ACh → Ca²⁺ release

What does cytoplasmic Ca²⁺ bind to?

Ca²⁺ binds to troponin C on the thin actin filament.

What happens when Ca²⁺ binds troponin?

Troponin changes shape and moves tropomyosin away from the myosin-binding sites on actin.

What does tropomyosin do when the muscle is relaxed?

It covers the myosin-binding sites on actin and prevents cross-bridge formation.

What happens when tropomyosin moves away from actin?

The myosin-binding sites on actin are exposed.

What is a cross-bridge?

A cross-bridge forms when an energized myosin head binds to actin.

What triggers the power stroke?

Release of inorganic phosphate, Pᵢ, from the myosin head triggers the power stroke.

What happens during the power stroke?

The myosin head pivots and pulls actin toward the M line at the center of the sarcomere.

What happens to the sarcomere when actin is pulled toward the M line?

The Z-discs move closer together, and the sarcomere shortens.

Do actin and myosin filaments themselves shorten?

No. They slide past one another; this is the sliding-filament theory.

What happens after the power stroke?

ADP is released from myosin, but myosin remains attached to actin.

What causes myosin to detach from actin?

A new ATP molecule binds to myosin.

What happens when myosin hydrolyzes ATP?

ATP becomes ADP + Pᵢ, and the released energy repositions the myosin head into its high-energy position.

When does cross-bridge cycling continue?

As long as Ca²⁺ remains bound to troponin and ATP is available.

What pump returns Ca²⁺ to the sarcoplasmic reticulum during relaxation?

SERCA, the sarco/endoplasmic reticulum Ca²⁺ ATPase

How does returning Ca²⁺ to the SR relax the muscle?

Ca²⁺ detaches from troponin, tropomyosin covers actin’s binding sites again, cross-bridge cycling stops, and the muscle relaxes.