22 Questions
What is the common molecular organization of skeletal muscle and cardiac muscle cells?
Both are constituted by sarcomeres and appear as striated muscle cells
What is the primary mechanism that produces contraction in all three types of muscle cells?
Interaction of actin and myosin filaments
How is myosin activity modulated in smooth muscle cells?
By phosphorylation
What regulates the contraction in striated muscle cells?
Availability of binding sites for myosin on the actin filament, dependent on the presence of calcium ions in the cytosol
Which type of muscle possesses myosin with ATPase activity that produces force for contraction?
All three types: skeletal, cardiac, and smooth muscles
What is a major difference between smooth muscle and striated muscle in terms of myosin activity regulation?
The regulation of striated muscle depends on calcium ion availability, while smooth muscle myosin activity is modulated by phosphorylation
What is the primary reason for the striated appearance of muscle fibers?
Organization of contractile material in sarcomeres
What maintains the inhibited position of myosin binding sites on actin?
Tropomyosin + troponin complex
What triggers the unmasking of myosin binding sites on actin?
Release of calcium ions
Which structure facilitates excitation-contraction coupling in skeletal muscle?
T-tubules
Why is the action potential duration longer in cardiac myocytes compared to skeletal muscle?
Presence of L-type calcium channels
What allows cardiac muscle to contract even in the absence of extracellular calcium?
Sustained calcium release from L-type calcium channels
In cardiac muscle, the force of contraction (contractility) can be modulated by all of the following EXCEPT:
Inhibiting the activity of MLCK
Which of the following is a difference between smooth muscle and striated muscle contraction regulation?
In striated muscle, regulation is on actin, while in smooth muscle, regulation is on myosin
Which factor influences the force of contraction in cardiomyocytes by affecting the probability for troponin to be activated and for the myosin binding site on actin to be available?
Level of cytosolic Ca$^{2+}$
What inhibits myosin light chain kinase (MLCK) in smooth muscle?
$cAMP$
Which mechanism produces smooth muscle relaxation?
$Gs$-coupled receptor activation leading to elevation of $cAMP$
What is a significant difference in the organization of contractile material between smooth and striated muscles?
$Actin filaments are not parallel and cross each other in smooth muscles only
Which enzyme activates myosin in smooth muscle cells?
Myosin light chain kinase (MLCK)
What slows down the myosin cycle in smooth muscle?
Release of ADP and inorganic phosphate
What produces vasoconstriction in smooth muscle?
Activation of Gq-coupled receptor leading to release of Ca$^{2+}$ from ER
How is myosin activity modulated in striated muscle cells?
By enhancing SERCA activity through $PKA$ phosphorylation of phospholamban
Study Notes
- Striated muscle fibers are multinucleated syncytia with contractile material organized in sarcomeres, giving a striated appearance.
- Each sarcomere consists of actin filaments attached to Z bands, myosin filaments at the M band, and alternating regions of overlap creating darker bands.
- Myosin is bound to ADP and inorganic phosphate at rest, but binds ATP and acquires a high affinity for actin upon cleavage.
- The myosin binding sites on actin are masked by the tropomyosin + troponin complex, which maintains the inhibited position.
- Calcium ions release troponin and unmask myosin binding sites, allowing myosin to bind and discharge energy.
- Myosin will bind ATP, lose affinity for actin, and repeat the cycle if ATP is available; if not, myosin remains attached to actin in rigor mortis.
- The striated muscle cannot shorten by more than some 30% of its maximum length due to actin and myosin filament organization.
- Excitation-contraction coupling in skeletal muscle is facilitated through the sarcoplasmic reticulum, T-tubules, and triads.
- The action potential invades the sarcolemma, reaches the T-tubules, and activates L-type calcium channels, Ca2+ is released into the cytosol, and Ca2+ triggers calcium-induced calcium release.
- Cardiac myocytes are not a syncytium, but have a higher surface to volume ratio and dyads for efficient calcium movement.
- The cardiac myocytes' action potential is much longer than skeletal muscle due to the presence of L-type calcium channels, which produce a large calcium current.
- The longer action potential allows for a more sustained calcium release, resulting in prolonged muscle contractions.
- Cardiac muscle can contract even in the absence of extracellular calcium.
- The contraction duration is influenced by the length of the action potential and the rate of calcium clearance.
- Repetitive stimulation is required to produce prolonged contractions.
Learn about the three types of muscle cells: skeletal muscle, cardiac muscle, and smooth muscle, and their specific contraction mechanisms. Understand the molecular organization of the cells and the interaction of actin and myosin filaments.
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