Tamara's muscle weakness and fatigue becomes progressively worse over the course of the day. this is a hallmark sign of myasthenia gravis, an autoimmune disorder that affects the ability of a motor neuron to communicate with a muscle fiber. what synaptic events must happen first for excitation to occur?

a. the action potential travels down the t tubules in the sarcolemma and triggers the release of calcium ions from the sarcoplasmic reticulum.
b. calcium binds to tropomyosin, shifting it from the active site on actin, allowing cross bridge formation to occur as myosin binds to the active site.
c. acetylcholine is released from the axon terminal and diffuses across the synapse to bind to a receptor in the surface of the motor end plate.
d. sodium ions from the extracellular fluid diffuse into the cytosol of the muscle fiber, triggering the generation of an action potential in the sarcolemma.