Compared with skeletal muscle, the position of the T tubules and the composition of the associated calcium-release complex differ in ventricular cardiac muscle. Which statement is correct?

• A T tubules lie at the A-I junction and form triads with two terminal cisterns
• B T tubules lie at the Z line and form diads with a single terminal cistern of sarcoplasmic reticulum ✓
• C T tubules are absent and calcium enters only through the sarcolemma
• D T tubules lie at the M line and form pentads with four terminal cisterns

Explanation

Cardiac muscle T tubules are larger than those of skeletal muscle and are positioned at the level of the Z line. Each associates with a single dilated terminal cistern of sarcoplasmic reticulum to form a diad, unlike skeletal muscle where T tubules sit at the A-I junction between two terminal cisterns forming a triad. Calcium influx through L-type channels also triggers further calcium release in cardiac muscle, but T tubules are definitely present.

Reference: Ross and Pawlina Histology: A Text and Atlas, 8th ed.

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