excitation–contraction coupling

Excitation–contraction coupling is the bridge between the heart's electrical command and its mechanical action — the chain of events that turns an electrical spark into an actual squeeze. The conduction system can deliver a perfect impulse, but it means nothing unless each cell translates that impulse into force. This is the translation step.

The key messenger is calcium. When a cell depolarizes, a small amount of calcium enters through channels in the membrane during the action potential's plateau. That trickle acts as a trigger, prompting the cell's internal calcium store (the sarcoplasmic reticulum) to release a much larger flood of calcium — a process called calcium-induced calcium release. The released calcium binds to the contractile proteins, allowing them to ratchet past one another and shorten the cell. When calcium is then pumped back into storage and out of the cell, the proteins release and the cell relaxes.

This calcium-handling machinery is also a dial for the strength of contraction. The more calcium released per beat, the stronger the squeeze — which is how sympathetic stimulation and drugs that boost calcium availability increase contractility. Conversely, when calcium handling is impaired, as in many forms of heart failure, the heart's force generation falters even when its electrical activity looks normal.