Muscle fiber

Muscle fiber Motor neuron

Nucleus

Sarcolemma MyofibrilsSarcoplasmic reticulum (SR)

T-tubule

SR Cistern

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)

Actin(thin filaments)

Myosin(thick filaments)

Sarcomere (contracted)

Actin(thin filaments)

Myosin(thick filaments)

Actin(thin filament)

Myosin(thick filaments)

Myosinheads

Actin Myosin head

ADP and phosphate

Protein complex

Note that ATP is hydrolyzed at this point, but still attached to myosin head.

Calcium ion

Myosin-binding sites (exposed)

Protein complex

ADP and phosphate

Actin Myosin head

ADP and phosphate

Actin Myosin head

Pi

ADP

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

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 Myosin headsProtein complex

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.

ActinMyosinCalcium

Sarcomere

(Ca+2 in green)