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General Biology: Muscle Structure and Function

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  • Three main muscle types in vertebrates

    Skeletal muscle, cardiac muscle, and smooth muscle.

  • Structure and function of skeletal muscle

    Striated, long fibers, multinucleated; responsible for voluntary movement; attached to bones.

  • Structure and function of cardiac muscle

    Striated, branched, single nucleus, intercalated discs; involuntary rhythmic contractions; located in the heart.

  • Structure and function of smooth muscle

    Non-striated, spindle-shaped, single nucleus; controls involuntary movements like peristalsis; found in walls of hollow organs.

  • Hierarchical organization of skeletal muscle

    Muscle → muscle fascicle → muscle fiber (myocyte) → myofibril → sarcomere → myofilaments (myosin and actin).

  • What is a sarcomere?

    The contractile unit within a myofibril, containing thick (myosin) and thin (actin) filaments.

  • Role of actin in muscle contraction

    Thin filament providing binding sites for myosin during contraction.

  • Role of myosin in muscle contraction

    Thick filament with heads that bind to actin and pull to shorten the sarcomere.

  • Function of tropomyosin in resting muscle

    Blocks myosin binding sites on actin to prevent contraction at rest.

  • Function of troponin in muscle contraction

    Binds calcium ions and causes tropomyosin to move, exposing myosin binding sites on actin.

  • Role of calcium ions (Ca 2+) in muscle contraction

    Bind to troponin, triggering tropomyosin to move and expose myosin binding sites on actin.

  • Role of ATP in muscle contraction

    Provides energy for the myosin head power stroke and detachment from actin.

  • Sequence of events in muscle contraction starting from action potential

    Action potential → acetylcholine release → muscle depolarization → Ca 2+ release → troponin binds Ca 2+ → tropomyosin moves → myosin binds actin → ATP hydrolysis → power stroke → cycle repeats.

  • Sliding filament theory

    Myosin heads attach to actin, pull (power stroke), detach, and reattach further along actin, shortening the sarcomere and causing contraction.

  • What happens when the action potential stops in muscle contraction?

    Ca 2+ is pumped back into the sarcoplasmic reticulum, tropomyosin blocks binding sites, and the muscle relaxes.

  • Definition of a motor unit

    A single motor neuron and all the muscle fibers it controls.

  • Innervation ratio in motor units for fine motor skills

    Low innervation ratio: few muscle fibers per neuron for precise control (e.g., writing).

  • Innervation ratio in motor units for gross motor skills

    High innervation ratio: many muscle fibers per neuron for powerful contractions (e.g., lifting weights).

  • What is motor unit recruitment?

    Increasing the number of active motor units to produce stronger muscle contractions.

  • Definition of antagonistic muscle pairs

    Pairs of muscles that work in opposition to move a joint, such as the biceps and triceps.

  • Flexion in antagonistic muscle pairs

    Bending a limb by decreasing the angle at a joint, e.g., biceps contract to flex the arm.

  • Extension in antagonistic muscle pairs

    Straightening a limb by increasing the angle at a joint, e.g., triceps contract to extend the arm.