t-tubes (t-tubules)

t-tubes (t-tubules)

in full transverse tubules. Tubules continuous with the surface membrane of a striated muscle fibre, which contain extracellular fluid yet penetrate in a network pattern the whole cellular cross-section, encircling every myofibril. The tubules are separated from the sarcoplasmic reticulum (SR) only by closely adjoining membranes, and are the route of excitation as an action potential spreads inward from the surface of the fibre to instigate Ca2+ release from the SR.