Q.What is triad system?

A triad = one T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum; it lets a nerve impulse trigger calcium release throughout the muscle fibre almost instantly.

Each skeletal muscle fibre contains numerous parallel contractile units called myofibrils, which run through the sarcoplasm. The sarcoplasm also contains an extensive membranous network called the sarcoplasmic reticulum (SR), which encircles each myofibril and acts as a store of Ca²⁺ ions.

At the junction of the A-band and I-band of each sarcomere, the sarcoplasmic reticulum forms enlarged sac-like chambers known as terminal cisternae. Running between two such terminal cisternae (belonging to adjacent segments of the SR) is a tubular inward extension of the plasma membrane (sarcolemma) called a transverse tubule or T-tubule.

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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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A triad is the set of three membranous elements in a muscle fibre - one T-tubule between two terminal cisternae of the sarcoplasmic reticulum - that couples the surface impulse to Ca2+ release for contraction.

What it is: In a skeletal muscle fibre, the plasma membrane (sarcolemma) dips inward as fine tubes called transverse tubules (T-tubules). The endoplasmic reticulum of the fibre, called the sarcoplasmic reticulum (SR), stores calcium ions and forms swollen sacs called terminal cisternae.

The triad: At the junction of the A-band and I-band, each T-tubule is closely bordered on both sides by two terminal cisternae of the SR. This grouping of two cisternae + one T-tubule = a triad.

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The triad system is the structural unit (T-tubule + two terminal cisternae) that couples membrane excitation to Ca2+-triggered muscle contraction.

In a skeletal muscle fibre, the sarcolemma (cell membrane) sends tubular invaginations, called transverse (T) tubules, deep into the fibre at regular intervals. The sarcoplasmic reticulum (SR), the muscle's internal Ca2+ store, surrounds each myofibril and expands into sac-like terminal cisternae on either side of a T-tubule.

The combination of one T-tubule sandwiched between two terminal cisternae of the SR is called a triad.

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