Final answer:
The t-tubules are not part of the neuromuscular junction.
Step-by-step explanation:
The correct answer is a. t-tubules. T-tubules, also known as transverse tubules, are invaginations of the sarcolemma that help transmit the electrical impulses deeper into the muscle fibers. They play a crucial role in excitation-contraction coupling, allowing the action potential to reach the sarcoplasmic reticulum and trigger the release of calcium ions.