Mechanism of muscle contraction.Thin filaments ratchet across thick filaments to pull the Z lines together and shorten the sarcomere. Myosin molecules on thick filaments attach to actin on the thin filament to form a crossbridge. It then bends inward, pulling the thin filament toward the center of the sarcomere, breaks the crossbridge, and forms a new crossbridge further down.
Energy for __ changes in __ comes from __.
hydrolysis of ATP (myosin head is an ATPase)
Skeletal muscles contract when?
when stimulated by lower motor neurons (via acetylcholine)
What happens in a muscle at rest?
myosin-binding sites on the actin are blocked by the regulatory protein strand (tropomyosin) in the thin filament. Attached to tropomyosin is a troponin complex.
__ in the muscle cell plasma membrane (__) carry the resulting __ deep into the muscle cell.
transverse tubulesgraded depolarization
The __ membrane becomes __ and releases its __.
sarcoplasmic reticulum depolarized
store of Ca2+
What do the calcium ions do?
bind to troponin causing conformational (shape) change that moves tropomyosin and exposes the binding sites on actin; myosin heads attach and muscle contract.
The contraction is terminated as __.
calcium is pumped out of the cytoplasm, back into the sarcoplasmic reticulum, by calcium pumps (calcium ATPase); as the calcium concentration falls, the tropomyosin-troponin complex again blocks the binding sites