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
Sinoatrial block delayed or blocked on the way to the atria, thus delaying the atrial beat. (An AV block, occurs in the AV node and delays ventricular depolarisation). SA blocks are categorized into three classes based on the length of the delay. Sinoatrial blocks are typically well-tolerated. They are not as serious as an AV block and most often do not require treatment. In some people, they can cause fainting, altered mental status, chest pain, hypoperfusion, and signs of shock. They can also lead to cessation of the SA node and more serious dysrhythmias. In a first degree sinoatrial block, there is