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+?
A) bound to troponin
B) within the sarcoplasmic reticulum of the muscle fiber
C) diffusing into the motor neuron through specialized channels in the plasma membrane
D) binding to the receptors at the motor endplate of the muscle cell
E) within the T-tubules