 Here students, in this topic we shall discuss what are triads and the membrane receptors in these triads. Here students, in a skeletal muscle fiber, a T-tubule is associated with two terminal cisternae of sarcoplasmic reticulum on its both sides. This arrangement, in which three tubes or sacs are closely associated with each other, this forms a structure which is called a triad. These triads are located at the junction between A and I bands of sarcoma, where A and I bands are joining, and these triads are found at the same place. There are thousands of triads like this near the partial skeletal muscle fiber, which are visible when we cut the longitudinal section of the muscle. Dear students, these triads form the anatomical basis of excitation-contraction coupling. Due to which the A stimulus excites the muscle and causes its contraction. Triads affect the close association of T-tubules and sarcoplasmic reticulum because of which the action potential of T-tubules affects the sarcoplasmic reticulum and releases calcium ions. Dear students, there are specialized receptors in this triad region, which are involved in the release of calcium ions. These specialized channel proteins in the membrane of sarcoplasmic reticulum are known as ryanodine receptors. When these ryanodine receptors open, calcium ions are released from the sarcoplasmic reticulum. Now we shall discuss the mechanism of opening of ryanodine receptors. When ryanodine receptors open, when action potential comes in T-tubules, it affects the ryanodine receptors. The mechanism for this opening is that there is a receptor in the membrane of T-tubules called dihydropyridine receptors. These receptors are actually a cluster of proteins and this cluster is a voltage-sensitive cluster of proteins and these receptors are activated when action potential is present in T-tubules. When action potential is in T-tubules, dihydropyridine receptors are activated which means that they change their conformation. In this activated state, these dihydropyridine receptors physically interact or mechanically interact with the ryanodine receptors present in the membrane of the terminal system of sarcoplasmic reticulum. As a result, they also change the conformation of these ryanodine receptors. Here we can see that in a triad, both the membrane which is closely associated with each other has the ryanodine receptors of the terminal system. Whereas in T-tubules, dihydropyridine receptors are present. Due to their close proximity, when dihydropyridine receptors are activated, they mechanically stimulate the ryanodine receptors which are activated then they are open as a result of this. Due to their opening, sarcoplasmic reticulum starts releasing calcium ions. The release of calcium ions causes the contraction to happen.