The primary role of cardiac muscles is to propel blood throughout the cardiovascular system. The cardiac muscle cells, or cardiomyocytes, exhibit specialized characteristics that allow them to perform this function.
Cardiac muscle cells are smaller than skeletal muscles, averaging 10–20 mm in diameter and 50–100 mm in length. However, they have large energy demands for continuous contraction and relaxation. This energy is almost exclusively derived from aerobic metabolism of energy reserves in the form of glycogen and lipid inclusions. Additionally, the sarcoplasm of these cells contains large numbers of mitochondria and abundant reserves of myoglobin, which store the oxygen needed to break down energy reserves during peak activity.
In contrast to skeletal muscles, cardiomyocytes can contract independently without relying solely on nerve stimulation. The pacemaker cells, a distinct type of cardiomyocyte, can generate action potentials and initiate contractions in cardiac muscles approximately 75 times per minute. This depolarization wave swiftly spreads through the muscle cells through gap junctions, prompting all the cells to contract and relax together as one unit, known as the functional syncytium.
Cardiomyocytes also differ from skeletal muscles in that the depolarization wave triggers the gradual release of calcium ions into the sarcoplasm of the contractile cells from both the sarcoplasmic reticulum and the interstitial fluid. This results in the contraction cycle of cardiac muscles that lasts for about 200 ms, which is longer than the skeletal muscle contractions that last only 40 to 120 ms. The extended cycle also increases the absolute refractory period of cardiac muscles, which coincides with the repolarization phase of the action potential. This feature enables the muscle to fully relax before another action potential is generated, preventing sustained or tetanic contractions.
