Myocardium electric current

Communication between neurons involves neurotransmitters. Up until the beginning of the last century, synaptic transmission was regarded as probably electrical. It was suggested that the close apposition of two neurons allowed the current to jump the synaptic cleft, rather like an electrical spark between two closely positioned wires. There is indeed evidence for electrical synapses in animal species where the synaptic cleft is particularly narrow (2 nm, or nanometres), as well as in the myocardium where the close coupling of cells allows electrical current to flow from one cell to the next,... 

Defibrillation—The therapeutic use of electric current in an attempt to completely depolarize the myocardium and provide an opportunity for the natural pacemaker centers of the heart to resume normal activity. 

Calculating the electrical axis is often considered one of the more complex elements of ECG interpretation. The electrical axis refers to the net or overall direction in which the electrical current travels in during ventricular depolarisation. This is achieved by adding together the sum of all of the individual vectors to determine the overall direction of the flow across the whole myocardium. Calculating the electrical axis is useful as it can provide additional evidence for the presence of various conditions. 

An electrical current is passed through the myocardium to spontaneously depolarise the heart, allowing the natural pacemaker to take over control. There are three main types of external defibrillator ... 

In the myocardium, automaticity is the ability of the cardiac muscle to depolarize spontaneously (i.e., without external electrical stimulation from the autonomic nervous system). This spontaneous depolarization is due to the plasma membrane within the heart that has reduced permeability to potassium (K+) but still allows passive transfer of calcium ions, allowing a net charge to build. Automaticity is most often demonstrated in the sinoatrial (SA) node, the so-called pacemaker cells. Abnormalities in automaticity result in rhythm changes. The mechanism of automaticity involves the pacemaker channels of the HCN (Hyperpolarization-activated, Cyclic Nucleotide-gated) family14 (e.g., If, "funny" current). These poorly selective cation channels conduct more current as the membrane potential becomes more negative, or hyperpolarized. They conduct both potassium and sodium ions. The activity of these channels in the SA node cells causes the membrane potential to slowly become more positive (depolarized) until, eventually, calcium channels are activated and an action potential is initiated. 

---