心筋の特殊な特性

The primary function of cardiac muscles is to pump blood through the cardiovascular system.

Unlike skeletal muscles, cardiomyocytes are autorhythmic, which means that they are not entirely dependent on neural stimulation for contraction.

Special cardiomyocytes called pacemaker cells can auto–generate action potentials and trigger cardiac muscle contractions about 75 times per minute.

This depolarization quickly travels through the contractile muscle cells via the gap junctions. So, the cells contract and relax as a single operational unit called the functional syncytium.

The depolarization wave also triggers the slow release of Ca²⁺ ions into the sarcoplasm of the contractile cells from both the sarcoplasmic reticulum and the interstitial fluid.

This results in a long contraction cycle that typically lasts about 200 ms compared to skeletal muscle contractions that last only 40 to 120 ms.

The long contraction cycle also increases the absolute refractory period of cardiac muscles, coinciding with the repolarization phase of the action potential.

This allows the muscle to fully relax before another action potential is generated, preventing tetanic or sustained contractions.