Question

the activity that directly triggers the sarcoplasmic reticulum to discharge calcium is when:
a sodium rushes into the cell
b potassium rushes into the cell
c an action potential propagates across the sarcoplasm and down the t tubules
d acetylcholine is released
e potassium leaves the cell

Explanation:

When an action potential propagates across the sarcoplasm and down the T - tubules, it directly triggers the sarcoplasmic reticulum to discharge calcium. Sodium rushing into the cell is related to depolarization of the cell membrane but not the direct trigger for sarcoplasmic reticulum calcium release. Potassium rushing into or leaving the cell is involved in repolarization and other membrane potential changes but not the direct trigger for this calcium release. Acetylcholine release is the initial step in muscle contraction (binding to receptors on the motor end - plate) but not the direct trigger for sarcoplasmic reticulum calcium release.

Answer:

C. an action potential propagates across the sarcoplasm and down the T tubules