The clear fibers in the figure above are myosin ATPase free and are all innervated by one ventral horn cell.  The striped fibers are the ATPase rich and would look similar under a microscope.  However, we want to illustrate that the ATPase rich fibers are innervated by two different ventral horn cells (a and b; hence the different orientations of the stripes)

Neuromuscular junction 

A muscle fiber is activated via a nerve impulse generated by an anterior horn cell. The impulse is conducted along the nerve fiber via saltatory conduction; that is an action potential is generated at one node of Ranvier and then jumps to the next node of Ranvier where another action potential is generated.  Once the impulse reaches the neuromuscular junction, voltage sensitive Ca2+ channels are opened which allow for the influx of Ca2+ into the nerve terminal.  Ca2+ entry into the nerve terminal initiates the fusion of acetylcholine containing vesicles with the presynaptic membrane and the subsequent release of acetylcholine into the synaptic cleft.  Acetylcholine binds to post-synaptic acetylcholine receptors on the muscle membrane.  This induces an end plate potential which subsequently results in the generation of an action potential in the muscle fiber membrane (figure 5).  The end result of this reaction is muscle fiber contraction.