Topic 22 - Characteristics of smooth muscle

Question 1

Words to include

Answer 1

• Visceral organs
• Protein lattices (dense bodies)
• Intermediate filaments
• Membrane plates
• Cytoskeleton
• Nucleus
• Blood supply
• Spindle shaped
• Non-striated
• Non-regular myofilaments
• Intermediate filaments
• T-tuble (ø)
• Sarcomere (ø)
• Sarcoplasmic reticulum (small)
• Multi-unit smooth muscle
  
  • Eyes
    
    • M. ciliaris
  • Skin
  • Individual fibers
  • Gap junctions (ø)
  • Local depolarization
  • Collagen
  • Glycogen
  • Autonomic neuron varicosity
• Single-unit smooth muscle
  
  • Hollow organs wall
  • Tubular organs wall
  • Spontaneous contraction
  • Gap junctions
  • Syncytium
  • Peristaltic movements
  • Bayliss effect
    
    • Self-reflection (myogenic response)
  • Autonomic neuron varicosity
  • Gastrointestinal muscles
  • Muscles of vessels
  • Bronchial muscles
  • Bladder of sphincters
• Varicosities
  
  • Axon terminal
  • Neurotransmitters
• Innervation
  
  • Serperately (multi)
  • Humoral regulation (single)
• Ligand
  
  • ACh (multi)
    
    • Parasympathetic
  • Norepinephrine (multi)
    
    • Sympathetic
  • ACh (single)
  • Norepinephrine (single)
  • VIP (single)
  • NO (single)
  • ATP (single)
• Receptor
  
  • Muscarinic (multi)
  • Alpha-1-R (multi) (single)
  • mACh-R (single)
  • Beta-2-R (single)
• Smooth muscle sarcomere
  
  • F-actin α-helix
  • G-actin subunit
  • Myosin binding-site
  • Tropomyosin
    
    • Inactive status
    • Active status
  • Calmodulin-Ca²⁺ complex
  • Caldesmon
• Sarcoplasmic reticulum
  
  • Action potential
  • Autonomic nerves
  • Caveoles
    
    • Rudimentary T-tubules
  • Sarcoplasmic tubule
• Myolemma
  
  • Voltage gated Ca²⁺-channels
  • Ligand gated Ca²⁺-channels
    
    • Hormone activated
• Actin
• Myosin
  
  • MLC subunit
    
    • P-LCh
      
      • ADP+P
      • Non-phosphorylated
      • Phosphorylated
• Prolonged tonic contraction
• Cross-bridge cycle (lower frequency)
  
  • ATPase activity (low)
  • ATP catabolism (slow)
  • Myosin head power-stroke (slow)
  • 1 ATP/ cross bridge cycle
• Action potential
  
  • Single-unit smooth muscles
  • Peak potential
    
    • Electric stimuli
    • Hormones
    • Spontaneously by pacemaker activity
  • AP with plateu
    
    • Prolonged repolarization
  • Humoral regulation
  • Metabolic regulation
• Contraction
  
  • Sympathetic (alpha-1) neural AP
  • Parasympathetic (mACh) neural AP
  • Myolemma
  • Voltage gated ion channel
  • Calcium influx
  • Chemical ligands
  • Ligand gated ion channels
  • IP3 release
  • G-protein
  • Phospholipase C (PLC)
  • Sarcoplasmic reticulum
  • Calcium sequester vesicle
• Relaxation
  
  • cAMP (↑)
  • CGMP (↑)
  • Calcium concentration (↓)
  • Sympathetic beta-2 receptor agonists
    
    • Epinephrine
    • VIP
  • NO
    
    • Capillary endothelium
  • ATP
    
    • Purinergic receptors
• Local chemical factors
  
  • O₂ Ø
  • O₂ excess
  • H⁺ (↑)
  • Lactic acid level (↑)
  • K⁺ (↑)
• Vasodilation
• Spontaneous generation of SM AP
  
  • Pacemaker
  • Slow wave potential
    
    • Local potential
  • Self-excitatory
  • Spike potential (AP)

Question 2

Where are smooth muscles found?

Answer 2

• Visceral organs
  
  • Gastrointestinal muscles (single-unit)
  • Muscles of vessels (single-unit)
  • Bladder of sphincters (single-unit)
• Bronchial muscles (single-unit)
• Eyes - m. ciliaris (multi-unit)
• Skin (multi-unit)
• Develop from one cell and have only one nucleus
• Can be formed throughout life

Question 3

Basics of smooth muscles

Answer 3

• Poor blood supply
• Small, spindle shaped cells
• Non-striated
• Non-regular myofilaments
• Intermediate filaments
• Dense bodies
• Small sarcoplasmic reticulum
• T-tubules Ø
• Sarcomere Ø
  
  • Each smooth muscle cell is like one giant sarcomere

Question 4

What are the major types of smooth muscle?

