Cardiac muscle is the specialized striated muscle of the heart. Its contractile cells are mechanically connected and electrically coupled at intercalated discs. Mechanical connections transmit force, while gap junctions help electrical excitation spread between cells.[ref]
Excitation and contraction are related but distinct stages. Changes in membrane potential regulate calcium handling, and calcium enables the actin-myosin machinery to generate force. A sinoatrial node normally sets the pace for the coordinated sequence, rather than an independent motor neuron commanding each beat.
This separation explains why an electrical recording and a measurement of pumping answer different questions. An electrocardiogram reports summed electrical activity; contraction also depends on cellular mechanics, filling and the loads against which the heart pumps.

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