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Anatomy & Physiology: Skeletal Muscle Structure and Contraction

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  • What is the structure of skeletal muscle from the outside in?

    Epimysium surrounds the entire muscle, Perimysium surrounds a fascicle, Endomysium surrounds each muscle fiber, then muscle fiber, myofibrils, sarcomeres, and myofilaments (actin and myosin).

  • What is another name for a muscle cell?

    Muscle fiber (muscle fiber cell).

  • What is the sarcomere?

    The sarcomere is the functional (contractile) unit of skeletal muscle, extending from one Z-disc to the next Z-disc.

  • Name the protein filaments of the sarcomere.

    Thin filament: Actin; Thick filament: Myosin; Also includes A band, I band, H zone, and Z-line.

  • What happens to sarcomere bands during contraction?

    Z-lines move inward, I bands and H zone narrow, while the A band stays fixed.

  • Name the three proteins that make up the thin filament (actin).

    Actin, Troponin, Tropomyosin.

  • Which thin filament protein binds calcium?

    Troponin binds Ca²⁺.

  • What triggers acetylcholine (ACh) release at the neuromuscular junction?

    An action potential arriving at the axon terminal opens voltage-gated calcium channels, allowing calcium to enter and trigger ACh release.

  • Where does acetylcholine (ACh) release occur?

    At the neuromuscular junction (NMJ), from the axon terminal into the synaptic cleft.

  • What is an action potential?

    A rapid electrical signal (nerve impulse) traveling along a neuron or muscle cell membrane.

  • Where is calcium released from in the muscle cell?

    From the sarcoplasmic reticulum (SR), specifically the terminal cisternae.

  • What is a cross-bridge in muscle contraction?

    A cross-bridge forms when a myosin head attaches to the binding site on actin during contraction.

  • What happens when calcium levels drop in the muscle cell?

    Calcium is pumped back into the sarcoplasmic reticulum, leaves troponin, tropomyosin covers myosin-binding sites on actin, stopping cross-bridge cycling and causing muscle relaxation.

  • What energy molecule powers the muscle contraction power stroke?

    ATP provides energy for the power stroke.

  • What neurotransmitter is involved in muscle contraction?

    Acetylcholine (ACh) is the neurotransmitter released at the neuromuscular junction.

  • What is the muscle cell membrane called?

    Sarcolemma is the muscle cell membrane.

  • What is the muscle cell cytoplasm called?

    Sarcoplasm is the muscle cell cytoplasm.

  • Where is calcium stored in muscle cells?

    Calcium is stored in the sarcoplasmic reticulum.

  • Through which structures does the action potential travel in muscle cells?

    Action potential travels through the sarcolemma and T-tubules.

  • What are the thick and thin filaments in muscle fibers?

    Thick filament: Myosin; Thin filament: Actin.