Anatomy and Physiology I Unit 11 ReviewThe Muscular System

Key Concepts and Terminology

• Myology studies the structure, function, and diseases of muscular tissue
• Muscle tissue consists of specialized cells called muscle fibers (myocytes) that can contract and generate force
• Muscles are highly vascularized to receive oxygen and nutrients for energy production
• Muscles are innervated by motor neurons that control their contraction and relaxation
• Muscle contraction requires the presence of calcium ions (Ca2+) and adenosine triphosphate (ATP)
• Muscles attach to bones via tendons, which are dense connective tissue structures
• Muscle tone refers to the constant state of partial contraction in muscles, maintaining posture and stability

Types of Muscle Tissue

• Skeletal muscle is voluntary, striated, and attached to bones, enabling movement and posture
  • Controlled by the somatic nervous system
  • Has a large number of mitochondria for energy production
• Cardiac muscle is involuntary, striated, and found only in the heart, responsible for pumping blood
  • Controlled by the autonomic nervous system
  • Has intercalated discs that allow synchronous contraction
• Smooth muscle is involuntary, non-striated, and found in hollow organs (blood vessels, digestive tract)
  • Controlled by the autonomic nervous system
  • Capable of sustained contraction with low energy expenditure
• Skeletal and cardiac muscle cells are multinucleated, while smooth muscle cells are uninucleated

Muscle Structure and Organization

• Muscle fibers are long, cylindrical cells containing multiple nuclei and mitochondria
• Sarcomeres are the basic functional units of muscle fibers, composed of thick and thin filaments
  • Thick filaments contain myosin, while thin filaments contain actin, troponin, and tropomyosin
• Myofibrils are bundles of sarcomeres arranged in parallel within muscle fibers
• Muscle fibers are organized into fascicles, which are bundles of fibers surrounded by perimysium
• Epimysium is a connective tissue layer that surrounds the entire muscle
• Endomysium is a delicate connective tissue layer that surrounds individual muscle fibers
• Neuromuscular junctions are specialized synapses where motor neurons innervate muscle fibers

Muscle Contraction Mechanism

• Sliding filament theory explains muscle contraction through the interaction of thick and thin filaments
  • Myosin heads on thick filaments bind to actin on thin filaments, forming cross-bridges
  • Cross-bridge cycling causes the sliding of thin filaments past thick filaments, shortening the sarcomere
• Excitation-contraction coupling is the process by which an action potential triggers muscle contraction
  • Acetylcholine released by motor neurons binds to receptors on the muscle fiber membrane
  • This triggers the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm
• Calcium ions bind to troponin, causing a conformational change that exposes the binding sites for myosin on actin
• ATP hydrolysis provides the energy for myosin heads to detach from actin and repeat the cross-bridge cycle
• Muscle relaxation occurs when calcium ions are actively pumped back into the sarcoplasmic reticulum

Energy Systems in Muscles

• Muscles require ATP for contraction and relaxation processes
• The phosphagen system provides immediate energy for short-duration, high-intensity activities
  • Creatine phosphate serves as a rapid source of ATP regeneration
• Glycolysis breaks down glucose or glycogen to produce ATP anaerobically
  • Produces lactic acid as a byproduct, which can lead to muscle fatigue
• Aerobic respiration in mitochondria produces ATP using oxygen and substrates from carbohydrates, fats, and proteins
  • Most efficient energy system for sustained muscle activity
• The relative contribution of each energy system depends on the intensity and duration of the muscle activity

Muscle Fiber Types

• Type I (slow-twitch) fibers are fatigue-resistant and adapted for prolonged, low-intensity activities (endurance)
  • Contain a high number of mitochondria and myoglobin for efficient aerobic metabolism
  • Appear red due to the high myoglobin content
• Type IIa (fast-twitch oxidative-glycolytic) fibers are moderately fatigue-resistant and adapted for intermediate-duration activities
  • Contain a moderate number of mitochondria and utilize both aerobic and anaerobic metabolism
• Type IIb (fast-twitch glycolytic) fibers are easily fatigued and adapted for short-duration, high-intensity activities (sprinting)
  • Contain fewer mitochondria and rely primarily on anaerobic glycolysis for energy production
  • Appear white due to the low myoglobin content
• Muscle fiber type composition is genetically determined but can be modified to some extent by training

Muscle Actions and Movements

• Agonist (prime mover) muscles are the primary muscles responsible for a specific movement
• Antagonist muscles oppose the action of the agonist muscles and provide smooth, controlled movements
• Synergist muscles assist the agonist muscles and stabilize the joint during movement
• Fixator muscles stabilize the origin of the agonist muscles to allow more efficient movement
• Isometric contractions occur when muscle tension increases without a change in muscle length (static hold)
• Concentric contractions occur when muscle tension increases, and the muscle shortens (lifting a weight)
• Eccentric contractions occur when muscle tension increases, and the muscle lengthens (lowering a weight)

Clinical Applications and Disorders

• Muscular dystrophy is a group of genetic disorders characterized by progressive muscle weakness and degeneration
  • Duchenne muscular dystrophy is the most common form, affecting primarily boys
• Myasthenia gravis is an autoimmune disorder that affects neuromuscular transmission, causing muscle weakness and fatigue
• Muscle strain (pulled muscle) occurs when muscle fibers are overstretched or torn due to excessive force or overuse
• Delayed onset muscle soreness (DOMS) is muscle pain and stiffness that occurs 24-72 hours after unaccustomed or strenuous exercise
• Electromyography (EMG) is a diagnostic tool that measures the electrical activity of muscles to assess neuromuscular function
• Muscle biopsies involve removing a small sample of muscle tissue for microscopic analysis to diagnose muscle disorders
• Strength training and physical therapy can help maintain and improve muscle function in various conditions