Comparative Vertebrate Physiology

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Comparative Vertebrate Physiology

• Smooth and cardiac muscle

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Smooth muscle

• Characteristics
• lack striations
• uninucleate
• occurs in layers

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Characteristics
Contracts uniquely because

• T tubules absent (SR touches sarcolemma)
• poorly develop SR (extracellular Ca)
• myosin, actin present lacks troponin

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Types of smooth muscle

• Single unit (visceral)
• occur in layers
• gap junctions (spontaneous depolarization)
• mechanical stimuli and innervated
• visceral organs (gut, bladder etc)

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Types of smooth muscle

• Multiunit
• discrete fibers
• no gap junctions
• innervated
• large airways, large arteries, arrector pilli
  muscles

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Potentials

• Single unit
• Pacemaker, slow wave and action potentials
• Multiunit
• Spike potentials

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Smooth muscle contraction
Mechanisms
1. Myosin phosphorylation Ca binds to
calmodulin and activates MLCK allowing myosin to
bind to actin 2. Regulation by caldesmon Ca-calm
odulin removes caldesmon from its blocking
position on actin 3. Direct Ca binding Binding
onto myosin, allowing it to bind to actin
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Calcium sources

• Intracellular from SR
• Extracellular
• Voltage operated channel (AP) in multiunit muscle
• Tension is maintained after Ca is gone

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Length-tension relationship

• Tension over a wide range of initial lengths

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Contract of smooth muscle

• Irregular myofilament arrangement

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Cardiac muscle

• Characteristics
• contains striations
• single innervation area by the ANS (pacemaker)

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Cardiac muscle

• Characteristics

• cells are uninucleated
• intercalated discs
• desmosomes
• gap junctions

disc
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Cardiac muscle

• Action potentials

• plateau phase
• prolonged refractory period

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Cardiac muscle

• Contraction
• regulated by Ca
• sources ECF and SR
• Following contraction
• Ca back into SR by Ca pump
• Ca back into ECF by Na/Ca proteins on
  sarcolemma
• Sources of Ca
• Differences between vertebrates (frogs vs.
  mammals)

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Intracellular calcium

• Factors effecting concentration
• 1. Degree of depolarization

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Intracellular calcium

• 2. Concentration of catecholamines
• Ex. epinephrine, norepinephrine, dopamine
• ?-adrenoreceptors activate IP3 increase Ca from
  SR

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Intracellular calcium

• 2. Concentration of catecholamines
• ?-adrenoreceptors activate adenyl cyclase
  increase Ca across sarcolemma

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Intracellular calcium

• 3. Temperature
• Why is heart rate in an ectotherm temperature
  dependent?
• reduction slows Ca pump in the SR
• reduction slows Na/Ca exchange across
  sarcolemma