Smooth muscle, found in hollow organs and passageways, plays a vital role in regulating bodily functions. Its unique structure, featuring dense bodies instead of Z-discs, allows for efficient force transmission during contraction.

Unlike skeletal and cardiac muscle, smooth muscle lacks striations and contracts involuntarily. It can be single-unit or multi-unit, with different properties suited to various functions throughout the body. Calcium ions are key to initiating smooth muscle contraction.

Smooth Muscle Structure and Function

Dense bodies in muscle contraction

• Structures within smooth muscle cells composed of α-actinin and other proteins that anchor actin filaments
• Allow for force transmission during contraction as actin filaments connect to dense bodies
• Functionally similar to Z-discs in skeletal muscle
• Contraction occurs when myosin heads pull on actin filaments, which transmit force to dense bodies and then to the cell membrane and extracellular matrix
• Contractile units in smooth muscle consist of actin and myosin filaments connected to dense bodies

Functions of smooth muscle

• Found in walls of hollow organs and passageways (digestive tract, respiratory tract, urinary tract, blood vessels)
• Contraction and relaxation regulate passage of substances like food through digestive system, air through respiratory system, urine through urinary system, and blood through blood vessels
• Aids in mixing and propulsion of contents within these systems
• Regulates blood pressure and blood flow distribution throughout the body
• Contributes to thermoregulation by controlling blood flow to skin surface

Comparison of Muscle Types

Smooth vs skeletal and cardiac muscle

• Similarities: contain actin and myosin filaments, contract via sliding filament mechanism, require calcium for contraction
• Differences:
  • Smooth muscle lacks sarcomeres and striations, while skeletal and cardiac muscle have them
  • Smooth muscle has dense bodies, skeletal muscle has Z-discs, and cardiac muscle has intercalated discs
  • Smooth muscle is involuntary, skeletal muscle is voluntary, and cardiac muscle is involuntary but autorhythmic
  • Smooth muscle contraction is slower and more sustained compared to faster and shorter contractions in skeletal and cardiac muscle
  • Smooth muscle contraction is regulated by phosphorylation of myosin light chains

Single-unit vs multi-unit smooth muscle

• Single-unit smooth muscle:
  • Cells electrically coupled via gap junctions allowing for synchronous contraction
  • Found in walls of hollow organs (digestive tract, uterus, ureters, small blood vessels)
  • Contraction and relaxation regulate passage of substances and aid in mixing and propulsion
• Multi-unit smooth muscle:
  • Cells not electrically coupled, each cell functions independently
  • Found in larger blood vessels, airways, and iris of the eye
  • Contraction and relaxation regulate blood flow, airflow, and pupil size
  • Innervated by autonomic nervous system allowing for more precise control compared to single-unit smooth muscle

Smooth Muscle Contraction Mechanism

• Calcium ions play a crucial role in initiating smooth muscle contraction
• Sarcoplasmic reticulum stores and releases calcium within smooth muscle cells
• Autonomic nervous system regulates smooth muscle contraction through neurotransmitter release