Lecture on Smooth Muscle

Introduction to Smooth Muscle

• Unlike skeletal muscle, smooth muscle exhibits more variation.
• Lacks a classical neuromuscular junction.
• Presence of varicosities along the axon to bathe smooth muscle in neurotransmitter.
• Gap junctions exist in smooth muscle cells, allowing cytosol sharing and signal penetration.
• Example: Muscles in the small intestine use gap junctions for rhythmic contractions.
• Example: Smooth muscle in the eye lacks gap junctions but is penetrated by varicosities.

Variation in Smooth Muscle

• Types of smooth muscle cells vary widely.
• Electrical properties differ across smooth muscle types.
• Multiple initiation methods for contraction and relaxation due to absence of classical neuromuscular junction.

Muscle Contraction Durations

• Skeletal Muscle: Muscle twitch lasts ~100 milliseconds.
• Cardiac Muscle: Twitch lasts ~250 milliseconds.
• Smooth Muscle: Contraction and relaxation last several seconds.
  • Slower and less energy-intensive process.
  • Supports sustained force of contraction over longer periods.
  • Linked to slow and steady myosin ATPase activity.

Types of Contractions

• Phasic Contractions:
  • Default state is relaxed.
  • Initiated by non-neural signals (e.g., stretch).
  • Example: Esophagus contraction initiated by signals, followed by relaxation.
  • Cyclical Phasic Contractions seen in intestines for rhythmic contraction.
• Tonic Contractions:
  • Default state is contracted.
  • Example: Sphincters (e.g., cardiac sphincter) prevent backflow by staying contracted.
  • Relaxation is triggered to allow passage of food/drink.
  • Distinction between phasic (default relaxation) and tonic (default contraction) contractions.

Histology and Structure

• Smooth muscle lacks striations seen in skeletal and cardiac muscle.
• Different organization of thin and thick filaments due to different contraction mechanisms.
  • Skeletal muscle: Contraction along a single plane.
  • Smooth muscle: Three-dimensional contraction for effective mechanical breakdown (e.g., stomach churning).
• Absence of troponin in smooth muscle affects contraction mechanism.
• Calcium release from the sarcoplasmic reticulum (SR) differs from skeletal muscle.

Conclusion

• The lecture highlights differences and unique aspects of smooth muscle.
• Future lectures will cover the calcium release mechanism in smooth muscle.