= Cardiac muscle
<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>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 4.4, Cardiac Muscle; section 19.2, cardiac-cell connections and electrical activity]{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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