Cardiac muscles differ fundamentally from skeletal muscles in their electrical and contractile properties. One important difference is that cardiac muscles cannot undergo tetany — a sustained contraction caused by rapid repeated stimuli — which is essential for the heart’s effective pumping function.

Reason Cardiac Muscles Do Not Undergo Tetany

The primary reason cardiac muscle cannot enter tetany is because of its long refractory period and auto-rhythmicity:

• The refractory period in cardiac muscle cells — the time during which the muscle cannot be re-excited — is almost as long as the contraction itself. This prevents the summation of contractions and sustained tetanic contractions.

• Auto-rhythmicity allows cardiac muscle cells to generate their own impulses rhythmically without external stimulation. This intrinsic pacemaker activity coordinates contraction and relaxation cycles.

Explanation of the Options

• (1) Resistant to tetanus toxin: Incorrect. The toxin affects skeletal muscle nerve terminals; it does not directly cause cardiac muscle’s inability to tetanize.

• (2) Auto-rhythmicity: Correct. Cardiac muscle’s intrinsic pacemaker nature prevents tetany.

• (3) Do not need calcium ions: Incorrect. Cardiac muscles require calcium ions for contraction.

• (4) Highly evolved muscles: Vague and not a scientific explanation.

Conclusion

Hence, cardiac muscles cannot undergo tetany primarily because of:

(2) Auto-rhythmicity