What is a triad in muscle fibers composed of?

A triad in muscle fibers is specifically composed of one T tubule and two terminal cisternae of the sarcoplasmic reticulum. The T tubules are invaginations of the sarcolemma that help transmit the action potential deep into the muscle fiber, while the terminal cisternae are enlarged areas of the sarcoplasmic reticulum that store calcium ions, which are crucial for muscle contraction.

This configuration is essential for the process of excitation-contraction coupling. When an action potential travels along the T tubule, it triggers the release of calcium from the terminal cisternae into the cytosol of the muscle cell, ultimately leading to muscle contraction. The arrangement of these components in a triad allows for efficient communication and rapid response in muscle contraction.