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Heart rate (HR) is determined by the rate of depolarization of the cardiac pacemaker which is found in the sinoatrial node, the atrioventricular node, and the Purkinje tissue. The intrinsic HR in absence of any neurohumoral influence is about 100 to 120 bpm. In the intact individual, HR at any time represents the net effect of the parasympathetic nerves which slow it and the sympathetic nerves which accelerate it. In most physiological conditions, sympathetic and parasympathetic (vagal) activities modulating HR undergo a reciprocal regulation, leading to the classic notion of sympathovagal balance [ABSTRACT FROM AUTHOR]