Sliding Filament Theory and Contraction Cycle - Anatomy and Physiology
- What ends contraction once stimulation stops?
- Ca2+-ATPase pumps (SERCA) return Ca2+ to the sarcoplasmic reticulum, and tropomyosin again blocks actin's binding sites.
- What causes a myosin head to detach from actin?
- Binding of a new ATP molecule to the myosin head.
- Why does rigor mortis occur after death?
- Without ATP, myosin heads cannot detach from actin, so cross-bridges stay locked and muscles stiffen.
- Which sarcomere bands shorten during contraction, and which stays the same length?
- The H zone and I band shorten; the A band stays the same, since it equals the thick filament length.
- What happens to the lengths of the thick and thin filaments during contraction?
- They stay the same. The filaments slide past each other, increasing their overlap and pulling Z discs together.
- A skeletal muscle fiber is stretched so far that thick and thin filaments barely overlap. How much tension can it develop?
- Very little. Few cross-bridges can form, so active tension falls far below that at optimal length.
- In excitation-contraction coupling, where does the Ca2+ for skeletal muscle contraction come from?
- The sarcoplasmic reticulum, which releases Ca2+ when T-tubule depolarization is sensed by DHP receptors linked to ryanodine receptors.
- What does Ca2+ bind to in skeletal muscle to start the cross-bridge cycle?
- Troponin. This shifts tropomyosin away from the myosin-binding sites on actin.
- What is the role of ATP hydrolysis in the cross-bridge cycle?
- It recocks the myosin head into its high-energy position, ready to bind actin again.
- A drug blocks the Ca2+ pump of the sarcoplasmic reticulum. How is muscle relaxation affected?
- Relaxation is impaired or delayed, because cytosolic Ca2+ stays high and keeps actin's binding sites exposed.
- What event in the cross-bridge cycle triggers the power stroke?
- Release of inorganic phosphate (Pi) from the myosin head after it binds actin.