Answer 4

1. Multi-unit smooth muscle
2. Single-unit smooth muscle

Question 5

Multi unit smooth muscle

Answer 5

• Individual fibers
• Ø gap junctions
• Contraction is independent in each fiber, and stimulated by nerves
• Not trough AP
• Spontaneous contraction is rare
• Transmitters cause local depolarization
• Fibers are isolated by:
  
  • Collagen
  • Glycogen
• Innervation:
  
  • Induvidal innervation
• Ligand:
  
  • Parasympathetic: ACh
  • Sympathetic: norepinephrine (and ATP)
• Receptor:
  
  • Muscarinic
  • Alpha-1-receptor
• Location:
  
  • M. ciliaris (eye)
    
    • Adjusting the diameter of the pupils according to the intensity of ligh
    • Altering the curvature of the lens in the eye
  • Skin
    
    • ​Changing the angle of hairs relative to skin surface

Question 6

Single-unit smooth muscle

Answer 6

• Millions of fibers contracting all at once, creating a functional system
• Spontaneous contraction may happen
• Lots of gap junctions
• Fast penetration of ions
• Fast spreading of AP
• Within single-unit cells, the whole bundle or sheet contracts as a syncytium
• Sustained contraction of the smooth muscle in walls transfers pressure to the content or the organ, while alternating contractions and relaxations lead to mixing of the content
• Can also create waves of contraction = peristaltic movements
• Bayliss effect: Extension of certain smooth muscles results in contraction
• Self-reflection (myogenic response) of smooth muscle
  
  • Unrelated to neural and hormonal influences
• Innervation:
  
  • Humoral regulation, active contraction and relaxation
• ​Ligand:
  
  • ACh
  • Norepinephrine
  • VIP
  • NO
  • ATP
• Receptor:
  
  • mACh receptor
  • Aplha-1 receptor
  • Beta-2 receptor
• Location:
  
  • Walls of hollow and tubular organs
    
    • ​Gastrointestinal muscles
    • Muscles of vessels
    • Bronchial muscles
    • Bladder of sphincters

Question 7

What is varicosities?

Answer 7

• Varicosities: the axon terminal
• A series of axon-like swelling, from autonomic neurons form motor units through the smooth muscle

Question 8

Molecular structure of smooth muscle sarcomere

Answer 8

• Myosin
  
  • ​Class II in SM
    
    • 2 heavy chains
      
      • 2 heads
    • 4 light chains
  • Thick filamet
  • MLC subunit:
    
    • P-LCh (ADP+P)
      
      • Non-phosphorylated: Ø bind to actin
      • Phosphorylated: can bind to actin → contraction
• Actin
  
  • ​Attached to protein lattices (dense bodies)
    
    • ​Analogous to the Z-disc
  • Thin filaments
  • G-actin
  • F-actin
• Tropomyosin
  
  • ​Inactive status: blocks 7 binding sites
  • Active status: in groove of α-helix lives binding sites
• Calmodulin
  
  • ​Calmodulin-Ca²⁺ complex
  • Removes tropomyosin from binding sites
• Caldesmon
  
  • ​Ø Ca²⁺ = caldemosin keeps tropomyosin on binding sites
• Caveoles
  
  • ​Rudimetary T-tubules

Question 9

Organization of acin-myosin filaments in smooth muscles

Answer 9

Question 10

Sarcoplasmic reticulum is smooth muscles

Answer 10

Question 11

Source of Ca²⁺

Answer 11

1. Sarcoplasmic reticulum
2. EC space

Question 12

Channels found on the myolemma of smooth muscles

Answer 12

• Voltage gated Ca²⁺ channels
  
  • ​More than in skeletal muscles
  • More than Na⁺ channels
• Ligand gated Ca²⁺ channels
  
  • ​Hormone activated

Question 13

Characteristics of smooth muscle contraction

Answer 13

• Prolonged tonic contraction
  
  • ​Hours, days
  • Length of contraction is 30x longer than in skeletal muscles
• Lower frequency of cross-bridge cycle
  
  • ​Reason:
    
    • Lower ATPase activity → Slower ATP catabolism → Slower myosin head power-stroke
• Energetically economic
  
  • 1 ATP/cross-bridge cycle
    
    • ​Irrespective of the length of contraction

Question 14

Factors causing contraction in smooth muscles

Answer 14

1. Spreading of sympathetic (alpha1) or parasympathetic (mACh) neural AP to the myolemma and its effect on voltage gated ion channel
2. Result: Ca²⁺ influx
3. Binding of chemical ligands to ligand gated calcium channels
4. IC IP3 release and G-protein or phospholipase C mediated, ligand gated, Ca²⁺ influx from sarcoplasmic reticulum or from a calcium sequester vesicle

Question 15

Local chemical factors influencing smooth muscle contraction

Answer 15

1. Lack of O₂
2. Excess of O₂
3. H⁺ ↑
4. Lactic acid level ↑
5. K⁺ ↑

Relaxation → vasodilation

Question 16

Relaxation of smooth muscles

Answer 16

• Relaxation is enhanced by every stimulus which can increase IC cAMP or CGMP level
  
  • Decreases IC Ca²⁺ concentration
• Most important signal are:
  
  • Sympathetic beta2 receptor agonists
    
    • Epinephrine
    • VIP
  • NO (direct, or diffusible from capillary endothelium)
  • ATP (acts on purinergic receptors)

Relaxation → vasodilation

Question 17

Action potential in smooth muscles

Answer 17

• AP occurs only in single-unit smooth muscles

​

• Types of AP:
  
  1. Peak potential
     
     • Can be initiated:
       
       • Electric stimuli
       • Hormones
       • Generated spontaneously (by pacemaker activity)
  2. AP with plateau
     
     • Prolonged repolarization
• Smooth muscle contraction functions under very intensive humoral and metabolic regulation (instead of intensive neural activation)
• Spontaneous generation of SM AP
  
  • Pacemaker of slow wave potential
    
    • ​Slow wave potential is not an AP, but a local potential. If it increases above -35 mV, it initiates AP
    • Cause of slow wave potential is unknown
  • AP (spike potential)
  • Some SM are self-excitatory