20 - unctional morphology of the smooth muscles – excitation, electrophysiological characteristics and mechanisms of contraction.

section

smooth muscle structure

smooth muscle contraction

smooth muscle regulation

smooth muscle structure

Are involuntary, Smooth muscle is made up cells arranged in sheets and contain three filament types:

• actin filaments with no troponin and tropomyosin used for contraction

• myosin filaments – used for contraction

• intermediate filaments -which do not take part in contraction

smooth muscle contraction

• In smooth muscle, calcium comes from:

  • The sarcoplasmic reticulum (SR)

  • The extracellular fluid (ECF) via calcium channels in the plasma membrane

• Calcium binds to calmodulin

• The calcium–calmodulin complex activates myosin light chain kinase (MLCK)

• MLCK phosphorylates myosin, enabling it to bind to actin

• Cross-bridge cycling begins using ATP, similar to skeletal muscle

• Smooth muscle lacks troponin, so contraction depends on myosin phosphorylation, not actin exposure

• Calcium acts as a second messenger, triggering a chemical cascade

• Relaxation occurs when:

  • Calcium is actively pumped back into the SR and out of the cell

  • Myosin is dephosphorylated and can no longer bind actin

• Key difference:

  • Smooth muscle: Calcium causes a chemical change in myosin

  • Skeletal/cardiac muscle: Calcium causes a physical change in actin

types of smooth muscle tissue

here are two types of smooth muscle tissue

1. Multi-unit 2. Single unit.

1. Multi-Unit Smooth Muscle:

   • Made of separate units that function independently

   • Each unit must be individually stimulated by the autonomic nervous system

   • Found in:

     • Large blood vessels

     • Large airways in the lungs

     • Eye muscles (for lens and pupil adjustment)

     • Base of hair follicles

     2. Single-Unit Smooth Muscle:

   • Muscle fibers are connected by gap junctions and contract together

   • Allows coordinated contraction of entire organs

   • Self-excitable (can contract without nerve input)

   • Found in walls of hollow organs:

     • Digestive system

     • Reproductive system

     • Urinary system