The action potential is an explosive release of charge between a nerve cell (neuron) and its surroundings. It moves along a neuron from a dendrite, through the soma and then the axon. It is part of the mechanism that moves nervous messages (nerve transmissions or impulses) along neurons.
An often used but slightly misleading analogy for how neurons work is to compare them to electrical wires along which nerve transmissions flow. This is not actually how they work. A better analogy is to imagine a skipping rope lying on the ground. If you take hold of one end of the rope and give it a quick vertical flick, a wave will move along the rope away from your hand. Nothing except energy actually moves down the rope and, when the wave has finished, the rope is in same position it was before you sent the energy along it.
The same principle applies to nerve impulses moving along axons, except that the wave consists of changes in electrical charge between the axon and its surroundings. These are changes in "electrochemical potential". The electrochemical potential is created when the area outside the axon is filled with an excess of positively charged particles (ions) and the axon itself contains an excess of negatively charged ones. Because negatively charged ions attract positively charged ones (and vice versa), the difference between the electrochemical charge in the nerve surroundings and the charge inside the cell itself creates an "urge" for the ions to flow between the two places. This is the normal state of a neuron and is known as the "resting potential".
The ions involved in setting up the resting potential are potassium (K+), sodium (Na+), calcium (Ca++) and chlorine (Cl-). Some of the cell proteins are also negatively charged. The resting potential is maintained by a "semi-permeable membrane" or "phospholipid bilayer" around the nerve cell which allows some ions to pass through it but not others. It is difficult for the negatively charged ions to move across this membrane and nerve impulses are transmitted by the movement of positively charged ions.
The membrane contains special selective "gates" or "channels" through which certain types of ions are allowed to pass. There are two main channels across the axonal membrane known as "sodium channels" which allow sodium ions to pass and "potassium channels" which allow potassium ions to pass. There is also a special "pump" that ejects three sodium ions for every two potassium ions that enter. During resting potential, the sodium channels are closed but potassium ions (K+) are allowed to freely move across the membrane. The net effect of this is that an electrical potential of about 70 millivolts exists between the nerve cell and its surroundings during resting potential. This is about a twentieth the charge of an ordinary 1.5V battery.
When the nerve cell receives a stimulus, it creates what is known as a "depolarising current". The depolarising current is effectively a force that tries to equalise the electrical potential between the cell and its surroundings. If the current is large enough (55 millivolts) an explosive release of charge is initialised as positive ions flood across the cell membrane causing the electrical potential to fall to 0 millivolts. This is the action potential. If the depolarising current is less than 55 millivolts then the action potential will not be initiated and the neuron will not "fire". There are no grades of nerve impulses - an action potential will either be released or it won't be, and all action potentials are of the same size.
The action potential passes down the length of the axon much as the wave passes down the skipping rope in our analogy. As it reaches each new part of the axon, it initiates the electrical release in the next part and so on until it reaches the end of the nerve. What is actually happening during the action potential is that the sodium channels are opening. Because of the charge difference at resting potential and because there are more sodium ions outside the cell than inside it, those that are outside pour into the cell thus equalising (depolarising) the charge across the cell membrane. As the action potential passes, the potassium channels open and the sodium channels close. When this happens potassium ions leave the cell. Now the ion pumps go into action, ejecting sodium ions, and gradually the cell returns to its resting potential. It take about 1.5 milliseconds for a neuron to return to its resting potential during which time it cannot send nerve impulses. This is called the refractory period.
When the action potential reaches the end of an axon, it causes special chemical messages called neurotransmitters to be released across the space between the neurons (the synapse).
An insulating fatty protein material called myelin sheaths normal axons. Myelin increases the speed at which an action potential can travel down a neuron. Without myelin, the diameter of an axon would need to be 100 times its size to transmit the implulse as the same speed. In multiple sclerosis, myelin is removed from the axon thereby slowing down the speed that the action potential can move down the axon.
Action Potential links:
Neuroscience for Kids - Action Potential
The Neuron & The Nervous System An Overview
Transmission of Nervous Impulses
The Action Potential - A Nerve Impulse
How Brain Cells Work. Part II The Action Potential
Modeling the Action Potential
Normal Function of a neuron
Lecture 4: Neuron and Action Potential