🫁Honors Anatomy and Physiology Unit 7 – Muscular System

Key Concepts and Terminology

• Myology studies the structure, function, and pathology of muscles
• Muscles classified as skeletal, cardiac, or smooth based on their structure and function
• Muscle contraction occurs through the sliding filament mechanism involving actin and myosin filaments
• Motor units consist of a motor neuron and all the muscle fibers it innervates
• Sarcomeres are the basic functional units of skeletal muscle fibers
• Myofibrils are long, cylindrical structures composed of repeating sarcomeres
• Sarcoplasmic reticulum stores and releases calcium ions necessary for muscle contraction
• Neuromuscular junction is the synapse between a motor neuron and a muscle fiber

Structure and Types of Muscles

• Skeletal muscles attach to bones via tendons and are under voluntary control
  • Responsible for body movements and maintaining posture
  • Exhibit cross-striations due to the arrangement of myofilaments
• Cardiac muscle is found only in the heart and has involuntary contractions
  • Intercalated discs connect adjacent cardiac muscle cells, allowing for coordinated contractions
  • Autorhythmic cells in the heart generate and propagate action potentials
• Smooth muscle lacks striations and is under involuntary control
  • Found in the walls of hollow organs (blood vessels, digestive tract, uterus)
  • Contractions are slower and more sustained compared to skeletal muscle

Muscle Fiber Anatomy

• Muscle fibers are long, cylindrical cells containing multiple nuclei
• Sarcolemma is the plasma membrane of a muscle fiber
• Transverse tubules (T-tubules) are invaginations of the sarcolemma that conduct action potentials into the fiber
• Myofibrils are composed of thick (myosin) and thin (actin) filaments arranged in sarcomeres
  • Myosin filaments have globular heads that bind to actin during contraction
  • Actin filaments have binding sites for myosin and regulatory proteins (troponin and tropomyosin)
• Sarcoplasmic reticulum surrounds each myofibril and stores calcium ions
  • Terminal cisternae are enlarged portions of the sarcoplasmic reticulum adjacent to T-tubules
• Sarcomeres are the repeating functional units of myofibrils
  • Consist of regions defined by the arrangement of thick and thin filaments (A-band, I-band, H-zone, M-line, Z-line)

Muscle Contraction Process

• Muscle contraction is triggered by an action potential from a motor neuron
• Acetylcholine released at the neuromuscular junction binds to receptors on the sarcolemma
• Action potential propagates along the sarcolemma and into T-tubules
• Calcium ions are released from the sarcoplasmic reticulum in response to the action potential
• Calcium binds to troponin, causing a conformational change that exposes myosin-binding sites on actin
• Myosin heads attach to actin, forming cross-bridges
• Myosin heads pivot, pulling the thin filaments towards the center of the sarcomere (power stroke)
• Myosin heads detach from actin when ATP binds, allowing for relaxation
• Calcium is actively pumped back into the sarcoplasmic reticulum, lowering intracellular calcium concentration

Energy Systems in Muscles

• ATP is the immediate energy source for muscle contraction
• Creatine phosphate serves as a rapid source of ATP replenishment
• Glycolysis breaks down glucose to produce ATP and pyruvate
  • Anaerobic glycolysis occurs in the absence of oxygen and produces lactic acid
  • Aerobic glycolysis occurs in the presence of oxygen and feeds into the Krebs cycle
• Oxidative phosphorylation in mitochondria produces the majority of ATP during sustained exercise
• Fatty acid oxidation provides a significant source of energy during prolonged, low-intensity exercise
• Muscle fiber types (slow-twitch and fast-twitch) have different metabolic profiles and fatigue resistance

Muscle Functions and Movements

• Muscles generate force through contraction, resulting in movement or stabilization of body parts
• Agonist muscles are the primary muscles responsible for a specific movement
• Antagonist muscles oppose the action of the agonist muscles
• Synergist muscles assist the agonist muscles in performing a movement
• Fixator muscles stabilize a joint or body part during movement
• Types of muscle contractions include concentric, eccentric, and isometric
  • Concentric contraction occurs when the muscle shortens while generating force
  • Eccentric contraction occurs when the muscle lengthens while under tension
  • Isometric contraction occurs when the muscle generates force without changing length

Disorders and Diseases of the Muscular System

• Muscular dystrophy is a group of genetic disorders characterized by progressive muscle weakness and degeneration
  • Duchenne muscular dystrophy is the most common form, caused by a mutation in the dystrophin gene
• Myasthenia gravis is an autoimmune disorder affecting the neuromuscular junction, causing muscle weakness and fatigue
• Rhabdomyolysis is the breakdown of muscle tissue, leading to the release of myoglobin into the bloodstream
  • Can be caused by trauma, excessive exercise, or certain medications
• Fibromyalgia is a chronic disorder characterized by widespread muscle pain, fatigue, and tender points
• Muscle strains occur when muscle fibers are overstretched or torn due to excessive force or improper use
• Muscle atrophy is the decrease in muscle mass and strength due to disuse, aging, or certain medical conditions

Clinical Applications and Research

• Electromyography (EMG) measures the electrical activity of muscles to diagnose neuromuscular disorders
• Biopsies of muscle tissue can help diagnose muscular dystrophies and other muscle pathologies
• Physical therapy and rehabilitation focus on improving muscle strength, flexibility, and function after injury or disease
• Gene therapy research aims to correct genetic defects responsible for muscular dystrophies
• Stem cell therapy is being investigated as a potential treatment for muscle regeneration and repair
• Pharmacological interventions target specific aspects of muscle function and disease
  • Acetylcholinesterase inhibitors (pyridostigmine) improve neuromuscular transmission in myasthenia gravis
  • Corticosteroids (prednisone) reduce inflammation and slow muscle degeneration in muscular dystrophy
• Exercise physiology research examines the effects of physical activity on muscle health, performance, and adaptation