Muscle Function and Contraction

Types of Muscle Tissues

• Cardiac and Smooth Muscle
  • Involuntary control
  • Not consciously controlled
• Skeletal Muscle
  • Voluntary control
  • Composed of bundles of muscle fibers

Muscle Fibers

• Made of long cylindrical cells with multiple nuclei
• Contract or relax upon receiving signals from the nervous system

Neuromuscular Junction

• Site of signal exchange
• Connection between the synaptic bulb of an axon terminal and a muscle fiber

Myofibrils and Sarcomeres

• Muscle fibers contain myofibrils
• Myofibrils consist of sarcomeres (contractile units)
  • Alternating thick and thin protein filaments
  • Give skeletal muscle a striated appearance

Sliding Filament Mechanism

• Thick Filaments
  • Composed of myosin
  • Anchored at the M-line (center of sarcomere)
• Thin Filaments
  • Composed of actin
  • Anchored to Z lines (outer edges of sarcomere)
• Contraction Process
  • Myosin pulls actin filaments
  • Myosin's cross bridges attach to actin, exerting force
  • Sarcomeres shorten as filaments slide past each other

Power Stroke

• ATP hydrolyzed to ADP and inorganic phosphate
• Myosin head extends, attaches to actin
• Myosin pulls actin toward M-line, shortening the sarcomere
• ADP and phosphate released
• Myosin detaches when a new ATP binds

Role of Calcium

• Regulatory Proteins
  • Tropomyosin blocks binding sites on actin when muscle is relaxed
  • Troponin binds to calcium, displaces tropomyosin, exposing binding sites
• Calcium Storage and Release
  • Stored in the sarcoplasmic reticulum
  • Released upon nerve signals
  • Triggers muscle contraction

Muscle Contraction

• Neurotransmitters depolarize muscle fiber membrane
• Electrical impulse travels down T-tubules
• Opens calcium stores, calcium ions flow to myofibrils
• Actin and myosin slide, shortening sarcomeres
• Muscle fibers shorten, generating force for movement (e.g., taking notes)