Cell physiology - Smooth muscles

Question 1

Components of smooth muscles and implication on its appearance

Answer 1

Has thick and thin filaments that are not arranged in sarcomeres; therefore, they appear homogeneous rather than striated

Question 2

Types of smooth muscle and which one is the most common type

Answer 2

• Multi-unit smooth muscle
• Unitary (single-unit) smooth muscle – most common type
• Vascular smooth muscle

Question 3

Multi-unit vs single unit smooth muscles

• Location
• Electrical coupling between cells
• Innervation

Answer 3

• Location
  
  • Multi-unit
    
    • Iris, ciliary muscle of the lens, and vas deferens
  • Single-unit
    
    • Uterus, gastrointestinal tract, ureter, and bladder
• Electrical coupling between cells
  
  • Multi-unit
    
    • Little or no electrical coupling between cells
  • Single-unit
    
    • High degree of electrical coupling between cells and, therefore, permits coordinated contraction of the organ (e.g., bladder)
• Innervation
  
  • Multi-unit
    
    • Densely innervated; contraction is controlled by neural innervation (e.g., autonomic nervous system) –>
  • Single-unit
    
    • Spontaneously active (exhibits slow waves) and exhibits “pacemaker” activity, which is modulated by hormones and neurotransmitters

Question 4

Properties of vascular smooth muscle

Answer 4

Properties of both multi-unit and single-unit smooth muscle

Question 5

Key difference of smooth vs skeletal muscle contraction

Answer 5

There is no troponin in smooth muscle; instead, Ca²⁺ regulates myosin on the thick filaments

Question 6

Steps in excitation–contraction coupling in smooth muscle

Answer 6

Depolarization of cell membrane opens voltage-gated Ca²⁺ channels

↓

Ca²⁺ flows into the cell down its electrochemical gradient

↓

Increase in the intracellular Ca²⁺

↓

Ca²⁺ binds to calmodulin

↓

Ca²⁺–calmodulin complex binds to and activates myosin

light-chain kinase

↓

Phosphorylates myosin and allows it to bind to actin

↓

Initiating cross-bridge cycling

Question 7

The amount of tension produced depends on

Answer 7

Proportional to the intracellular Ca²⁺ concentration

Note: Decrease in intracellular Ca²⁺ produces relaxation

Question 8

Other mechanisms that increase intracellular Ca²⁺ in smooth muscles

Answer 8

• Hormones and neurotransmitters
  
  • may open ligand-gated Ca²⁺ channels in the cell membrane
  • directly release Ca²⁺ from the SR through inositol 1,4,5-triphosphate (IP3)-gated Ca²⁺ channels