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^+ inflow and 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.
• 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.