Current aspects of the basic concepts of the electrophysiology of the sinoatrial node

Cardiac pacemaker cells, also named P-cells (pale cytoplasm, pacemaker, phylogenetically primitive), including cells of the sinoatrial node, are heterogeneous in size, morphology, and electrophysiological characteristics. The exact extent to which these cells differ electrophysiologically in the human heart is unclear, yet it is critical for the understanding of normal cellular function. In this review, we describe major ionic currents and Ca²⁺ clocks acting on Ca²⁺ release in the sarcoplasmic reticulum. We also explain the external regulation of the heart rate controlled by the two branches of the autonomic (involuntary) nervous system: the sympathetic and the parasympathetic nervous system. Vagal stimulus causes bradycardia, rapid and short-duration modulation, and controls rapid responses, and increases heart rate variability. A typical example is constituted by phasic or respiratory sinus arrhythmia, characterized by pronounced vagal activity, more frequent in children and young individuals.