Sinoatrial node atrial cells, with the average cell being around 8 micrometers in diameter and 20-30 micrometers in length (1 micrometer= 0.000001 meter). Unlike the atrial cells, SA node cells contain fewer mitochondria (the power plant of the cell), fewer myofibers (the contractile machinery of the cell), and a smaller sarcoplasmic reticulum (a calcium storage organelle that releases calcium for contraction). This means that the SA node cells are less equipped to contract compared to the atrial and ventricular cells. Action potentials pass from one cardiac cell to the next through pores known as gap junctions. These gap junctions are made of
Artificial cardiac pacemaker which is the mode of choice when atrioventricular conduction is intact but the natural pacemaker the sinoatrial node is unreliable – sinus node disease (SND) or sick sinus syndrome. Where the problem is atrioventricular block (AVB) the pacemaker is required to detect (sense) the atrial beat and after a normal delay (0.1–0.2 seconds) trigger a ventricular beat, unless it has already happened – this is VDD mode and can be achieved with a single pacing lead with electrodes in the right atrium (to sense) and ventricle (to sense and pace). These modes AAIR and VDD are unusual in the US