The sinus node is a collection of fast depolarizing cells, of all the pacemaking cells in the heart these have the highest frequency. It is located in the wall of the right atrium, near the entrance of the superior vena cava. The sinus node is the primary pacemaker of the heart, it is responsible for the rest of the heart's electrical activity. In the absence of extrinsic neural and hormonal control, cells in the sinoatrial node will naturally discharge at about 60-100 beats/minute. The parasympathetic and sympathic nerveous system fibers innervating the sinus node can adjust the frequency according to what is necessary. If the influence of the parasympathetic vagus exceeds the normal, one speaks of a so-called bradycardia (about 40-50 bpm). If the influence of the sympathetic nervous system exceeds the normal we call it a sinus tachycardia (100-150 bpm). The frequency of the sinus node is also controllable by natural substances such as adrenaline or administered drugs.
