In neurophysiology, an action potential (also known as a nerve impulse or spike) is a pulse-like wave of voltage that travels along several types of cell membranes. The best-unerstood example is generated on the membrane of the axon of a neuron, but also appears in other types of excitable cells, such as cardiac muscle cells, and even plant cells. The resting voltage across the axonal membrane is typically −70 millivolts (mV), with the inside being more negative than the outside. As an action potential passes through a point, this voltage rises to roughly +40 mV in one millisecond, then returns to −70 mV. The action potential moves rapidly down the axon, with a conduction velocity as high as 100 meters/second (225 miles per hour). Because they are able to transmit information so fast, the flow of action potentials is a very efficient form of data transmission, considering that each neuron the signal passes through can be up to a meter in length.[note 1]