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. muscular action potentials main articles: neuromuscular junction and muscle contraction 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. plant action potentials plant and fungal cells are also electrically excitable. the fundamental difference from 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. in 1906, j. c. bose published the first measurements of action potentials in plants, which had previously been discovered by burdon-sanderson and darwin. an increase in cytoplasmic calcium ions may be the cause of anion release into the cell. this makes calcium a precursor to ion movements, such as the influx of negative chloride ions and efflux of positive potassium ions, as seen in barley leaves. the initial influx of calcium ions also poses a small cellular depolarization, causing the voltage-gated ion channels to open and allowing full depolarization to be propagated by chloride ions. some plants (e.g. dionaea muscipula) use sodium-gated channels to operate movements and essentially "count". dionaea muscipula, also known as the venus flytrap, is found in subtropical wetlands in north and south carolina. when there are poor soil nutrients, the flytrap relies on a diet of insects and animals. despite research on the plant, there lacks an understanding behind the molecular basis to the venus flytraps, and carnivore plants in general. however, plenty of research has been done on action potentials and how they affect movement and clockwork within the venus flytrap. to start, the resting membrane potential of the venus flytrap (-120mv) is lower than animal cells (usually -90mv to -40mv). the lower resting potential makes it easier to activate an action potential. thus, when an insect lands on the trap of the plant, it triggers a hair-like mechanoreceptor. this receptor then activates an action potential which lasts around 1.5 ms. ultimately, this causes an increase of positive calcium ions into the cell, slightly depolarizing it. however, the flytrap doesn't close after one trigger. instead, it requires the activation of 2 or more hairs. if only one hair is triggered, it throws the activation as a false positive. further, the second hair must be activated within a certain time interval (0.75 s - 40 s) for it to register with the first activation. thus, a buildup of calcium starts and slowly falls from the first trigger. when the second action potential is fired within the time interval, it reaches the calcium threshold to depolarize the cell, closing the trap on the prey within a fraction of a second. together with the subsequent release of positive potassium ions the action potential in plants involves an osmotic loss of salt (kcl). whereas, the animal action potential is osmotically neutral because equal amounts of entering sodium and leaving potassium cancel each other osmotically. the interaction of electrical and osmotic relations in plant cells appears to have arisen from an osmotic function of electrical excitability in a common unicellular ancestors of plants and animals under changing salinity conditions. further, the present function of rapid signal transmission is seen as a newer accomplishment of metazoan cells in a more stable osmotic environment. it is likely that the familiar signaling function of action potentials in some vascular plants (e.g. mimosa pudica) arose independently from that in metazoan excitable cells. unlike the rising phase and peak, the falling phase and after-hyperpolarization seem to depend primarily on cations that are not calcium. to initiate repolarization, the cell requires movement of potassium out of the cell through passive transportation on the membrane. this differs from neurons because the movement of potassium does not dominate the decrease in membrane potential; in fact, to fully repolarize, a plant cell requires energy in the form of atp to assist in the release of hydrogen from the cell – utilizing a transporter commonly known as h+-atpase. taxonomic distribution and evolutionary advantages 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. comparison of action potentials (aps) from a representative cross-section of animals animalcell typeresting potential (mv)ap increase (mv)ap duration (ms)conduction speed (m/s) squid (loligo)giant axon−601200.7535 earthworm (lumbricus)median giant fiber−701001.030 cockroach (periplaneta)giant fiber−7080–1040.410 frog (rana)sciatic nerve axon−60 to −80110–1301.07–30 cat (felis)spinal motor neuron−55 to −8080–1101–1.530–120 given its conservation throughout evolution, the action potential seems to confer evolutionary advantages. one function of action potentials is rapid, long-range signaling within the organism; the conduction velocity can exceed 110 m/s, which is one-third the speed of sound. for comparison, a hormone molecule carried in the bloodstream moves at roughly 8 m/s in large arteries. part of this function is the tight coordination of mechanical events, such as the contraction of the heart. a second function is the computation associated with its generation. being an all-or-none signal that does not decay with transmission distance, the action potential has similar advantages to digital electronics. the integration of various dendritic signals at the axon hillock and its thresholding to form a complex train of action potentials is another form of computation, one that has been exploited biologically to form central pattern generators and mimicked in artificial neural networks. the common prokaryotic/eukaryotic ancestor, which lived perhaps four billion years ago, is believed to have had voltage-gated channels. this functionality was likely, at some later point, cross-purposed to provide a communication mechanism. even modern single-celled bacteria can utilize action potentials to communicate with other bacteria in the same biofilm. experimental methods