Action Potential

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-understood 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 of this high speed, action potentials are used to transmit information, with this being particularly important in neurons, as these cells can be more than a meter long.

An action potential is provoked on a patch of membrane when the membrane is depolarized sufficiently strongly, i.e., when the voltage of the cell's interior relative to the cell's exterior is raised above a threshold. Such a depolarization opens voltage-sensitive channels, which allow positive current to flow into the axon, further depolarizing the membrane. This will cause the membrane to "fire", initiating a positive feedback loop that suddenly and rapidly causes the voltage inside the axon to become more positive. After this rapid rise, the membrane voltage is restored to its resting value by a combination of effects: the channels responsible for the initial inward current are inactivated, while the raised voltage opens other voltage-sensitive channels that allow a compensating outward current. Because of the positive feedback, an action potential is all-or-none; there are no partial action potentials. In neurons, a typical action potential lasts for just a few thousandths of a second at any given point along their length. The passage of an action potential can leave the ion channels in a non-equilibrium state, making them more difficult to open, and thus inhibiting another action potential at the same spot: such an axon is said to be refractory.

The principal ions involved in an action potential are sodium and potassium cations; sodium ions enter the cell, and potassium ions leave, restoring equilibrium. Relatively few ions need to cross the membrane for the membrane voltage to change drastically. The ions exchanged during an action potential, therefore, make a negligible change in the interior and exterior ionic concentrations. The few ions that do cross are pumped out again by the continual action of the sodium–potassium pump, which, with other ion transporters, maintains the normal ratio of ion concentrations across the membrane. Calcium cations and chloride anions are involved in a few types of action potentials, such as the cardiac action potential and the action potential in the single-celled alga Acetabularia, respectively..

The action potential "travels" along the axon without fading out because the signal is regenerated at each patch of membrane. This happens because an action potential at one patch raises the voltage at nearby patches, depolarizing them and provoking a new action potential there. In unmyelinated neurons, the patches are adjacent, but in myelinated neurons, the action potential "hops" between distant patches, making the process both faster and more efficient. The axons of neurons generally branch, and an action potential often travels along both forks from a branch point. The action potential stops at the end of these branches, but usually causes the secretion of neurotransmitters at the synapses that are found there. These neurotransmitters bind to receptors on adjacent cells. These receptors are themselves ion channels, although—in contrast to the axonal channels—they are generally opened by the presence of a neurotransmitter, rather than by changes in voltage. The opening of these receptor channels can help to depolarize the membrane of the new cell (an excitatory channel) or work against its depolarization (an inhibitory channel). If these depolarizations are sufficiently strong, they can provoke another action potential in the new cell.