Muscle fiber Nucleus Motor neuron Sarcolemma Myofibrils Sarcoplasmic

Muscle fiber Nucleus Motor neuron

Sarcolemma Myofibrils Sarcoplasmic reticulum (SR) SR Cistern T-tubule

Action potential reaches synaptic terminal of motor neuron

New AP generated at motor end-plate and propogated down T-tubules

Myofibrils (relaxed) Sarcomere (Sarcolemma and other structures not shown)

Myofibrils (contracted) Sarcomere

Sarcomere (relaxed) Myosin (thick filaments) Actin (thin filaments)

Sarcomere (contracted) Myosin (thick filaments) Actin (thin filaments)

Myosin heads Myosin (thick filaments) Actin (thin filament)

Actin Protein complex Myosin head ADP and phosphate Note that ATP is hydrolyzed at this point, but still attached to myosin head.

Protein complex ADP and phosphate Calcium ion Myosin-binding sites (exposed)

Actin ADP and phosphate Myosin head

Actin Myosin head ADP Pi 1. Release of Pi initiates the power stroke. 2. At the end of the power stroke, ADP is released. 3. A new ATP is required for myosin to release its hold on actin.

Actin Myosin head ATP binds to the myosin head, causing it to detach from actin.

Actin Myosin head ADP and phosphate When the ATP is broken down to ADP and phosphate, the myosin head extends.

Actin Myosin heads The sequence repeats as long as calcium ions are present. The combined work of many myosin heads causes the actin filaments to slide past the myosin filaments.

Actin Protein complex Myosin heads When the action potentials stop, calcium ions are pumped back into the ER, the myosin-binding sites on actin are again blocked, and the muscle relaxes.

Calcium (Ca+2 in green) Actin Myosin Sarcomere