What causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum within a muscle cell?

Answer: Muscle contractions stimulate the release of calcium

from sarcoplasmic reticulum.

Explanation:

During muscle contractions, calcium is released from its store into the sarcoplasma, where it goes and bind to troponin protein complex. This causes the troponin to change its shape and remove the tropomyosin from the myosin-binding site for exposure. Myosin then binds to actin active site and form a cross-bridge or power stroke due the presence of ATP.

Calcium functions in regulating muscle contractions and relaxations in response to stimulus.