<P> The cardiac action potential plays an important role in coordinating the contraction of the heart . The cardiac cells of the sinoatrial node provide the pacemaker potential that synchronizes the heart . The action potentials of those cells propagate to and through the atrioventricular node (AV node), which is normally the only conduction pathway between the atria and the ventricles . Action potentials from the AV node travel through the bundle of His and thence to the Purkinje fibers . Conversely, anomalies in the cardiac action potential--whether due to a congenital mutation or injury--can lead to human pathologies, especially arrhythmias . Several anti-arrhythmia drugs act on the cardiac action potential, such as quinidine, lidocaine, beta blockers, and verapamil . </P> <P> The action potential in a normal skeletal muscle cell is similar to the action potential in neurons . Action potentials result from the depolarization of the cell membrane (the sarcolemma), which opens voltage - sensitive sodium channels; these become inactivated and the membrane is repolarized through the outward current of potassium ions . The resting potential prior to the action potential is typically − 90mV, somewhat more negative than typical neurons . The muscle action potential lasts roughly 2--4 ms, the absolute refractory period is roughly 1--3 ms, and the conduction velocity along the muscle is roughly 5 m / s . The action potential releases calcium ions that free up the tropomyosin and allow the muscle to contract . Muscle action potentials are provoked by the arrival of a pre-synaptic neuronal action potential at the neuromuscular junction, which is a common target for neurotoxins . </P> <P> Plant and fungal cells are also electrically excitable . The fundamental difference to animal action potentials is that the depolarization in plant cells is not accomplished by an uptake of positive sodium ions, but by release of negative chloride ions . Together with the following release of positive potassium ions, which is common to plant and animal action potentials, the action potential in plants infers, therefore, an osmotic loss of salt (KCl), whereas the animal action potential is osmotically neutral, when equal amounts of entering sodium and leaving potassium cancel each other osmotically . The interaction of electrical and osmotic relations in plant cells indicates an osmotic function of electrical excitability in the common, unicellular ancestors of plants and animals under changing salinity conditions, whereas the present function of rapid signal transmission is seen as a younger accomplishment of metazoan cells in a more stable osmotic environment . It must be assumed that the familiar signalling function of action potentials in some vascular plants (e.g. Mimosa pudica), arose independently from that in metazoan excitable cells . </P> <P> Action potentials are found throughout multicellular organisms, including plants, invertebrates such as insects, and vertebrates such as reptiles and mammals . Sponges seem to be the main phylum of multicellular eukaryotes that does not transmit action potentials, although some studies have suggested that these organisms have a form of electrical signaling, too . The resting potential, as well as the size and duration of the action potential, have not varied much with evolution, although the conduction velocity does vary dramatically with axonal diameter and myelination . </P>

Part of neuron where action potential is generated