Q.What is triad system?

The triad system links the muscle fibre's surface excitation to the release of calcium ions from the sarcoplasmic reticulum, coupling nerve stimulation to contraction.

A skeletal muscle fibre's sarcolemma (plasma membrane) forms tubular invaginations called transverse tubules (T-tubules) that run deep into the fibre, close to the junction of the A and I bands of each myofibril.

Running parallel to the myofibrils, and surrounding them, is the sarcoplasmic reticulum (SR), a modified endoplasmic reticulum that stores Ca2+ ions. The SR is enlarged at intervals into sac-like terminal cisternae.

At each such interval, one T-tubule is flanked on both sides by a terminal cisterna of the SR - this three-part structure (cisterna-tubule-cisterna) is called a triad.

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