Muscle Physiology

Learning Outcomes

  • List the characteristic features of skeletal muscle.
  • Describe the structural components of a muscle fibre.
  • Name the major proteins in muscle fibres and state the function of each.
  • Describe the structures & processes involved in the stimulation & contraction of skeletal muscle fibers.
  • Describe the stages of a muscle twitch.

Types of Muscle Tissue

  • Muscle Tissue Types:
    • Skeletal Muscle
    • Cardiac Muscle
    • Smooth Muscle

Functions of Muscles

  • Specialized for contraction.
  • Functions include:
    • Movement: Skeletal muscles move the body by pulling on bones.
    • Control: Cardiac and smooth muscles control movements inside the body.
  • Common properties:
    • Excitability: Responsiveness to stimuli.
    • Contractility: Ability of cells to shorten.
    • Extensibility: Capacity to be stretched.
    • Elasticity: Ability to return to original shape.

Functions of Skeletal Muscle

  • Producing body movement.
  • Maintaining posture and body position.
  • Supporting soft tissues.
  • Guarding body entrances and exits.
  • Maintaining body temperature.
  • Storing nutrients.

Organization of Skeletal Muscle

  • Components of Skeletal Muscles:
    • Primarily skeletal muscle tissue.
    • Connective tissues.
    • Blood vessels.
    • Nerves.
Connective Tissue Layers
  • Epimysium:
    • Layer of collagen fibers surrounding the muscle.
    • Connects to deep fascia, separating muscle from surrounding tissues.
  • Perimysium:
    • Surrounds bundles of muscle fibers (fascicles).
    • Contains collagen & elastic fibers, blood vessels & nerves.
  • Endomysium:
    • Surrounds individual muscle cells (muscle fibers).
    • Contains capillary networks, myosatellite cells, and nerve fibers.
Muscle Fiber Structure
  • Bulk of skeletal muscle fibers are large, multinucleate cells formed by the fusion of embryonic cells (myoblasts).
  • Striated appearance due to organized sarcomeres.

Components of Muscle Fibers

  • Sarcolemma: Plasma membrane of muscle fiber, surrounding the sarcoplasm.
  • Transverse Tubules (T Tubules):
    • Extend from surface into the sarcoplasm, conducting action potentials into the cell.
  • Sarcoplasmic Reticulum (SR):
    • Tubular network surrounding myofibrils, important for calcium ion storage and release.

Myofibrils and Myofilaments

  • Myofibrils: Subdivisions responsible for muscle contraction, contain myofilaments.
  • Two types of myofilaments:
    • Thin Filaments: Composed mainly of actin.
    • Thick Filaments: Composed mainly of myosin.
Sarcomere Structure
  • Sarcomeres: Smallest functional units of a muscle fiber.
    • A Band: Contains thick filaments (myosin).
    • I Band: Contains thin filaments (actin).
    • Z Lines: Boundaries between sarcomeres.
Sliding-Filament Theory
  • Concept that explains muscle contraction:
    • H bands and I bands narrow.
    • Zones of overlap widen.
    • Z lines move closer together as thin filaments slide toward the center.

Neuromuscular Junction (NMJ)

  • Synapse between a motor neuron and a muscle fiber.
  • Action potential triggers the release of acetylcholine (ACh), leading to depolarization and muscle contraction.
Events at NMJ
  1. ACh release from the neuron.
  2. ACh binds to receptors on the muscle fiber, leading to membrane depolarization.
  3. Generation of action potential in the muscle fiber.
  4. ACh is removed from the synaptic cleft by breakdown or diffusion.

Excitation-Contraction Coupling

  • Process linking action potentials to muscle contraction:
    1. Action potential travels down T-tubules.
    2. Release of Ca2+ from SR.
    3. Ca2+ binds to troponin, exposing active sites on actin.
    4. Cross-bridge formation and contraction cycles initiated.
Contraction Cycle Steps
  1. Active-site exposure: Ca2+ binds to troponin, exposing binding sites on actin.
  2. Cross-bridge formation: Myosin binds to actin.
  3. Myosin head pivoting: Myosin pulls actin inward (power stroke).
  4. Cross-bridge detachment: New ATP binds to myosin, breaking the link.
  5. Myosin reactivation: ATP is split to re-cock the myosin head.

Muscle Contraction Phases

  • Twitch phases:
    1. Latent Phase: Time between stimuli and contraction.
    2. Contraction Phase: Active tension increase.
    3. Relaxation Phase: Calcium levels fall, tension decreases.

Frequency of Stimulation

  • Treppe: Increasing tension due to rapid stimulation.
  • Wave Summation: Increased tension due to overlapping twitches.
  • Tetanus: Sustained contraction with no relaxation.

Energy for Muscle Contraction

  • ATP is the only direct energy source.
  • At rest, excess ATP is converted to creatine phosphate (CP) to store energy.
  • ATP levels must be replenished during contraction:
    • Anaerobic metabolism (glycolysis) generates ATP from glucose.
    • Aerobic metabolism is the primary energy source during rest, utilizing fatty acids.