Sino-Atrial Action Potential
The heart depends upon organised electrical activity to contract its various chambers in a coordinated fashion and produce blood flow throughout the body. The electrical activity of the heart originates in a cluster of cells in the wall of the right atrium known as the the Sino-Atrial Node (SA node). The SA Node is the hearts main pace- maker, delivering regular and organised action potentials within the heart, which coordinates cardiac contraction.
Phases of the Sino-Atrial Action Potential
Phase 4: Hyper-polarisation following a previous action potential initiates the funny current (If) via the opening of Hyperpolarisation-activated Cyclic Nucleotide-gated Na+ (HCN) channels. These channels allow a slow influx of Na+ causing a gradual depolarisation to the threshold potential of -40mV.
Phase 0: At -40mV L-type Ca2+ channels open causing a rapid influx of Ca2+, causing rapid depolarisation. The membrane potential spikes at 20mV at which point L-type Ca2+ channels close.
Phase 3: At 20mV voltage gated K+ channels open, allowing movement of K+ ions out, causing gradual repolarisation of the membrane potential and closure of voltage gated K+ channels. Hyperpolarisation occurs at -60mV again initiating If and causing the cycle to restart.
You may have noticed the seemingly illogical numerical order of the above phases. The phases of the SA Node action potential are numbered to maintain congruence with the formerly described cardiac myocyte action potential.
Interruptions of SA Node Pacemaking
Various pathologies can cause the heart’s normal electrical activity to become disorganised (ie ventricular fibrillation). These include congenital defects, electrolyte disturbances, drug intoxication and, of course, acute myocardial infarction. Each of these can result in a decrease or cessation of blood flow.
In the case of ventricular fibrillation and other arrhythmias, the SA Node may still be actively trying to coordinate cardiac contractions, but its signal is drowned out by the disorganised electrical activity of other myocytes.
Automated External Defibrillators (AEDs) are devices designed to interrupt disorganised electrical activity in a person’s heart, allowing the conduction of the SA Node to coordinate cardiac electrical activity once again.