You measure levels of Ca2+ in various locations within a motor neuron and a skeletal muscle fiber when the motor neuron is NOT depolarized, and the muscle fiber is at rest. Where do you expect to find high levels of Ca2+?
1. diffusing into the axon terminus of a motor neuron through specialized channels in the plasma membrane
2. bound to troponin
3. within the sarcoplasmic reticulum
4. in vesicles within the motor neuron
5. binding to the receptors at the motor endplate of the muscle cell
6. within the T-tubules
7. bound to myosin
8. bound to tropomyosi

Muscular contraction is a highly regulated process that depends on free calcium concentration in the cytoplasm. Amounts of cytoplasmic calcium are regulated by sarcoplasmic reticulum that functions as a storage of the ion.

When a nerve impulse reaches the membrane of a muscle fiber, through acetylcholine release, the membrane depolarizes producing the entrance of calcium from extracellular space. The impulse is transmitted along the membrane to the sarcoplasmic reticulum, from where calcium is released. At this point, tropomyosin is obstructing binding sites for myosin on the thin filament. The calcium channel in the sarcoplasmic reticulum controls the ion release, that activates and regulates muscle contraction, by increasing its cytoplasmic levels. When calcium binds to the troponin C, the troponin T alters the tropomyosin by moving it and then unblocks the binding sites, making possible the formation of cross-bridges between actin and myosin filaments. When myosin binds to the uncovered actin-binding sites, ATP is transformed into ADP and inorganic phosphate.

Z-bands are then pulled toward each other, thus shortening the sarcomere and the I-band, and producing muscle fiber contraction.