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 node slowly). These channels allow a flow of calcium ions (Ca) into the cell, making the membrane potential more positive. Phase 0 This is the depolarisation phase. When the membrane potential reaches a threshold value (around -20 to -50 mV), it begins to rapidly depolarise (become more positive). This is mainly due to the flow of Ca through the L-type calcium channels, which are now fully open. During this stage the T-type calcium channels and HCN channels deactivate. Phase 3 This phase is the repolarisation phase. This occurs due to the inactivation of the L-type calcium channels (preventing the movement of