Why can't a cardiac muscle be completely tetanized?

Answer 1

Cardiac muscle doesn't go into tetanic contraction due to the effect of calcium ions which are slowly released into the cell during an action potential, prolonging the action potential. As a result, the graph for electrical activity in cardiac cells has a 'plateau' area because of the release of calcium and the decreased membrane permeability to potassium. By the time the action potential is over, the refractory period (channel inactivation period) for the ions are over, and the cardiac cell is relaxed and ready to be excited again. skeletal action potential = 5-10 msec. cardiac action potential = 250 msec.

Answer 2

Cardiac Muscle and Tetany

In cardiac muscle, the mechanism of contraction is essentially the same as in skeletal, but the excitation-contraction coupling mechanism differs slightly. T-tubules invaginate at the level of the Z-lines. The SR is relatively poorly developed (cisternae are small or absent - in most EM sections 'diads' and not 'triads' are seen), and provides insufficient Ca2+ to fully activate the contractile apparatus. Unlike those in skeletal muscle the ryanodine channels in cardiac muscle are activated by Ca2+ in the cytosol (calcium activated calcium release) and Ca2+ entry through the dihydropyridine channels is an important trigger of Ca2+ release. Significant amounts of Ca2+ enter the fibre from the ECF during the AP, which consequently has a long plateau phase caused by slowly inactivating Ca2+ channels in the sarcolemma, prolonging AP (c. 200 ms). Ca2+ is also released from sub-sarcolemmal binding sites during the AP. Because the AP lasts almost as long as the twitch, heart muscle cannot be tetanized.

Answer 3

Cardiac muscle conducts its own action potentials (in a sense). It has a very long Refractory Period. A refractoy period is when the voltage gated Na channels are blocked in a way that is not related to voltage. This means that the action potential cannot be continued, at least until the gate is unblocked. This long refractory period in cardiac tissue means that there has to be a gap long enough for the tissue to relax before the next contaction occurs. As a result Tetanus of the heart is impossible. What can occur though it that the heart goes into Fibrillation. This is where the conduction of the cardiac tissue is disrupted in some way and each of the cells contracts whenever it pleases, i.e. not under the control of the SA node. Fibrilation means there is no contaction of the heart walls as a structure and so no overall movement of the walls. This appears like tetanus of the heart and no blood is pumped.

Answer 4

Heart muscle can't tetanize because it has a very long refractory period. By the time the refractory period is over, the muscle has already started relaxing.

Answer 5

The refractory period lasts until the muscle is completely relaxed.