Muscle Physiology

Types of Muscles

  • Smooth:
    • Involuntary, non-striated, one nucleus per cell.
    • Found in blood vessel walls and internal organs.
    • Functions: respiration, digestion, blood circulation.
  • Cardiac:
    • Self-controlled, striated, one nucleus per cell.
    • Only in the heart.
    • Function: blood circulation.
  • Skeletal:
    • Voluntary, striated, multinucleate.
    • Attached to the skeleton.
    • Functions: locomotion, posture, body temperature.

Muscle Properties

  • Contractility: Ability to shorten and generate tension.
  • Extensibility: Ability to be stretched beyond resting length.
  • Elasticity: Ability to return to original shape.
  • Excitability: Ability to respond to stimuli.

Skeletal Muscle Structure

  • Epimysium: Surrounds the entire muscle.
  • Fascicle: Bundle of muscle fibers, surrounded by perimysium.
  • Perimysium: Surrounds each fascicle.
  • Endomysium: Surrounds individual muscle fibers.
  • Myofibrils: Contain sarcomeres.
  • Myofilaments: Thin (actin, troponin, tropomyosin) and thick (myosin) filaments.

Sarcolemma and T-Tubules

  • Sarcolemma: Encloses muscle fiber and invaginates to form T-tubules.
    • Function: Carries depolarization from action potentials to the interior of the fiber

Sarcomere

  • Basic contractile unit between Z lines.
  • Arrangement creates striation pattern.

Myofilaments

  • Thin filament: actin, troponin complex, tropomyosin
  • Thick filament: myosin

Myosin Filament (Thick Filament)

  • Composed of myosin molecules with tails and heads.
  • Heads bind ATP and actin, function as ATPase.

Actin Filament (Thin Filament)

  • Composed of actin, tropomyosin, and troponin complex.
  • Actin contains myosin binding sites.
  • Troponin subunits: TnC (calcium-binding), TnI (inhibitory), TnT (tropomyosin-binding).

Organelles of the Muscle Cell (Fiber)

  • Mitochondria: Generates ATP for muscle contraction.
  • Sarcoplasmic Reticulum: Regulates calcium storage, release, and reuptake.
  • T-Tubules: Transmit action potentials to the interior of the fiber.

Muscle Fiber Components

  • Sarcolemma: Plasma membrane enclosing muscle fiber.
  • Sarcoplasmic Reticulum (SR): Stores, releases, and reuptakes calcium ions.
  • T-tubules: Rapidly transmit action potentials and regulate calcium concentration.

Muscle Contraction

  • Myofibrils are made up of sarcomeres, the smallest functional units of a muscle.
  • Sarcomere is composed of Myosin and actin.

Elements of muscle contraction

  • Action Potential: Rapid change in voltage across a membrane.
    • Na+Na^+ inflow and K+K^+ outflow cause voltage change.

Sliding Filament Model

  • Thick and thin filaments slide over each other without changing length.
  • Calcium ions bind to troponin, uncovering actin sites.
  • Myosin heads bind to actin, forming cross-bridges.
  • Myosin heads bend, sliding actin toward the M line (power stroke).

Muscle Fiber Action

  • Nerve impulse initiates muscle action.
  • Ca2+Ca^{2+} ions bind with troponin, exposing actin sites.
  • Myosin heads bind to actin; ATP provides energy.
  • Muscle action ends when calcium is pumped back into the sarcoplasmic reticulum.

Types of Muscle Fibers

  • Type I (Red):
    • High aerobic endurance, uses more oxygen.
  • Type II (White):
    • Better for anaerobic activities.
      • Type IIa
      • Type IIb

TYPE I MUSCLE FIBERS

  • High aerobic capacity and fatigue resistance
  • Rich in mitochondria (high ATP)
  • Slow contractile speed (110 ms) = Slow Twitch
  • 10–180 fibers per motor neuron

TYPE IIa MUSCLE FIBERS

  • Moderate aerobic (oxidative) capacity and fatigue resistance
  • High anaerobic (glycolytic) capacity
  • Fast contractile speed (50 ms) = Fast Twitch
  • 300–800 fibers per motor neuron

TYPE IIb MUSCLE FIBERS

  • Low aerobic capacity and fatigue resistance
  • High anaerobic capacity and motor unit strength
  • Fast contractile speed (50 ms)
  • 300–800 fibers per motor neuron

Functional Classification of Muscles

  • Agonists: Responsible for movement.
  • Antagonists: Oppose agonists.
  • Synergists: Assist agonists.

Types of Muscle Contraction

  • Isometric: No change in muscle length.
  • Concentric: Muscle fibers shorten.
  • Eccentric: Muscle fibers lengthen.

Satellite Cell Function

  • Involved in muscle growth and repair.
  • Activated upon injury to form new muscle fibers.
  • MGF (Mechano Growth Factor) in mechanically overloaded muscles

Myostatin Function

  • Regulates muscle growth by inhibiting muscle cell proliferation and differentiation.