Question

The T tubules of cardiac muscle cells are wider than those of skeletal muscle cells and are lined by external lamina (a basal lamina-like material). In contrast to skeletal muscle, where are the T tubules located and what do they often form?

1) At the Z disk, where they often form dyads
2) At the M line, where they often form triads
3) At the A band, where they often form dyads
4) At the I band, where they often form triads

Answer 1

The T tubules in cardiac muscle cells are found at the Z discs and often form dyads. This contrasts with skeletal muscle, where T tubules are at the A and I band junction and form triads. The correct answer is option 1) At the Z disk, where they often form dyads.

Step-by-step explanation:

In cardiac muscle cells, or cardiomyocytes, the T tubules are found at the Z discs, as opposed to skeletal muscle cells where T tubules are located at the junction of the A and I bands. In the cardiac muscle, T tubules often form dyads, which involve a pairing structure with the sarcoplasmic reticulum, responsible for calcium storage and release during muscle contraction. This anatomical arrangement contributes to the mechanisms by which cardiomyocytes contract, utilizing calcium ions mostly from outside the cell, leading to a slower onset of muscle contraction compared to skeletal muscle.

The accurate answer to the question is option 1) At the Z disk, where they often form dyads. This is because cardiac muscle cells possess fewer T tubules than skeletal muscle, and the calcium required for contraction is predominantly sourced from the extracellular space, as the sarcoplasmic reticulum in cardiac muscle cells stores fewer calcium ions.