GLOSSARY

Calcium release is the outflow of calcium ions (Ca²⁺) from the sarcoplasmic reticulum into the sarcoplasm and is the key trigger for muscle contraction.

Mechanism

• Trigger: the action potential reaches the T-tubules and activates the DHPR.
• Release channel: the ryanodine receptor (RyR1) in the terminal cisternae opens and calcium flows out down its concentration gradient.
• Rise in concentration: sarcoplasmic calcium rises from about 0.1 micromolar at rest to about 10 micromolar.

Role of Calcium in Contraction

Calcium binds troponin C, moves tropomyosin away from actin’s binding sites, and allows cross-bridge formation. As long as calcium remains high, cross-bridge cycling continues.