Muscle Contraction

reduces the length of each sarcomere

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. 

• conformational changes
• myosin heads
• hydrolysis of ATP (myosin head is an ATPase)

when stimulated by lower motor neurons (via acetylcholine)

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. 

• Infoldings
• transverse tubules
• graded depolarization

• sarcoplasmic reticulum 
• depolarized
• store of Ca2+

bind to troponin causing conformational (shape) change that moves tropomyosin and exposes the binding sites on actin; myosin heads attach and muscle contract. 

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

indirectly provides most of the energy