Muscular System Histology and Physiology Notes

Muscular System Histology and Physiology Notes

9.1 Functions of the Muscular System

• Types of Muscle Tissue:
  • Skeletal Muscle:
  • Responsible for locomotion, facial expressions, posture, and respiratory movements.
  • Voluntary control, regulated by the nervous system.
  • Smooth Muscle:
  • Found in the walls of hollow organs (e.g., digestive tract, blood vessels).
  • Involved in propelling urine, mixing food, and regulating blood flow.
  • Controlled involuntarily by the endocrine and autonomic nervous systems.
  • Cardiac Muscle:
  • Located in the heart, responsible for blood movement.
  • Autorhythmic and controlled involuntarily.

9.2 General Properties of Muscle Tissue

• Contractility: Ability of a muscle to shorten with force.
• Excitability: Capacity of muscle to respond to stimuli (typically from nerves).
• Extensibility: Muscle can stretch beyond its normal resting length and still contract.
• Elasticity: Ability to recoil to resting length after being stretched.

9.3 Skeletal Muscle Anatomy

• Connective Tissue Coverings:
  • Epimysium: Surrounds whole muscle, merges with muscular fascia.
  • Perimysium: Envelopes groups of muscle fibers; allows passage for blood vessels/nerves.
  • Endomysium: Surrounds individual muscle fibers within fascicles.
• Myofibrils: Composed of actin and myosin myofilaments, essential for contraction.

Sarcomeres

• Basic functional unit of muscle fiber.
• Components include:
  • Z disk: Attachment point for actin.
  • I bands: Light regions, contain Z disk and actin only.
  • A bands: Dark central regions, overlap of actin/myosin.
  • H zone: Center of A band with no overlapping filaments.
  • M line: Middle of H zone, holds myosin in place.

9.4 Neuromuscular Junction Structure

• Function:
  • Action potentials stimulate muscle fiber action potentials, leading to contraction.
  • Components:
  • Presynaptic terminal: Contains synaptic vesicles with acetylcholine (ACh).
  • Synaptic cleft: Space between neuron and muscle.
  • Postsynaptic membrane: Motor end-plate where ACh binds.

Contraction Mechanism

• Sliding Filament Model: During contraction, actin slides over myosin, shortening the sarcomeres.
• Cross-Bridge Cycle:
  1. Exposure of Active Sites: Calcium binds to troponin, exposing actin's active sites.
  2. Binding of Myosin Heads: Myosin heads bind to active sites on actin.
  3. Power Stroke: Myosin heads pivot, pulling actin filaments inward.
  4. Cross-Bridge Detachment: ATP binds myosin, allowing it to detach from actin.
  5. ATP Hydrolysis: Provides energy for myosin head to return to high-energy state.

Muscle Contraction Types

• Isometric: No change in length but increases tension (e.g., postural muscles).
• Isotonic: Change in length at constant tension (e.g., lifting weights).
  • Concentric: Muscle shortens as it contracts.
  • Eccentric: Muscle lengthens while maintaining tension.

Muscle Fibers Types

• Slow-twitch (Type I):
  • Contract slowly, fatigue-resistant, high myoglobin content.
  • Predominantly in postural muscles.
• Fast-twitch (Type II):
  • Contract quickly, more powerful contractions, low endurance.
  • Includes both glycolytic and oxidative types.

Energy Sources for Muscle Contraction

• ATP Uses:
  1. Immediate energy from ATP and phosphocreatine.
  2. Anaerobic respiration (lactic acid formation).
  3. Aerobic respiration (efficient production of ATP).

Muscle Fatigue

• Causes:
  • Acidosis, ATP depletion, oxidative stress, local inflammation, and psychological factors.

Rigor Mortis

• Caused by calcium influx leading to sustained contraction post-death, preventing muscle relaxation.

Aging Effects on Muscle

• Reduction in muscle mass and strength, increased recovery time, decreased endurance.

Duchenne Muscular Dystrophy (DMD)

• Characterized by muscle weakness and atrophy, affecting skeletal, cardiac, and smooth muscles.
• No effective treatment exists; physical therapy may help manage symptoms.