Skeletal, cardiac, and smooth muscle contraction mechanisms and the sliding filament model tested on USMLE Step 1.
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- What is the smallest contractile unit of skeletal muscle, defined by the region between two Z lines?
- The sarcomere
- What protein makes up the thin filament of the sarcomere?
- Actin
- What protein makes up the thick filament of the sarcomere?
- Myosin
- What two regulatory proteins are associated with the actin thin filament and control cross-bridge formation?
- Tropomyosin and troponin
- What is the function of tropomyosin at rest?
- It physically blocks the myosin-binding sites on actin, preventing cross-bridge formation
- What troponin subunit binds calcium to trigger the contraction cycle?
- Troponin C
- What happens to tropomyosin when calcium binds troponin C?
- The troponin-tropomyosin complex shifts position, exposing the myosin-binding sites on actin
- According to the sliding filament model, what happens to the lengths of the thick and thin filaments themselves during contraction?
- They stay the same length; contraction occurs because the thin filaments slide past the thick filaments, shortening the sarcomere
- What sarcomere zone shortens during contraction while the A band stays the same length?
- The I band (and the H zone) shorten, while the A band length remains constant
- What is the source of the calcium that triggers skeletal muscle contraction?
- The sarcoplasmic reticulum
- What structure allows an action potential on the muscle cell surface to rapidly reach the interior of the muscle fiber?
- The T tubules (transverse tubules)
- What voltage-sensing protein in the T tubule membrane physically couples to the sarcoplasmic reticulum calcium release channel in skeletal muscle?
- The dihydropyridine receptor
- What is the name of the calcium release channel on the sarcoplasmic reticulum that the dihydropyridine receptor is mechanically linked to in skeletal muscle?
- The ryanodine receptor
- What ATP-dependent process terminates muscle contraction by returning calcium to the sarcoplasmic reticulum?
- The SERCA (sarco/endoplasmic reticulum calcium ATPase) pump
- What happens to a muscle fiber during rigor mortis at the molecular level?
- Depleted ATP leaves myosin heads unable to detach from actin, causing sustained, involuntary cross-bridge binding and muscle stiffness
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