What is the primary reason cardiac muscle has prolonged contraction?

The primary reason cardiac muscle has prolonged contraction is due to sodium-induced calcium release. In cardiac muscle cells, when an action potential is initiated, there is a rapid influx of sodium ions, which depolarizes the cell membrane. This triggers not just the quick contraction that occurs from this initial depolarization, but also leads to a significant release of calcium ions from the sarcoplasmic reticulum through a process known as calcium-induced calcium release.

The calcium released from the sarcoplasmic reticulum contributes to prolonged contraction by binding to troponin, allowing for the interaction between actin and myosin filaments in the muscle fibers, thus sustaining the contraction. Unlike skeletal muscle, where contractions can be very rapid and short-lived, the interplay of this sodium-induced calcium release system allows cardiac muscle to maintain its contraction for a longer duration, necessary for the heart to effectively pump blood throughout the body.

This mechanism is vital to the heart's function as it allows enough time for blood to be ejected from the chambers before the muscle relaxes, supporting the rhythmic and sustained contractions needed for circulation.