triad

in muscle

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July 14, 2026

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In muscle cells, a triad is a crucial structure formed by the junction of the transverse tubule (T-tubule) and two terminal cisternae of the sarcoplasmic reticulum. The sarcoplasmic reticulum is a specialized endoplasmic reticulum that stores and releases calcium ions, which are essential for muscle contraction.123

When a nerve impulse triggers an action potential, it travels down the T-tubule. This electrical signal causes the terminal cisternae to rapidly release calcium ions into the sarcoplasm (the muscle cell's cytoplasm). This influx of calcium ions initiates the interaction between actin and myosin filaments, leading to muscle contraction. The sarcoplasmic reticulum then actively pumps calcium ions back into its cisternae, allowing the muscle to relax.134

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muscle
The myofibril
…a three-element complex called a triad. The number of triads per sarcomere depends on the species; for example, in frog muscle there is one per triad, and in mammalian muscle there are two. In fishes and crustaceans, only one cisterna is associated with each transverse tubule, thus forming a dyad.…
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…a three-element complex called a triad. The number of triads per sarcomere depends on the species; for example, in frog muscle there is one per triad, and in mammalian muscle there are two. In fishes and crustaceans, only one cisterna is associated with each transverse tubule, thus forming a dyad.…
Read More