinucleated cell formed by fusion (syncytium).
  • What is the sarcolemma?
    The plasma membrane of a muscle fiber.
  • What causes the striated appearance of skeletal muscle?
    Alternating dark A bands and light I bands in myofibrils due to arrangement of thick (myosin) and thin (actin) filaments.
  • Define a sarcomere.
    The smallest contractile unit of a muscle cell, composed of overlapping actin and myosin filaments.
  • What proteins compose thick and thin filaments?
    Thick filaments are myosin; thin filaments are actin with associated tropomyosin and troponin.
  • What is the role of troponin in muscle contraction?
    Troponin binds Ca++ and causes tropomyosin to move, exposing myosin binding sites on actin.
  • What is the function of the sarcoplasmic reticulum?
    Stores and releases Ca++ to trigger muscle contraction.
  • What is the sliding filament model?
    Muscle contraction occurs by myosin heads sliding along actin filaments, shortening sarcomeres.
  • Describe the power stroke in muscle contraction.
    Myosin head bends, pulling actin filament and releasing ADP + Pi.
  • What causes detachment of myosin from actin?
    Binding of ATP to myosin head causes detachment.
  • What is rigor mortis?
    Muscle stiffness after death due to myosin heads remaining attached to actin without ATP.
  • What is the neuromuscular junction?
    The synapse where a motor neuron communicates with a muscle fiber using acetylcholine (ACh).
  • How does acetylcholine trigger muscle excitation?
    ACh binds receptors opening Na+ channels, causing depolarization of the sarcolemma.
  • What is the refractory period in muscle cells?
    Time after an action potential when the muscle cell repolarizes and cannot be re-excited immediately.
  • What enzyme breaks down acetylcholine in the synaptic cleft?
    Acetylcholinesterase (AChE) rapidly degrades ACh to stop stimulation.
  • What is a motor unit?
    A single motor neuron and all the muscle fibers it innervates.
  • What is a muscle twitch?
    The response of a muscle to a single brief electrical stimulus, including latent, contraction, and relaxation phases.
  • What is tetanus in muscle physiology?
    A sustained smooth muscle contraction caused by high-frequency stimulation without relaxation.
  • What is the difference between isometric and isotonic contractions?
    Isometric: muscle tension without length change; Isotonic: muscle changes length to move a load.
  • What factors determine the force of muscle contraction?
    Number of motor units activated, muscle size, series-elastic elements, and degree of muscle stretch.
  • Describe red slow-twitch muscle fibers.
    Small, red fibers with many mitochondria and myoglobin; slow contraction; aerobic and fatigue-resistant.
  • Describe white fast-twitch muscle fibers.
    Large, pale fibers; fast contraction; anaerobic metabolism; fatigue quickly.
  • What is muscle hypertrophy?
    Increase in muscle size due to enlargement of individual muscle fibers, often from resistance exercise.
  • What is the immediate energy source for muscle contraction?
    Creatine phosphate donates phosphate to ADP to rapidly regenerate ATP.
  • What happens during anaerobic respiration in muscle?
    Glucose is converted to lactic acid, producing 2 ATP per glucose without oxygen.
  • What is oxygen debt?
    Extra oxygen required after exercise to restore ATP, creatine phosphate, and convert lactic acid back to pyruvate.
  • What causes muscle fatigue?
    Depletion of ATP, lactic acid buildup lowering pH, ionic imbalances, and impaired enzyme function.
  • How does the sarcoplasmic reticulum release calcium?
    Depolarization of T tubules triggers Ca++ release from terminal cisternae of the SR.
  • What is the latent period in muscle contraction?
    The delay between muscle stimulation and the start of contraction, during excitation-contraction coupling.