A researcher is comparing the ultrastructure of skeletal and cardiac muscle. In skeletal muscle, the T-tubules are located at the junction of the A and I bands. Where are the T-tubules located in cardiac muscle?

• A A-I junction
• B H-zone
• C M-line
• D Z-line ✓

Explanation

In skeletal muscle, T-tubules are located at the A-I junction, while in cardiac muscle, they are located at the Z-line. This difference affects the excitation-contraction coupling timing and calcium release kinetics between the two muscle types. The M-line and H-zone are components of the sarcomere but do not contain T-tubules.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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