 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 eticulum 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 on the same place, these triads are found. There are thousands of triads like this in the part skeletal muscle fiber, which are visible in 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 eticulums because of which the action potential of T-tubules affects the sarcoplasmic eticulum 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 eticulum are known as ryanodine receptors. When these ryanodine receptors open, calcium ions are released from the sarcoplasmic eticulum. Now we shall discuss the mechanism of opening of ryanodine receptors. 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 at that time when action potential is present in T-tubules. When action potential is present in T-tubules, then dihydropyridine receptors are activated which means they change their confirmation. In this activated state, these dihydropyridine receptors physically interact or mechanically interact with the ryanodine receptors present in the membrane of terminal cisternae of sarcoplasmic eticulum. As a result, they also change the confirmation of these ryanodine receptors. Here we can see that in a triad, both the membranes which are closely associated with each other have the ryanodine receptors in the membrane of terminal cisternae. Whereas in the membrane of T-tubules, there are dihydropyridine receptors. Due to their close proximity, when dihydropyridine receptors are activated, they mechanically stimulate the ryanodine receptors which are activated, then as a result, they become open. Due to their opening, sarcoplasmic eticulum starts releasing calcium ions. The release of calcium ions causes the contraction to happen.