Heart muscles cannot be tetanised why

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Cardiac muscle cannot be tetanised because of its very long refractory period, which is tied directly to the long duration of its action potential (the plateau).

The mechanism

A skeletal muscle fiber has a brief action potential (a few milliseconds) and a very short refractory period. If stimuli arrive fast enough, a new contraction can be triggered before the previous one relaxes, causing summation and eventually a smooth, sustained contraction (tetanus).
Cardiac muscle fibers, in contrast, have a long action potential with a plateau phase (phase 2), caused by sustained calcium influx through voltage-gated Ca2+ channels balanced against reduced K+ efflux. This plateau keeps the membrane depolarized for roughly 200-300 ms.
During phases 0 through 2, and about half of phase 3 (until the membrane repolarizes to about -50 mV), the cardiac cell is in its absolute refractory period - no stimulus, however strong, can generate a new action potential. This is followed by a short relative refractory period until phase 4, during which only a very strong stimulus could re-excite the tissue.
Because the refractory period (about 0.25-0.30 sec in the ventricle) lasts almost as long as the mechanical contraction itself, a second action potential (and thus a second contraction) cannot be initiated until the muscle has already begun to relax. This prevents wave summation, so premature stimuli produce isolated extra beats rather than a fused, tetanic contraction.
Guyton and Hall Textbook of Medical Physiology, "Refractory Period of Cardiac Muscle"
Ganong's Review of Medical Physiology, 26th Ed., "Resting Membrane & Action Potentials"

Why this matters physiologically

This is not incidental - it is essential for survival. The heart must relax between beats to allow ventricular filling and to pump blood rhythmically. As Ganong's puts it, "tetanization of cardiac muscle for any length of time would have lethal consequences," since a heart locked in sustained (tetanic) contraction could not fill with blood or eject it, and circulation would stop.

Heart muscles cannot be tetanised why reasoning questions in easy words for 2 mark

Cardiac muscle cannot be tetanised because it has a very long refractory period, almost as long as the contraction itself.

Reason (2 marks):

Cardiac muscle has a long action potential with a plateau phase, caused by slow calcium influx into the cell. This keeps the muscle in its absolute refractory period for about 250-300 milliseconds - almost the whole duration of contraction.
Because of this long refractory period, a new stimulus cannot excite the heart muscle again until it has already relaxed. So the contractions never add up (summate) into one continuous contraction, unlike skeletal muscle.

This is also important for the body - if the heart got tetanised (locked in continuous contraction), it could never relax to fill with blood, and it would stop pumping, which would be fatal.

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