Figure 10-1 Skeletal Muscle Fiber

Use Figure 10-1 to answer the following questions:
-What physiological process occurs in the structure labeled "7"?

A) release of neurotransmitter
B) conduction of the action potential into the cell interior
C) activity of acetylcholinesterase
D) release of protein and calcium ions into the muscle fiber
E) opening of sodium channels and subsequent influx of sodium