Myology: Muscle Tissues and Structure

• Myologie is the study of muscles, their structure, function, and diseases
• Muscles facilitate movement, maintain posture and generate heat

Types of Muscle Tissue

• Skeletal muscle attaches to bones, enabling voluntary movements
  • Identified by its striated appearance when viewed microscopically
  • Governed by the somatic nervous system
• Smooth muscle lines the walls of internal organs like the stomach, bladder and blood vessels
  • Facilitates involuntary actions such as peristalsis and vasoconstriction
  • Characterized by the absence of striations and regulation by the autonomic nervous system
• Cardiac muscle solely resides in the heart
  • Responsible for blood circulation
  • Displays striated texture, operates involuntarily, and is regulated by the autonomic nervous system along with its own intrinsic mechanisms

Skeletal Muscle Structure

• Muscle fibers (cells) appear as elongated, cylindrical, multinucleated structures
• Sarcolemma denotes the plasma membrane of a muscle fiber
• Sarcoplasmic reticulum represents the endoplasmic reticulum within a muscle fiber, crucial for storing calcium ions
• Myofibrils constitute lengthy, contractile components inside muscle fibers, comprising sarcomeres
• Sarcomere defines the fundamental functional unit of a muscle fiber, essential for contraction
  • Characterized by Z discs (lines) that secure thin filaments (actin)
  • Includes thick filaments (myosin) at its center (A band), overlapping with actin
  • The I band exclusively contains thin filaments
  • The H zone solely contains thick filaments, noticeable during muscle relaxation
• T tubules are invaginations of the sarcolemma that propagate action potentials into the muscle fiber's core

Muscle Contraction

• Neuromuscular junction serves as the synapse where a motor neuron meets a muscle fiber
• Action potential refers to an electrical signal transmitted along the motor neuron to the neuromuscular junction
• Neurotransmitter release happens as the motor neuron discharges acetylcholine (ACh) into the synaptic cleft.
• ACh binding occurs when ACh attaches to receptors on the sarcolemma, triggering an action potential within the muscle fiber
• Calcium release occurs as the action potential moves along the T tubules, prompting the sarcoplasmic reticulum to liberate calcium ions (Ca2+) into the sarcoplasm
• Cross-bridge formation is initiated when Ca2+ binds to troponin, causing tropomyosin to move away from the myosin-binding sites on actin, allowing myosin heads to bind to actin and form cross-bridges
• Power stroke: involves the pivoting of the myosin head, drawing the actin filament toward the sarcomere's center, thus shortening the sarcomere
• ATP binding and detachment: ATP's attachment to the myosin head causes its detachment from actin
• Myosin reactivation: ATP is broken down into ADP and inorganic phosphate, which provides energy to reposition the myosin head. The cycle continues if Ca2+ remains present
• Muscle relaxation ensues when the action potential ceases, Ca2+ is actively transported back into the sarcoplasmic reticulum, and tropomyosin obstructs the myosin-binding sites on actin, preventing cross-bridge formation and facilitating muscle relaxation

Muscle Metabolism

• ATP (adenosine triphosphate) acts as the primary energy fuel for muscle contraction.
• Creatine phosphate functions as an energy-rich molecule capable of rapidly replenishing ATP
• Glycolysis involves breaking down glucose to produce ATP and pyruvate
  • Anaerobic glycolysis transpires without oxygen, yields less ATP, and leads to lactic acid accumulation
  • Aerobic glycolysis occurs with oxygen, generates a greater amount of ATP, and produces less lactic acid
• Oxidative phosphorylation signifies the ATP production process within mitochondria, utilizing oxygen
• Muscle fatigue defines the diminished capacity of a muscle to sustain force during prolonged activity
  • Factors contributing to fatigue include ATP and glycogen depletion, lactic acid accumulation, and electrolyte imbalances
• Oxygen debt represents the oxygen quantity needed post-exercise to restore metabolic conditions to their resting levels

Muscle Fiber Types

• Type I (slow oxidative) fibers:
  • Characterized by high endurance and resistance to fatigue
  • Possess a red hue due to elevated myoglobin content (an oxygen-binding protein)
  • Primarily rely on aerobic metabolism
• Type IIa (fast oxidative glycolytic) fibers:
  • Display moderate endurance and strength
  • Exhibit a pink coloration
  • Utilize both aerobic and anaerobic metabolic pathways
• Type IIb (fast glycolytic) fibers:
  • Exhibit high strength but are prone to fatigue
  • Appear white due to minimal myoglobin content
  • Predominantly depend on anaerobic glycolysis

Muscle Actions

• Agonist (prime mover): This muscle is chiefly accountable for initiating a specific movement
• Antagonist: This muscle resists or reverses the action of the agonist
• Synergist: This muscle aids the agonist by either stabilizing a joint or contributing additional force
• Fixator: This muscle secures the origin of the agonist, thus enhancing its efficiency

Muscle Disorders

• Muscle strain: Damage to a muscle or tendon resulting from excessive stretching or tearing
• Muscular dystrophy: A collection of genetic conditions marked by gradual muscle deterioration and weakness
• Myasthenia gravis: An autoimmune disorder impacting the neuromuscular junction, resulting in muscle weakness
• Cramps: Abrupt, uncontrollable muscle contractions
• Fibromyalgia: A persistent ailment defined by widespread muscle tenderness and fatigue

Muscle Growth and Adaptation

• Hypertrophy: Refers to the enlargement of muscle fibers resulting from increased synthesis of proteins (actin and myosin)
• Atrophy: Denotes the reduction in muscle fiber size due to inactivity or immobilization
• Resistance training: Entails exercises where muscles contract against opposition, promoting hypertrophy and boosting strength
• Endurance training: Consists of sustained, rhythmic exercises that improve cardiovascular performance and enhance endurance

Muscle Naming Conventions

• Muscle names often derive from:
  • Location (e.g., tibialis anterior)
  • Shape (e.g., deltoid)
  • Size (e.g., gluteus maximus)
  • Direction of fibers (e.g., rectus abdominis)
  • Number of origins (e.g., biceps brachii)
  • Action (e.g., flexor carpi ulnaris)