An action potential is the neuron's way of carrying information, like a signal along a telephone wire. Action potentials are unidirectional electrical signals; that is, they travel in only one direction, from the dendrite, through the soma, and along the axon. An action potential begins when receptors on the neuron's soma or dendrites receive a signal in the form of neurotransmitters from another neuron. The action potential then moves down the axon of the neuron. When the action potential reaches the terminal bouton, special channels open to allow calcium ions to flow into the cell. More calcium ions can also be released from stores inside the cell. These calcium ions carry out many important functions in the neuron, one of which is to cause the release of neurotransmitters from the bouton. The release of neurotransmitters sends a signal to the next neuron to begin an action potential, fulfilling its role as a link in the chain of communication.

When the neuron's receptors find neurotransmitters to bind, they open ion channels in the neuron's membrane that allow sodium ions to flow out of the cell and potassium ions to flow in. Normally, these ions exist in unequal proportions on the inside and the outside of the cell. This is an unfavorable situation, since ions always want to be distributed equally. The separation of charged ions across the membrane generates voltage. The voltage generated by the ion imbalance is called the membrane potential, because the difference in charge lies across the neuron's membrane. The resting membrane potential for a neuron varies between -40 and -90 millivolts, depending on the type of neuron being studied.

When the channels open as a result of the neurotransmitters' signal, the ions are allowed to distribute themselves more equally across the membrane to correct the unfavorable imbalance. When the ions flow in and out of the cell, the membrane potential changes. The resulting changes in voltage and current constitute an action potential. The changes are generally the same for every neuron. The action potential moves from the dendrites or soma down the axon toward the terminal bouton. As it moves, channels along the axon open to let ions flow in and out. You can graph the ions' movements as the change in voltage or the flow of current during an action potential. The voltage graph is the more common of the two. A sample voltage graph and a sample current graph can be seen below.

The positive surge in voltage at the beginning of the action potential is called depolarization. It is caused by fast-opening ion channels that only let sodium ions through. Because there are more sodium ions outside the cell than inside, sodium ions will flow into the cell when the ion channels open, depolarizing the cell. The downward swing, back to baseline levels, is called repolarization. Repolarization is caused by the sodium ion channels beginning to close, and the slow-opening potassium ion channels beginning to open. Because there is more potassium inside the cell than outside, potassium ions will flow out of the cell when these ion channels are opened. Note that the current graph shows the same events, but in a different way; the flow of positive ions (like sodium) into the cell is recorded as negative current, so the current becomes more negative as the voltage becomes more positive.