Triad (physiology)

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Triads occur in skeletal muscle cells and are used for the rapid transmission of motor stimuli, which lead to the active shortening of the muscle fibers . So that the nerve stimulus for muscle contraction can be transferred into the interior of the muscle cell as quickly as possible, the cell membrane is turned inside out at various points. These invaginations are called transverse tubules (T-tubules).

In order for a muscle cell to contract , calcium ions must be released inside. These are stored in the sarcoplasmic reticulum . The sarcoplasmic reticulum is a specialized endoplasmic reticulum (ER) of muscle cells. One speaks of a triad if two sac-like chambers of the sarcoplasmic reticulum (terminal cisterns) are located near a transverse tubule (1 tubule + 2 cisterns = 3 = 1 triad). If only a terminal cistern borders the tubule, it is called a dyad .

Physiological process

If the electrical stimulus of a (motor) nerve reaches the transverse tubule, its voltage-dependent L-type calcium channels (also called dihydropyridine receptors) are opened. Calcium ions flow into the interior of the cell (sarcoplasm = cytoplasm), diffuse to the nearby terminal cisterns of the sarcoplasmic reticulum and activate its calcium channels (ryanodine receptors) . The calcium ions stored in high concentrations in the sarcoplasmic reticulum also flow out into the sarcoplasm and now cause the muscles to contract .

Due to the short diffusion distances between the tubule and the sarcoplasmic reticulum (triad), the transmission of stimuli occurs very quickly.

literature

  • Rainer Klinke, Hans-Christian Pape, Stefan Silbernagl (Hrsg.): Textbook of Physiology. 5th edition. Thieme, Stuttgart 2005, ISBN 3-13-796003-7