Sinus pacemaker cells

Effect of epinephrine on the transmembrane potential of a pacemaker cell in the frog heart. The arrowed trace was recorded after the addition of epinephrine. Note the increased slope of diastolic depolarization and decreased interval between action potentials. This pacemaker acceleration is typical of i-stimulant drugs. (Modified and reproduced, with permission, from Brown H, Giles W, Noble S Membrane currents underlying rhythmic activity in frog sinus venosus. In The Sinus Node Structure, Function, and Clinical Relevance. BonkeFIM [editor], MartinusNijhoff, 1978.)... 

In addition, these cells, as well as cells from the conducting system, normally display the phenomenon of spontaneous diastolic, or phase 4, depolarization and thus spontaneously reach threshold for regeneration of action potentials. The rate of spontaneous firing usually is fastest in sinus node cells, which therefore serve as the natural pacemaker of the heart. Specialized K channels underlie the pacemaker current in the heart. 

A number of ionic currents have been described in single pacemaker cells from the sinus node region (15). In contrast to atrial and ventricular cells. 

Sutiagin, P. V. Pylaev, A. S. (1983). "[Identification of rat sinus node pacemaker cells by intracellular injection of lanthanum ions]." Biull Eksp Biol Med, 95(5), 93-5. 

In the following, the cardiac action potential is explained (Fig. 1) An action potential is initiated by depolarization of the plasma membrane due to the pacemaker current (If) (carried by K+ and Na+, which can be modulated by acetylcholine and by adenosine) modulated by effects of sympathetic innervation and (3-adrenergic activation of Ca2+-influx as well as by acetylcholine- or adenosine-dependent K+-channels [in sinus nodal and atrioventricular nodal cells] or to dqjolarization of the neighbouring cell. Depolarization opens the fast Na+ channel resulting in a fast depolarization (phase 0 ofthe action potential). These channels then inactivate and can only be activated if the membrane is hyperpolarized... 

Tachyarrhythmias (sinus rate more than 100 per minute) are produced by a disturbances of impulse generation or of impulse conduction in the heart. Tachyarrhythmias due to disturbed impulse formation are associated with irregular and rhythmic discharge from ectopic pacemaker activity in areas of the heart other than the SA node. The characteristic of myocardial cells, which enables them to generate spontaneous depolarization, is called automaticity. 

Enhanced automaticity may occur in cells that normally display spontaneous diastolic depolarization— the sinus andAVnodes and the His-Purkinje system. [) Adrenergic stimulation, hypokalemia, and mechanical stretch of cardiac muscle cells increase phase 4 slope and so accelerate pacemaker rate, whereas acetylcholine reduces pacemaker rate both by decreasing phase 4 slope and by hyperpolarization (making the maximum diastolic potential more negative). In addition, automatic behavior may occur in sites that ordinarily lack spontaneous pacemaker activity e.g., depolarization of ventricular cells by ischemia) may produce such abnormal" automaticity. 

---