Question

What is a triad system?

Answer 1

Hint: It is present in the muscle fibres. It is a structure formed by a T tubule with a sarcoplasmic reticulum. It plays an important role in muscle contraction.

Complete answer: The triad system is the characteristic of muscle cells. It is a part of the skeletal muscle. The skeletal muscle consists of thousands of triads. It is visible when the longitudinal section of muscle fibre is taken. The muscle cells consist of many transverse tubules present on the sarcolemma. The plasma membrane of the muscle cell is called sarcolemma. The muscle cell also consists of endoplasmic reticulum which is known as sarcoplasmic reticulum. Sarcoplasmic reticulum has calcium ion pumps, which continuously pumps calcium ions from the sarcoplasm into the sarcoplasmic reticulum. The wall of sarcoplasmic reticulum also consists of calcium ion channels from which calcium ions leak out into the sarcoplasm. Inside the sarcoplasmic reticulum, calcium ions are held by a protein calsequestrin. This protein has a high binding capacity towards the calcium ions. In muscle, there are thousands of small invaginations of the sarcolemma, called T-tubules or transverse tubules. T tubules are filled with interstitial fluid as their opening is present outside. The muscle action potential from neuromuscular junctions travels along the sarcolemma and through transverse tubules (T-tubules) and rapidly spread throughout the muscle fibre. Due to this arrangement, it can ensure that an action potential spreads evenly and excites all parts of the muscle fibre at the same instant. The end sac of sarcoplasmic reticulum is dilated and is called terminal cisterns. These act as reservoirs. This terminal cistern hits against the T-tubule from both sides. Thus, two-terminal cisterns on either side of the T tubule form a triad.

Note: Due to the presence of transverse tubules, the surface area of a muscle cell or sarcolemma increases. A single muscle cell consists of many sarcomeres. The sarcomere is a structural and functional unit of the muscle cell.