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Cardiac excitation-contraction coupling describes the series of events, from the production of an electrical impulse to the contraction of muscles in the heart. This process is of vital importance as it allows for the heart to beat in a controlled manner, without the need for conscious input. EC coupling results in the sequential contraction of the heart muscles that allows blood to be pumped, first to the lungs and then around the rest of the body at a rate between 60 and 100 beats every minute, when the body is at rest. This rate can be altered, however, by nerves that work to either increase heart rate or decrease it , as the body's oxygen demands change. Ultimately, muscle contraction revolves around a charged atom , calcium , which is responsible for converting the electrical energy of the action potential into mechanical energy of the muscle. This is achieved in a region of the muscle cell, called the transverse-tubule during a process known as calcium induced calcium release.