Final answer:
The tubular extensions of the sarcolemma that contact the terminal cisternae of the sarcoplasmic reticulum are known as T-tubules, and they are fundamental in triggering muscle fiber contractions.
Step-by-step explanation:
The tubular extensions of sarcolemma that extend deep into the sarcoplasm and contact the terminal cisternae of the sarcoplasmic reticulum are T-tubules. These T-tubules, or transverse tubules, play a critical role in muscle fiber contractions. They enable the propagation of an action potential deep into the muscle fiber, ensuring that the signal reaches the sarcoplasmic reticulum which releases calcium ions, initiating muscle contraction. The T-tubules are found at the junctions of the A bands and I bands in skeletal muscle. The I bands and A bands are the alternating light and dark stripes seen in striated muscle fibers and are attributed to the precise arrangement of actin (thin filaments) and myosin (thick filaments), creating the striated appearance of muscle fibers. The T-tubules are essential for coordinating this process and ensuring effective muscle contractions.