Lecture Notes: Smooth Muscle in the Muscular System

Introduction

• Final module on the muscular system focuses on smooth muscle.
• Not covered in as much detail as skeletal muscle.
• Key structural and functional characteristics to note.

Structural Characteristics of Smooth Muscle

• Location: Found throughout the body

  • Walls of hollow organs: stomach, intestines, bladder, uterus.
  • Passageways: blood vessels, lymph vessels.
  • Tracks: respiratory, urinary, and reproductive systems.
  • Eye muscles: dilate/contract pupil, alter lens shape.
  • Muscles causing goosebumps.
  • Involuntary sphincters throughout the body.

• Structure: Spindle-shaped fibers

  • Shape: Tapered at ends, fatter in the middle.
  • Nucleus: Single, centrally located.
  • Non-striated: Lacks visible striations unlike skeletal muscle.

• Arrangement of Filaments

  • Contains thick and thin filaments, not in sarcomeres.
  • Thin filaments anchored to dense bodies instead of Z discs.
  • Dense bodies pulled together during contraction.

• Layer Arrangement

  • Typically in two layers or sheets.
  • Inner circular layer: forms sphincters.
  • Outer longitudinal layer: works with circular layer for peristalsis.

Contraction and Innervation

• Innervation Differences

  • Skeletal muscle: Innervated by a single motor neuron forming neuromuscular junctions.
  • Smooth muscle: Innervated by autonomic motor neurons using varicosities.

• Diffuse Junctions

  • Varicosities: Swellings in the neuron containing vesicles with neurotransmitters.
  • Neurons lie on a sheet of muscle for simultaneous contraction.
  • Junctions are broad/diffuse, unlike well-defined neuromuscular junctions.

• Action Potential Propagation

  • Skeletal muscle: Action potential travels via T tubules.
  • Smooth muscle: Propagates through gap junctions.
  • Gap junctions: Allow ions to pass directly, facilitating rapid spread of action potential.

• Calcium's Role

  • Released into sarcoplasm for contraction.
  • Source of Calcium
    • Skeletal muscle: Sarcoplasmic reticulum.
    • Smooth muscle: Less developed sarcoplasmic reticulum, also extracellular fluid.

• Molecular Mechanism

  • Skeletal muscle: Calcium binds to troponin.
  • Smooth muscle: Calcium binds to calmodulin.
  • Activation of enzyme for myosin-actin binding and cross-bridge formation.

Conclusion

• Structural and functional distinctions are crucial for understanding smooth muscle behavior.
• Focus on how these differences impact muscle contraction and function throughout the body.