Anatomy and Physiology I Unit 10 Review: Muscle Tissue

Key Concepts and Terminology

• Muscle tissue specialized for contraction and force generation enables movement, posture maintenance, and organ function
• Myocytes (muscle cells) contain myofilaments (actin and myosin) that slide past each other during contraction
• Sarcomeres are the basic functional units of muscle fibers consisting of overlapping myofilaments
  • Sarcomeres are bordered by Z-lines and contain M-lines in the center
  • Sarcomeres shorten during muscle contraction resulting in muscle tension
• Motor neurons innervate muscle fibers at neuromuscular junctions (NMJs) to initiate muscle contraction
• Excitation-contraction coupling links electrical stimulation to mechanical contraction in muscle fibers
• Adenosine triphosphate (ATP) provides energy for muscle contraction through hydrolysis by myosin ATPase
• Muscle tone refers to the continuous partial contraction of muscles maintaining posture and joint stability

Types of Muscle Tissue

• Skeletal muscle attaches to bones via tendons, is under voluntary control, and enables movement and posture
  • Skeletal muscle fibers are long, cylindrical, multinucleated cells with striations
  • Skeletal muscle is innervated by somatic motor neurons from the central nervous system
• Cardiac muscle is found in the heart, is involuntary, and pumps blood through the circulatory system
  • Cardiac muscle fibers are shorter, branched, and interconnected by intercalated discs
  • Cardiac muscle has autorhythmicity (self-excitation) and can contract without neural stimulation
• Smooth muscle is found in hollow organs (blood vessels, digestive tract), is involuntary, and enables organ function
  • Smooth muscle fibers are spindle-shaped, uninucleated cells without striations
  • Smooth muscle contraction is slower and more sustained compared to skeletal and cardiac muscle

Muscle Structure and Organization

• Muscle fibers (cells) are the basic structural units of muscle tissue surrounded by endomysium
• Muscle fibers are bundled into fascicles surrounded by perimysium
  • Fascicles are grouped together to form whole muscles surrounded by epimysium
  • Connective tissue sheaths (endomysium, perimysium, epimysium) provide structural support and transmit force
• Myofibrils are long, cylindrical structures within muscle fibers composed of myofilaments (actin and myosin)
  • Myofibrils contain repeating units called sarcomeres, the basic functional units of muscle contraction
  • Sarcomeres give skeletal and cardiac muscle their striated appearance under microscopy
• Sarcoplasmic reticulum (SR) surrounds myofibrils and stores calcium ions (Ca2+) essential for muscle contraction
• Transverse tubules (T-tubules) are invaginations of the muscle fiber membrane that conduct action potentials into the fiber interior
• Myofilaments consist of thick filaments (myosin) and thin filaments (actin, troponin, tropomyosin)
  • Myosin filaments have globular heads that interact with actin during cross-bridge cycling

Muscle Contraction Mechanism

• Muscle contraction occurs through the sliding filament mechanism within sarcomeres
  • Myosin heads attach to actin filaments forming cross-bridges and pull actin towards the center of the sarcomere
  • Shortening of sarcomeres along the length of myofibrils results in muscle fiber contraction and tension generation
• Excitation-contraction coupling links electrical stimulation (action potentials) to mechanical contraction
  • Action potentials propagate along the muscle fiber membrane and into T-tubules
  • Voltage-gated calcium channels (dihydropyridine receptors) in T-tubules interact with ryanodine receptors in the SR
  • Calcium is released from the SR into the sarcoplasm, binding to troponin on thin filaments
• Calcium binding to troponin exposes myosin-binding sites on actin, allowing cross-bridge formation
  • Myosin heads undergo a power stroke, pulling actin filaments towards the center of the sarcomere
  • Myosin heads detach from actin, rebind to new sites, and repeat the cycle (cross-bridge cycling) as long as calcium and ATP are available
• Muscle relaxation occurs when calcium is actively pumped back into the SR by SERCA (sarco/endoplasmic reticulum calcium ATPase)
  • Lowered sarcoplasmic calcium levels cause troponin to block myosin-binding sites on actin, preventing cross-bridge formation

Muscle Metabolism and Energy Systems

• Muscle contraction requires energy in the form of ATP for myosin ATPase activity and SERCA function
• Muscle fibers rely on three main energy systems to generate ATP depending on the intensity and duration of activity
  • Phosphagen system provides immediate ATP using creatine phosphate (CP) stores for short, high-intensity activities (weightlifting)
  • Glycolytic system generates ATP through anaerobic breakdown of glucose for moderate-intensity activities lasting several minutes (sprinting)
  • Oxidative system produces ATP through aerobic metabolism of carbohydrates and fats for low-intensity, long-duration activities (marathon running)
• Muscle fibers store glycogen as a local energy source that can be rapidly mobilized during exercise
• Oxidative metabolism in muscle mitochondria generates the majority of ATP during prolonged exercise
  • Mitochondrial density and enzyme content determine the oxidative capacity of muscle fibers
• Lactate accumulation during high-intensity exercise can lead to muscle fatigue and acidosis
  • Lactate is produced by anaerobic glycolysis when pyruvate cannot enter the mitochondria fast enough
• Muscle fibers adapt their metabolic properties in response to training, enhancing the capacity of specific energy systems

Muscle Fiber Types and Properties

• Muscle fibers are classified into different types based on their contractile and metabolic properties
  • Type I (slow-twitch) fibers have slow contraction speed, high oxidative capacity, and high fatigue resistance (postural muscles)
  • Type IIa (fast-twitch) fibers have fast contraction speed, high oxidative and glycolytic capacity, and moderate fatigue resistance (endurance athletes)
  • Type IIx (fast-twitch) fibers have fast contraction speed, high glycolytic capacity, and low fatigue resistance (sprinters)
• Fiber type composition is genetically determined but can be modified to some extent by training
  • Endurance training increases the proportion of Type I and IIa fibers, while resistance training increases Type IIa and IIx fibers
• Motor units consist of a motor neuron and all the muscle fibers it innervates, determining the contractile properties of the unit
  • Small motor units (Type I) are recruited for fine motor control and low-force activities
  • Large motor units (Type II) are recruited for high-force, rapid movements
• Muscle fiber type proportions vary among different muscles and individuals, influencing athletic performance and susceptibility to fatigue

Muscle Growth and Adaptation

• Muscle hypertrophy is the increase in muscle fiber size and cross-sectional area in response to resistance training
  • Hypertrophy occurs through the addition of myofilaments and sarcomeres within existing muscle fibers
  • Satellite cells (muscle stem cells) fuse with muscle fibers to donate nuclei and support protein synthesis
• Muscle atrophy is the decrease in muscle fiber size and strength due to disuse, immobilization, or aging (sarcopenia)
  • Atrophy results from a net breakdown of muscle proteins and loss of myofilaments and sarcomeres
• Resistance training stimulates muscle hypertrophy through mechanical tension, metabolic stress, and muscle damage
  • Mechanical tension activates mechanosensitive pathways (mTOR) that promote protein synthesis
  • Metabolic stress (lactate, hypoxia) and muscle damage (microtrauma) trigger the release of growth factors and inflammatory responses
• Endurance training enhances the oxidative capacity of muscle fibers through mitochondrial biogenesis and capillary density
  • Mitochondrial biogenesis is stimulated by the activation of PGC-1α, a master regulator of oxidative metabolism
  • Increased capillary density improves oxygen and nutrient delivery to muscle fibers during prolonged exercise
• Muscle fiber type transitions can occur in response to specific training stimuli, altering the contractile and metabolic properties of muscles

Clinical Applications and Disorders

• Muscular dystrophies are genetic disorders characterized by progressive muscle weakness and wasting
  • Duchenne muscular dystrophy is caused by mutations in the dystrophin gene, leading to membrane instability and muscle degeneration
  • Becker muscular dystrophy is a milder form caused by partial dystrophin deficiency
• Myopathies are muscle disorders caused by abnormalities in muscle structure or metabolism
  • Mitochondrial myopathies result from defects in mitochondrial function, leading to exercise intolerance and muscle weakness
  • Glycogen storage diseases (McArdle disease) cause muscle cramps and fatigue due to impaired glycogen utilization
• Neuromuscular disorders affect the communication between motor neurons and muscle fibers
  • Myasthenia gravis is an autoimmune disorder targeting acetylcholine receptors at the NMJ, causing muscle weakness and fatigability
  • Amyotrophic lateral sclerosis (ALS) involves the degeneration of motor neurons, leading to progressive muscle atrophy and paralysis
• Sarcopenia is the age-related loss of muscle mass and strength, increasing the risk of frailty and falls in the elderly
  • Resistance training and adequate protein intake can help preserve muscle mass and function in older adults
• Rhabdomyolysis is the breakdown of muscle tissue due to injury, overexertion, or certain medications
  • Muscle damage releases myoglobin into the bloodstream, potentially leading to kidney damage if untreated
• Exercise physiology principles are applied in the prevention and rehabilitation of musculoskeletal disorders
  • Resistance training is used to counteract muscle atrophy and weakness in various conditions (post-surgery, immobilization)
  • Aerobic exercise is prescribed to improve cardiovascular health and endurance in patients with chronic diseases (heart failure, COPD)